jev
07-01-2009, 9:24pm
The big flash equipment tutorial
This tutorial is meant as an introduction to flash equipment. It describes the basic knowledge required to understand the tutorial on the creative use of flash that will be posted shortly (stay tuned for more! ;)). Its content might be interesting for those that want to buy a new flashgun too.
Disclaimer: some of the information presented in this tutorial may be Canon specific. If you use another brand, your camera and flash combination might react differently. I've tried to minimize the brand-specific stuff except for chapter 4 which really is brand specific.
On to the contents...
1. Various types of flash and their features
1.1 The internal / popup flash thingy
1.2 The system flash or flashgun
1.3 Studio flash
1.4 Exotic miscellaneous
2. Power
2.1 The guide number
2.2 Joules
3. Synchronization
3.1 The minimal synchronization time
3.2 Synchronize on rear curtain
3.3 High-speed flash
3.4 Stroboscopic flash
4 Main light versus fill light
4.1 Fully automatic mode (P)
4.2 Half automatic mode with user-set aperture (Av)
4.3 Half automatic mode with user-set exposure (Tv)
4.4 Fully manual (M)
A. Frequently Asked Questions
B. Bibliography
1. Various types of flash and their features
http://www.ausphotography.net.au/gallery/files/1/5/2/3/flashtypes_thumb.jpg (http://www.ausphotography.net.au/gallery/showimage.php?i=3290&catid=member&imageuser=1523)
Some typical flash equipment (click for larger image)
In this tutorial, I (admittedly completely arbitrary) distinguish the following categories: the internal flash, the system flash, the studio flash and last but not least the repository of "exotics".
1.1 The internal / popup flash thingy
Probably everyone is familiar with it: the internal or popup flash. Treated by most as "useless", it sometimes can save the day. Features: it is built in so you can't leave home without it (duhhh... :rolleyes:), you can't aim it anywhere but straight ahead, it is not so powerful (guide number varies per body, but usually somewhere around the 12 or 13) and it uses power provided by the body (which is both, an advantage and a disadvantage!). I guess this thing doesn't really need any more introduction.
1.2 The system flash or flashgun
Flashguns are available in several sizes and types. Nikon calls them "speedlight", Canon varies on that and calls them "Speedlite". Other manufacturers give 'm a type number only. Canon these days makes three different types: the 220EX, the 430EX II and the 580 EX II, Nikon makes five: SB-R200, SB-400, SB-600, SB-800 and the largest of the pack: the SB-900. Other manufacturers like Metz and Sigma make excellent and often relatively cheap alternatives for all of the top brands. In addition, some manufacturers (Metz specifically) make external flash systems that do not sit on the flash shoe of your camera but instead are hand held.
System flashes in general provide more light than the internal flash, the flash head swivels and turns and they use their own set of batteries. You often can get a couple of accessories like external power packs, gels and other light modifiers for them too. In short, system flashes are becoming a viable alternative to studio flashes these days
1.3 Studio flash
A whole different category of flash contains the studio flash systems. Basically, there are two different types: the flash heads that connect to a big external generator pack and the so-called "monoblocks" that usually hook up directly to a wall socket. These things typically sit on top of a special stand and a triggered from the camera either by a cable hooked up to the X-sync terminal on your camera, a remote control (either radio or infrared) or they can be used in a slave configuration where they act on the light from another flash.
The light the smaller studio flashes generate often can be compared to that of a system flash; the bigger studio flashes however generate a lot more light. They usually are used in combination with some type of light modifier (an umbrella, softbox, snoot, beautydisc or whatever).
1.4 The exotics
Now, this is a totally arbitrary category; I put all the flash types not mentioned earlier in this category: ring flash, macro flash, slave flash, you name it. Flashes in this category usually provide a mix from the above types: they often (but not always) connect to a hotshoe, the can or can not be aimed etc. etc. The flashes in this category fall outside the scope of this tutorial (even though some information is valid for them too!).
2 Power
On first sight, one of the most important features of a flash is the amount of light it can produce - the more light, the better. How much light often is expressed in one of two units: the guide number ("GN" for short) and energy (in Joules).
2.1 The Guide Number
The maximum amount of light an internal flash or a system flash can produce usually is expressed using the Guide Number. This number is the outcome of the formula RG = aperture x distance, specified at 100 ISO. Distance can be expressed in meters or in feet (the latter is somewhat strange and in reality used in the USA only - Yanks specify focal distance in mm as does the rest of the world and set the sensitivity in ISO, but use feet to express subject distance with... now that is plain silly, isn't it?). For the sake of simplicity, I assume all distances and GN's in the remainder of this tutorial to use the metric system.
The internal flash on a typical camera body has a GN = 13, meaning you can use it up to a distance of just over 3 meters if you use f/4 and ISO 100. If you close the aperture to, say, f/8, this distance is reduced to 1.5 meters.
When using an external flashgun, chances are you can derive the GN from the type identifier (Nikonians: you're out of luck here!). Canon's Speedlite 580EX has a GN of 58, the 430EX has a GN of 43.
Caveat... check the specifications of the GN carefully since things can be deceiving. To re-use George Orwell's words: all guide numbers are equal, but some guide numbers are more equal than others. Modern system flashes adjust the opening angle of the reflector to the focal distance of the lens used and some even take the sensor size into account. The idea is that you don't need to light the same area when shooting at a longer focal length than you’d need when using shorter lenses. The longer the lens, the smaller the area, the smaller the angle. Of course, the covered area is different for smaller sensors too. If you don't have to spread out the light, you can concentrate it and thus get more light on the same area. The bundling of light is done by moving the reflector in the flash backwards when shooting at longer focal lenses.
In the end, this means guide numbers are depending on the focal length you’re shooting on. Some manufacturers specify the GN at 105mm (Canon, Metz and Sigma), others at 80mm (Pentax), 50mm (Sony/Minolta) or 35mm (Nikon). Be sure to check the manual for a table representing the GN's at different focal length. For example: here is the table from the Canon's 580EX's user manual:
http://www.ausphotography.net.au/gallery/files/1/5/2/3/guide_numbers_thumb.jpg (http://www.ausphotography.net.au/gallery/showimage.php?i=3291&catid=member&imageuser=1523)
Guide numbers for a Canon SpeedLite 580EX as specified in its manual (click for larger image)
Except from the focal length, the ISO settings are important too. After all, if you increase the sensitivity of your sensor by a factor 2 (going from 100 to 200 ISO), you can take 1 stop back in your aperture settings. This means that the guide number increases if you increase the sensitivity; each time you quadruple your ISO setting, the guide number is doubled.
2.2 Joules
When checking the specification of studio flashes, you will notice light production hardly ever is specified as a guide number. Instead, they specify power in Ws, Watt Second (note: Watt x second, not Watt per second!), also known as "Joule". The idea is that a 100 Ws monoblock generates the same amount of light in a fraction of a second as a "standard" (whatever that may be) 100 Watt household bulb generates in 1 second. The reason for expressing "light power" in this unit instead of a GN is that studio blocks hardly ever are used bare bulb (it usually is modified using softboxes, umbrella's etc), plus typically the distance between camera and subject is very different from the distance between studio flash and subject.
Now, it is not easy to compare different brands based on their specifications in Ws. Some brands are more energy efficient than others and thus effectively generate more light from the same amount of power. There also are differences in attributes that make it hard to compare brands. An increasing number of manufacturers actually specify a guide number in addition to the power rating (LinkStar specifies a 400 Ws monoblock with GN = 65, Elinchrom specifies 64.5 for the same power), but how this is measured and under what circumstances is not clear.
Recap:
- Guide Number GN = distance x aperture at ISO 100
- The GN increases by a factor 2 if the ISO increases by 4
- Modern system flash units change GN with focal lenght and sensor size
- Studio flashes are specified in power x time => thus WattSeconds, not Watt / second as is often written
3. Synchronization
3.1 Minimal flash synchronization time
Modern single lens reflex camera's use a shutter known as a curtain- or focal plane shutter. The mechanism used in such a shutter consists of two "curtains", consisting of light metal fins. When at rust, the sensor is covered by the first curtain. When you trigger the shutter, the first curtain opens. After a set period of time (close to the exposure time), the second curtain (also known as the "read" curtain) closes, after which both curtains are moved to the original position.
The speed at which the curtains move is not infinite; it takes a curtain some 4 milliseconds to move from completely closed to completely travel from "closed" to "open" and vice versa. If you dial in an exposure time that is less than those 4 milliseconds (1/250"), the rear curtain already starts closing whilst the first curtain is not even completely open. Thus, a split of light moves across the sensor and there is no time at which the sensor is completely exposed. If you would generate a flash in this case, part of the sensor will be shielded from the light.
This problem exists with all focal plane shutter cameras with exposure times shorter than 1/200 or 1/250 of a second. The minimum time at which the whole sensor is visible is also known as the X-sync time or the minimal flash synchronization time. If you do not use any tricks, it therefore is not possible to use shorter exposure than X-sync in combination with a flash.
3.2 Rear curtain synchronization
Instead of taking a short exposure, you could dial in a longer exposure time too, for example to show movement. If the flash fires as soon as the first curtain is open, the subject is nicely lit at the start of the movement. Take, for example, a car that runs from left to right through the image area. The car is "frozen" at the beginning of the movement. After, say, 1/1000", the flash is gone... but the curtains remain open and the car is moving. Thus, a ghost trail due to the subject's motion is generated. The effect: the car seems to drive backwards.
http://www.ausphotography.net.au/gallery/files/1/5/2/3/first_curtain_sync_thumb.jpg (http://www.ausphotography.net.au/gallery/showimage.php?i=3287&catid=member&imageuser=1523)
First curtain synchronization (Click for larger image)
This may or may not be what you want. A rally car that seems to drive backwards is a strange thing to see. The solution is rather simple: make sure that the flash fires just before the second curtain closes and the frozen action follows the trail. This feature is known as "read curtain synchronization" (or variations thereof). Be aware that on some camera / flash combinations you need to set this feature on the flash. The Canon EOS system for example provides settings on the camera body to control this feature, but in combination with some flashguns the settings on the flash overrides this setting on the camera.
http://www.ausphotography.net.au/gallery/files/1/5/2/3/second_curtain_sync_thumb.jpg (http://www.ausphotography.net.au/gallery/showimage.php?i=3288&catid=member&imageuser=1523)
Example of rear curtain sync (Click for larger image)
Caveat: rear curtain synchronization needs time. In theory, you could use it on any exposure speed less than X-sync. However, where X-sync on Canon bodies is around 1/200", rear sync won't work with exposure time shorter than 1/40"!
Note: this use of flash with longer exposure times to create an action blur is a balancing act. If you don't expose long enough, hardly any motion might be visible (depending on the speed of the object). In addition, if your flash generates too much light in comparison to the ambient light, chances are the trail is just barely (if at all) visible, but if the flash does not generate enough light, you will see the motion blur but the freezing point won't show).
3.3 high-speed flash
Sometimes, for whatever reason, you may want to use very short exposure times, maybe even shorter than X-sync. Most manufacturers offer a special feature for this purpose known as "High Speed Sync" ("HSS") or "Focal Plane" sync ("FP"). When selected, the flash starts generating pulses of light before the first curtain opens and continues pulsing until the second curtain is closed. Because just a small part of the sensor is exposed at a certain time, this costs quite a lot of energy and the GN will not be met in FP mode. How much energy is lost and what GN is available in this mode is hardly ever specified - use it at your own risk.
Note: as with rear curtain sync, this feature can be set on both, the flash and in the menu on certain Canon bodies. And as with rear-sync, the setting on the flash might override what you set on the body.
3.4 Stroboscopic flash
A feature that is available on some flashguns is "stroboscopic" flash. With this feature enabled, the flash fires several times during the time your shutter is open. In combination with longer exposure times, this gives you the possibility to create images in which you see how an object moves. It is a fun feature that is very nice with sports like motocross and ski board-jumping. However, as with most of these things, the effect soon wears of its novelty. To me, it is more a gadget than a serious feature (except maybe for scientific or sport analysis purposes...).
http://www.ausphotography.net.au/gallery/files/1/5/2/3/stroboscopic_thumb.jpg (http://www.ausphotography.net.au/gallery/showimage.php?i=3289&catid=member&imageuser=1523)
Example of stroboscopic flash (Click for larger image)
Recap:
- flash with shutter speeds < X-sync will generate a partly exposed image only
- rear curtain sync makes motion move forward"
- caveat: rear curtain sync might not work on shutter speeds < 1/40" (Canon!)
- FP flash costs a lot of energy; you won't be able to use the full GN
- Settings on the flash often override settings on the body (Canon!)
4. Main light versus fill light
Note: this chapter probably is quite Canon specific. Would appreciate though if some kind soul adds similar information for other brands!
A flash can be used for several reasons. You could use it simply because there is not enough light (use as main light!), but you could also use it to control shadows (fill light). Other create applications include contrast control and motion stopping. Canon tries to help its users to "guess" how the flash should behave. Thus, the behaviour of the flash and the camera depend on the mode the camera is set to as well as the amount of ambient light available. An explanation...
4.1. Fully automatic (P mode)
In 'P' mode, the camera will choose a shutter speed somewhere between X-sync and 1/60" - it will try to choose such a speed that you could shoot without the use of a tripod. If there is enough light (> 13 Ev*), the aperture will match the ambient light and the flash will be used as fill-light. If it's quite dark (< 10 Ev), the aperture is set wide open and the flash will function as main light source. In between, the flash will balance in ½ stops between main and fill light.
So, what this means in reality:
- outside, when light conditions are quite good, a shutter speed is selected that matches the scene. The flash will make sure the subject (that is close by) is lit well.
- inside, when light conditions are much worse, the shutter speed is fixed to 1/60". The result of this choice is that the background can become quite dark, but the subject should be well lit.
*): Ev = Exposure Value. This is the number of stops required to get a "good" exposure measured from an aperture f/1, shutter speed 1 second at 100 ISO. 10 Ev is ca. 1/60" at f/4, 13 Ev is ca. 1/125 second at f/8.
4.2. Half automatic with aperture priority (Av)
In Av mode you choose the aperture (duh..!). The body will choose a shutter speed that somewhat* matches the ambient light as if no flash is used and the flash will work as fill light only.
*): If there is very little light (< 8 Ev), the camera will choose about half the shutter speed it should use without flash. If there is more light( > 9 Ev) the shutter speed is not changed.
4.3. Half automatic with shutter speed priority (Tv)
In Tv mode you choose the shutter speed and the camera chooses the correct aperture to get as good an exposure of the background as possible. The flash will function as fill light, until the aperture is not enough to get good exposure of the foreground. In that case, the flash will add as much light as needed to get good exposure on the subject.
4.4. Manual (M)
In manual mode, you are in control. If you set the exposure very closely to match the ambient light, the flash will function as a fill light. If there is a large discrepancy between what the camera thinks is correct and what you dialed in, the flash will try to compensate for the missing light.
More on main light and fill light will follow in another tutorial.
A. Frequently Asked Questions
Last but not least, an answer to the most frequently asked questions:
Why does my flash under expose?
Usually, the following things cause underexposure:
The flash is not capable of lighting the complete distance. Check the settings of aperture, ISO and focal length against the guide numbers mentioned in your manual. Increase ISO or choose a larger aperture.
The preflash is bounced against something highly reflective (windows!)
There is a lot of white or light in the scene
Head is not aimed correctly (especially when bouncing is used)
Camera in Av or Tv mode (flash works as fill light only)
Camera in P mode whilst light conditions are reasonable (flash works as fill light!)
Flash has no power (are the batteries okay?)
Why does the flash overexpose?
Over exposure usually is caused by:
A lot of black in the scene
Your subject is too close
Diffused light causes indirect lighting through your omnibounce
Why does rear-curtain sync not work?
A couple of possibilities:
Rear sync set on camera, not on the flash
Shutter speed < X-sync
Focal Plane (a.k.a. "HSS") mode set
Can I use my flash to trigger my studio strobes with?
Yes, you can. But (yes, there always is a 'but' ;))... modern flashes generate a pre-flash to find the correct exposure settings. Your studio strobes may trigger on thus preflash and are not ready when making the real photo. Thus, they won't flash at all or flash without nay power. If you cover the flash and press '*' (FEL) you can make the photo without preflash. If you're using an external (system) flash, you should switch it to manual which will stop the flash from generating a preflash.
How do I prevent the background from turning all black?
Consider a flash photo as a double exposure; once from the flashlight and once from the ambient light. The flash exposure is not (or hardly) affected by the shutter speed, the ambient light exposure however does depend on the shutter speed. If you increase the exposure, the ambient light that probably lights the background becomes more effective. This technique is often called "slow sync" or "dragging the shutter".
Hoe do I get a black background?
Light decreases quadratic with distance. If the subject at 2 meter distance is nicely lit by the flash, it is underexposed by two stops at 4 meter
How do I prevent "red eyes"?
"Red eyes" originate from the flashlight that goes at a straight angle through a person's pupil and illuminate the blood vessels in the back of the eye. If you look at almost the same angle through the pupil, you'll see the reflection of these blood vessels. If you increase the angle between the light coming in and the optical line of the lens, this effect decreases. Thus, the solution is to increase the distance between flashlight and lens (bounce!). Now, if you're using an internal flash, you can't change that angle. Most camera's provide a "red eye flash" feature that triggers the flash a couple of times before taking the picture. The eye's reflex is to close the pupil, thus effectively removing the light from getting into the eye. A disadvantage is that your model will probably squeeze his/her eyes which is not a pretty sight. Better to remove red eyes in post processing IMHO.
The flash head does not zoom anymore, is it broken?
If the head of a flash does not zoom anymore with the focal length of the lens, the most likely causes are:
the head is not aimed straight forward. If the head points up or down or is turned left or right, it automatically zooms to 50mm. Note that "down" can be very subtle - Canon's SpeedLites for example can be pointed down just 7 degrees for close-up photography!
the diffuser for wide angle is (partly) folded out (note: if you're using an omnibounce you can have a hell of a time looking for this!)
some (usually old) lenses do not communicate a focal length to the camera
the head on most flashguns can't zoom more than 105mm. With modern flash/camera combinations, that can include the cropfactor from the sensor. Thus, if you're using a APS-C camera (Canon's xxxD en xxD series for example) and you have a 70-200 mounted, the flash will always be in the 105mm setting (when pointing straight forward!).
en last but not least: yes, it can be broken...
B. Bibliography
Further reading:
[1] NK Guy: Flash Photography with Canon EOS Cameras , Version 1.7. 23 February, 2007, http://photonotes.org/articles/eos-flash
[2] Neil van Niekerk: flash photography techniques, http://planetneil.com/tangents/flash-photography-techniques
[3] Herbert Waldhecker & Stefan Gross: besser blitzen mit Canon-DSLR, http://www.traumflieger.de/desktop/blitzreport/blitzreport_teil1.php (note: German language)
[4] J. Voorhaar: Licht, belichting en verlichting, Maart 2004, Photographic Memories (note: Dutch language)
[5] Canon USA: Speedlite System Tips, http://www.usa.canon.com/dlc/controller?act=GetArticleAct&articleID=288&fromTips=1
[6] Canon USA: Fill-in flash use with EOS cameras and speedlites, http://www.usa.canon.com/dlc/controller?act=GetArticleAct&articleID=106
[7] Canon Japan: http://web.canon.jp/imaging/flashwork/, http://web.canon.jp/imaging/flashwork/
[8] Kevin Björke: Strobe Trigger Voltages, Dated 2004/04/06 04:51:29, http://www.botzilla.com/photo/strobeVolts.html
[9] M. Buschman: Basisboek licht meten, Focus Media Groep, ISBN 9072216598
[10] M. Buschman: Basisboek flitstechniek, Focus Media Groep , ISBN 9072216822 (Dutch language)
[11] Steve Edgell: Licht en belichting, Cantecleer, ISBN 9021323443 (Dutch language)
[12] Hicks, R.: Licht!, Cantecleer, ISBN 9021328526 (Dutch language)
[13] Toomas Tamm: Electronic Flash Information, 28 August 1999, Internet http://www.chem.helsinki.fi/~toomas/photo/flash-faq.html
[14] Bob Atkins: CANON EOS FAQ, Version 2.4, http://www.bobatkins.com/photography/eosfaq
A special thanks to Rick for his assistance in getting this tutorial cleaned up - thanks mate!
This tutorial is meant as an introduction to flash equipment. It describes the basic knowledge required to understand the tutorial on the creative use of flash that will be posted shortly (stay tuned for more! ;)). Its content might be interesting for those that want to buy a new flashgun too.
Disclaimer: some of the information presented in this tutorial may be Canon specific. If you use another brand, your camera and flash combination might react differently. I've tried to minimize the brand-specific stuff except for chapter 4 which really is brand specific.
On to the contents...
1. Various types of flash and their features
1.1 The internal / popup flash thingy
1.2 The system flash or flashgun
1.3 Studio flash
1.4 Exotic miscellaneous
2. Power
2.1 The guide number
2.2 Joules
3. Synchronization
3.1 The minimal synchronization time
3.2 Synchronize on rear curtain
3.3 High-speed flash
3.4 Stroboscopic flash
4 Main light versus fill light
4.1 Fully automatic mode (P)
4.2 Half automatic mode with user-set aperture (Av)
4.3 Half automatic mode with user-set exposure (Tv)
4.4 Fully manual (M)
A. Frequently Asked Questions
B. Bibliography
1. Various types of flash and their features
http://www.ausphotography.net.au/gallery/files/1/5/2/3/flashtypes_thumb.jpg (http://www.ausphotography.net.au/gallery/showimage.php?i=3290&catid=member&imageuser=1523)
Some typical flash equipment (click for larger image)
In this tutorial, I (admittedly completely arbitrary) distinguish the following categories: the internal flash, the system flash, the studio flash and last but not least the repository of "exotics".
1.1 The internal / popup flash thingy
Probably everyone is familiar with it: the internal or popup flash. Treated by most as "useless", it sometimes can save the day. Features: it is built in so you can't leave home without it (duhhh... :rolleyes:), you can't aim it anywhere but straight ahead, it is not so powerful (guide number varies per body, but usually somewhere around the 12 or 13) and it uses power provided by the body (which is both, an advantage and a disadvantage!). I guess this thing doesn't really need any more introduction.
1.2 The system flash or flashgun
Flashguns are available in several sizes and types. Nikon calls them "speedlight", Canon varies on that and calls them "Speedlite". Other manufacturers give 'm a type number only. Canon these days makes three different types: the 220EX, the 430EX II and the 580 EX II, Nikon makes five: SB-R200, SB-400, SB-600, SB-800 and the largest of the pack: the SB-900. Other manufacturers like Metz and Sigma make excellent and often relatively cheap alternatives for all of the top brands. In addition, some manufacturers (Metz specifically) make external flash systems that do not sit on the flash shoe of your camera but instead are hand held.
System flashes in general provide more light than the internal flash, the flash head swivels and turns and they use their own set of batteries. You often can get a couple of accessories like external power packs, gels and other light modifiers for them too. In short, system flashes are becoming a viable alternative to studio flashes these days
1.3 Studio flash
A whole different category of flash contains the studio flash systems. Basically, there are two different types: the flash heads that connect to a big external generator pack and the so-called "monoblocks" that usually hook up directly to a wall socket. These things typically sit on top of a special stand and a triggered from the camera either by a cable hooked up to the X-sync terminal on your camera, a remote control (either radio or infrared) or they can be used in a slave configuration where they act on the light from another flash.
The light the smaller studio flashes generate often can be compared to that of a system flash; the bigger studio flashes however generate a lot more light. They usually are used in combination with some type of light modifier (an umbrella, softbox, snoot, beautydisc or whatever).
1.4 The exotics
Now, this is a totally arbitrary category; I put all the flash types not mentioned earlier in this category: ring flash, macro flash, slave flash, you name it. Flashes in this category usually provide a mix from the above types: they often (but not always) connect to a hotshoe, the can or can not be aimed etc. etc. The flashes in this category fall outside the scope of this tutorial (even though some information is valid for them too!).
2 Power
On first sight, one of the most important features of a flash is the amount of light it can produce - the more light, the better. How much light often is expressed in one of two units: the guide number ("GN" for short) and energy (in Joules).
2.1 The Guide Number
The maximum amount of light an internal flash or a system flash can produce usually is expressed using the Guide Number. This number is the outcome of the formula RG = aperture x distance, specified at 100 ISO. Distance can be expressed in meters or in feet (the latter is somewhat strange and in reality used in the USA only - Yanks specify focal distance in mm as does the rest of the world and set the sensitivity in ISO, but use feet to express subject distance with... now that is plain silly, isn't it?). For the sake of simplicity, I assume all distances and GN's in the remainder of this tutorial to use the metric system.
The internal flash on a typical camera body has a GN = 13, meaning you can use it up to a distance of just over 3 meters if you use f/4 and ISO 100. If you close the aperture to, say, f/8, this distance is reduced to 1.5 meters.
When using an external flashgun, chances are you can derive the GN from the type identifier (Nikonians: you're out of luck here!). Canon's Speedlite 580EX has a GN of 58, the 430EX has a GN of 43.
Caveat... check the specifications of the GN carefully since things can be deceiving. To re-use George Orwell's words: all guide numbers are equal, but some guide numbers are more equal than others. Modern system flashes adjust the opening angle of the reflector to the focal distance of the lens used and some even take the sensor size into account. The idea is that you don't need to light the same area when shooting at a longer focal length than you’d need when using shorter lenses. The longer the lens, the smaller the area, the smaller the angle. Of course, the covered area is different for smaller sensors too. If you don't have to spread out the light, you can concentrate it and thus get more light on the same area. The bundling of light is done by moving the reflector in the flash backwards when shooting at longer focal lenses.
In the end, this means guide numbers are depending on the focal length you’re shooting on. Some manufacturers specify the GN at 105mm (Canon, Metz and Sigma), others at 80mm (Pentax), 50mm (Sony/Minolta) or 35mm (Nikon). Be sure to check the manual for a table representing the GN's at different focal length. For example: here is the table from the Canon's 580EX's user manual:
http://www.ausphotography.net.au/gallery/files/1/5/2/3/guide_numbers_thumb.jpg (http://www.ausphotography.net.au/gallery/showimage.php?i=3291&catid=member&imageuser=1523)
Guide numbers for a Canon SpeedLite 580EX as specified in its manual (click for larger image)
Except from the focal length, the ISO settings are important too. After all, if you increase the sensitivity of your sensor by a factor 2 (going from 100 to 200 ISO), you can take 1 stop back in your aperture settings. This means that the guide number increases if you increase the sensitivity; each time you quadruple your ISO setting, the guide number is doubled.
2.2 Joules
When checking the specification of studio flashes, you will notice light production hardly ever is specified as a guide number. Instead, they specify power in Ws, Watt Second (note: Watt x second, not Watt per second!), also known as "Joule". The idea is that a 100 Ws monoblock generates the same amount of light in a fraction of a second as a "standard" (whatever that may be) 100 Watt household bulb generates in 1 second. The reason for expressing "light power" in this unit instead of a GN is that studio blocks hardly ever are used bare bulb (it usually is modified using softboxes, umbrella's etc), plus typically the distance between camera and subject is very different from the distance between studio flash and subject.
Now, it is not easy to compare different brands based on their specifications in Ws. Some brands are more energy efficient than others and thus effectively generate more light from the same amount of power. There also are differences in attributes that make it hard to compare brands. An increasing number of manufacturers actually specify a guide number in addition to the power rating (LinkStar specifies a 400 Ws monoblock with GN = 65, Elinchrom specifies 64.5 for the same power), but how this is measured and under what circumstances is not clear.
Recap:
- Guide Number GN = distance x aperture at ISO 100
- The GN increases by a factor 2 if the ISO increases by 4
- Modern system flash units change GN with focal lenght and sensor size
- Studio flashes are specified in power x time => thus WattSeconds, not Watt / second as is often written
3. Synchronization
3.1 Minimal flash synchronization time
Modern single lens reflex camera's use a shutter known as a curtain- or focal plane shutter. The mechanism used in such a shutter consists of two "curtains", consisting of light metal fins. When at rust, the sensor is covered by the first curtain. When you trigger the shutter, the first curtain opens. After a set period of time (close to the exposure time), the second curtain (also known as the "read" curtain) closes, after which both curtains are moved to the original position.
The speed at which the curtains move is not infinite; it takes a curtain some 4 milliseconds to move from completely closed to completely travel from "closed" to "open" and vice versa. If you dial in an exposure time that is less than those 4 milliseconds (1/250"), the rear curtain already starts closing whilst the first curtain is not even completely open. Thus, a split of light moves across the sensor and there is no time at which the sensor is completely exposed. If you would generate a flash in this case, part of the sensor will be shielded from the light.
This problem exists with all focal plane shutter cameras with exposure times shorter than 1/200 or 1/250 of a second. The minimum time at which the whole sensor is visible is also known as the X-sync time or the minimal flash synchronization time. If you do not use any tricks, it therefore is not possible to use shorter exposure than X-sync in combination with a flash.
3.2 Rear curtain synchronization
Instead of taking a short exposure, you could dial in a longer exposure time too, for example to show movement. If the flash fires as soon as the first curtain is open, the subject is nicely lit at the start of the movement. Take, for example, a car that runs from left to right through the image area. The car is "frozen" at the beginning of the movement. After, say, 1/1000", the flash is gone... but the curtains remain open and the car is moving. Thus, a ghost trail due to the subject's motion is generated. The effect: the car seems to drive backwards.
http://www.ausphotography.net.au/gallery/files/1/5/2/3/first_curtain_sync_thumb.jpg (http://www.ausphotography.net.au/gallery/showimage.php?i=3287&catid=member&imageuser=1523)
First curtain synchronization (Click for larger image)
This may or may not be what you want. A rally car that seems to drive backwards is a strange thing to see. The solution is rather simple: make sure that the flash fires just before the second curtain closes and the frozen action follows the trail. This feature is known as "read curtain synchronization" (or variations thereof). Be aware that on some camera / flash combinations you need to set this feature on the flash. The Canon EOS system for example provides settings on the camera body to control this feature, but in combination with some flashguns the settings on the flash overrides this setting on the camera.
http://www.ausphotography.net.au/gallery/files/1/5/2/3/second_curtain_sync_thumb.jpg (http://www.ausphotography.net.au/gallery/showimage.php?i=3288&catid=member&imageuser=1523)
Example of rear curtain sync (Click for larger image)
Caveat: rear curtain synchronization needs time. In theory, you could use it on any exposure speed less than X-sync. However, where X-sync on Canon bodies is around 1/200", rear sync won't work with exposure time shorter than 1/40"!
Note: this use of flash with longer exposure times to create an action blur is a balancing act. If you don't expose long enough, hardly any motion might be visible (depending on the speed of the object). In addition, if your flash generates too much light in comparison to the ambient light, chances are the trail is just barely (if at all) visible, but if the flash does not generate enough light, you will see the motion blur but the freezing point won't show).
3.3 high-speed flash
Sometimes, for whatever reason, you may want to use very short exposure times, maybe even shorter than X-sync. Most manufacturers offer a special feature for this purpose known as "High Speed Sync" ("HSS") or "Focal Plane" sync ("FP"). When selected, the flash starts generating pulses of light before the first curtain opens and continues pulsing until the second curtain is closed. Because just a small part of the sensor is exposed at a certain time, this costs quite a lot of energy and the GN will not be met in FP mode. How much energy is lost and what GN is available in this mode is hardly ever specified - use it at your own risk.
Note: as with rear curtain sync, this feature can be set on both, the flash and in the menu on certain Canon bodies. And as with rear-sync, the setting on the flash might override what you set on the body.
3.4 Stroboscopic flash
A feature that is available on some flashguns is "stroboscopic" flash. With this feature enabled, the flash fires several times during the time your shutter is open. In combination with longer exposure times, this gives you the possibility to create images in which you see how an object moves. It is a fun feature that is very nice with sports like motocross and ski board-jumping. However, as with most of these things, the effect soon wears of its novelty. To me, it is more a gadget than a serious feature (except maybe for scientific or sport analysis purposes...).
http://www.ausphotography.net.au/gallery/files/1/5/2/3/stroboscopic_thumb.jpg (http://www.ausphotography.net.au/gallery/showimage.php?i=3289&catid=member&imageuser=1523)
Example of stroboscopic flash (Click for larger image)
Recap:
- flash with shutter speeds < X-sync will generate a partly exposed image only
- rear curtain sync makes motion move forward"
- caveat: rear curtain sync might not work on shutter speeds < 1/40" (Canon!)
- FP flash costs a lot of energy; you won't be able to use the full GN
- Settings on the flash often override settings on the body (Canon!)
4. Main light versus fill light
Note: this chapter probably is quite Canon specific. Would appreciate though if some kind soul adds similar information for other brands!
A flash can be used for several reasons. You could use it simply because there is not enough light (use as main light!), but you could also use it to control shadows (fill light). Other create applications include contrast control and motion stopping. Canon tries to help its users to "guess" how the flash should behave. Thus, the behaviour of the flash and the camera depend on the mode the camera is set to as well as the amount of ambient light available. An explanation...
4.1. Fully automatic (P mode)
In 'P' mode, the camera will choose a shutter speed somewhere between X-sync and 1/60" - it will try to choose such a speed that you could shoot without the use of a tripod. If there is enough light (> 13 Ev*), the aperture will match the ambient light and the flash will be used as fill-light. If it's quite dark (< 10 Ev), the aperture is set wide open and the flash will function as main light source. In between, the flash will balance in ½ stops between main and fill light.
So, what this means in reality:
- outside, when light conditions are quite good, a shutter speed is selected that matches the scene. The flash will make sure the subject (that is close by) is lit well.
- inside, when light conditions are much worse, the shutter speed is fixed to 1/60". The result of this choice is that the background can become quite dark, but the subject should be well lit.
*): Ev = Exposure Value. This is the number of stops required to get a "good" exposure measured from an aperture f/1, shutter speed 1 second at 100 ISO. 10 Ev is ca. 1/60" at f/4, 13 Ev is ca. 1/125 second at f/8.
4.2. Half automatic with aperture priority (Av)
In Av mode you choose the aperture (duh..!). The body will choose a shutter speed that somewhat* matches the ambient light as if no flash is used and the flash will work as fill light only.
*): If there is very little light (< 8 Ev), the camera will choose about half the shutter speed it should use without flash. If there is more light( > 9 Ev) the shutter speed is not changed.
4.3. Half automatic with shutter speed priority (Tv)
In Tv mode you choose the shutter speed and the camera chooses the correct aperture to get as good an exposure of the background as possible. The flash will function as fill light, until the aperture is not enough to get good exposure of the foreground. In that case, the flash will add as much light as needed to get good exposure on the subject.
4.4. Manual (M)
In manual mode, you are in control. If you set the exposure very closely to match the ambient light, the flash will function as a fill light. If there is a large discrepancy between what the camera thinks is correct and what you dialed in, the flash will try to compensate for the missing light.
More on main light and fill light will follow in another tutorial.
A. Frequently Asked Questions
Last but not least, an answer to the most frequently asked questions:
Why does my flash under expose?
Usually, the following things cause underexposure:
The flash is not capable of lighting the complete distance. Check the settings of aperture, ISO and focal length against the guide numbers mentioned in your manual. Increase ISO or choose a larger aperture.
The preflash is bounced against something highly reflective (windows!)
There is a lot of white or light in the scene
Head is not aimed correctly (especially when bouncing is used)
Camera in Av or Tv mode (flash works as fill light only)
Camera in P mode whilst light conditions are reasonable (flash works as fill light!)
Flash has no power (are the batteries okay?)
Why does the flash overexpose?
Over exposure usually is caused by:
A lot of black in the scene
Your subject is too close
Diffused light causes indirect lighting through your omnibounce
Why does rear-curtain sync not work?
A couple of possibilities:
Rear sync set on camera, not on the flash
Shutter speed < X-sync
Focal Plane (a.k.a. "HSS") mode set
Can I use my flash to trigger my studio strobes with?
Yes, you can. But (yes, there always is a 'but' ;))... modern flashes generate a pre-flash to find the correct exposure settings. Your studio strobes may trigger on thus preflash and are not ready when making the real photo. Thus, they won't flash at all or flash without nay power. If you cover the flash and press '*' (FEL) you can make the photo without preflash. If you're using an external (system) flash, you should switch it to manual which will stop the flash from generating a preflash.
How do I prevent the background from turning all black?
Consider a flash photo as a double exposure; once from the flashlight and once from the ambient light. The flash exposure is not (or hardly) affected by the shutter speed, the ambient light exposure however does depend on the shutter speed. If you increase the exposure, the ambient light that probably lights the background becomes more effective. This technique is often called "slow sync" or "dragging the shutter".
Hoe do I get a black background?
Light decreases quadratic with distance. If the subject at 2 meter distance is nicely lit by the flash, it is underexposed by two stops at 4 meter
How do I prevent "red eyes"?
"Red eyes" originate from the flashlight that goes at a straight angle through a person's pupil and illuminate the blood vessels in the back of the eye. If you look at almost the same angle through the pupil, you'll see the reflection of these blood vessels. If you increase the angle between the light coming in and the optical line of the lens, this effect decreases. Thus, the solution is to increase the distance between flashlight and lens (bounce!). Now, if you're using an internal flash, you can't change that angle. Most camera's provide a "red eye flash" feature that triggers the flash a couple of times before taking the picture. The eye's reflex is to close the pupil, thus effectively removing the light from getting into the eye. A disadvantage is that your model will probably squeeze his/her eyes which is not a pretty sight. Better to remove red eyes in post processing IMHO.
The flash head does not zoom anymore, is it broken?
If the head of a flash does not zoom anymore with the focal length of the lens, the most likely causes are:
the head is not aimed straight forward. If the head points up or down or is turned left or right, it automatically zooms to 50mm. Note that "down" can be very subtle - Canon's SpeedLites for example can be pointed down just 7 degrees for close-up photography!
the diffuser for wide angle is (partly) folded out (note: if you're using an omnibounce you can have a hell of a time looking for this!)
some (usually old) lenses do not communicate a focal length to the camera
the head on most flashguns can't zoom more than 105mm. With modern flash/camera combinations, that can include the cropfactor from the sensor. Thus, if you're using a APS-C camera (Canon's xxxD en xxD series for example) and you have a 70-200 mounted, the flash will always be in the 105mm setting (when pointing straight forward!).
en last but not least: yes, it can be broken...
B. Bibliography
Further reading:
[1] NK Guy: Flash Photography with Canon EOS Cameras , Version 1.7. 23 February, 2007, http://photonotes.org/articles/eos-flash
[2] Neil van Niekerk: flash photography techniques, http://planetneil.com/tangents/flash-photography-techniques
[3] Herbert Waldhecker & Stefan Gross: besser blitzen mit Canon-DSLR, http://www.traumflieger.de/desktop/blitzreport/blitzreport_teil1.php (note: German language)
[4] J. Voorhaar: Licht, belichting en verlichting, Maart 2004, Photographic Memories (note: Dutch language)
[5] Canon USA: Speedlite System Tips, http://www.usa.canon.com/dlc/controller?act=GetArticleAct&articleID=288&fromTips=1
[6] Canon USA: Fill-in flash use with EOS cameras and speedlites, http://www.usa.canon.com/dlc/controller?act=GetArticleAct&articleID=106
[7] Canon Japan: http://web.canon.jp/imaging/flashwork/, http://web.canon.jp/imaging/flashwork/
[8] Kevin Björke: Strobe Trigger Voltages, Dated 2004/04/06 04:51:29, http://www.botzilla.com/photo/strobeVolts.html
[9] M. Buschman: Basisboek licht meten, Focus Media Groep, ISBN 9072216598
[10] M. Buschman: Basisboek flitstechniek, Focus Media Groep , ISBN 9072216822 (Dutch language)
[11] Steve Edgell: Licht en belichting, Cantecleer, ISBN 9021323443 (Dutch language)
[12] Hicks, R.: Licht!, Cantecleer, ISBN 9021328526 (Dutch language)
[13] Toomas Tamm: Electronic Flash Information, 28 August 1999, Internet http://www.chem.helsinki.fi/~toomas/photo/flash-faq.html
[14] Bob Atkins: CANON EOS FAQ, Version 2.4, http://www.bobatkins.com/photography/eosfaq
A special thanks to Rick for his assistance in getting this tutorial cleaned up - thanks mate!