Flash Sync Speed Question

[wideangle]

My camera says it syncs with the flash at 1/200th. Can someone please explain what happens when my shutter speed symbol on the LCD flashes, how is the photo effected when I use a shutter speed faster than 1/200th?

Cheers

[I @ M]

Thomas, from memory you use Canon gear?
If it is anything similar to the Nikon high speed flash synch then what happens is that the flash is purely acting as a fill light and instead of one intense flash over the duration of the shutter opening it "fires" a series of very small flashes during the shutter open period.
I may be wrong but my understanding of it is that when the flash discharges just the once when above the synch speed it can create uneven lighting due to the time it is released in conjuntion with the shutter curtains position.
When several small pulses are fired they are spread accross the shutter opening to ensure that the available light isn't mistimed with the shutter curtains.

[arthurking83]

I still don't fully comprehend how images are affected in FP mode, myself.

But what camera are you using Tom?(5D from memory :confused:)

All I really understand is that the flash operates as a pseudo shutter.
if the shutter speed is set too fast(say 1/1000s or so) you may get a dark line across the scene, where the shutter itself is exposed in the scene.

I've mucked about with it, not really using it as a means to an end, more to see what it is and what it does.

I think you get different results with different combinations of equipment, so what model camera and flash you have may be significant.

[wideangle]

Thanks mate, yeah I use Canon gear, looking at using the Canon Speedlite 580 EXII

[Lani]

With my Nikon stuff I get a dark line along the edge of the image, which I understand is the capturing of the last (or first) bit of the shutter.
Here is an example of some shots I took when mucking around with off camera flash and ss was 1/320th from memory
http://www.ausphotography.net.au/forum/showthread.php?t=32923

[I @ M]

My ideas on using high speed synch
Not wanting to blow or darken the background but to have the subject properly exposed.
Poor example below but -----
Spot meter the sky = F/5 @ 1/1000
Spot meter the subject = F/5 @ 1/15
Solution, set the camera on F/5 @ 1/1000
Set the flash to Manual FP ( high speed synch fill ) at full power.
Result, sky and subject exposed correctly.
http://i259.photobucket.com/albums/hh294/ImagesAtMarlo/FP_07-27-200913-34-29.jpg

Lani, the black line along the side of the frame is usually the sign of the trigger not being able to go above the flash synch speed of the camera. My cheap Yong Nuo triggers sometimes don't do the job at 1/250 with monoblocs, other times they do.
Canon users can buy Pocket Wizards now that are allowing very very high synch speeds by fooling the flash with a time difference, Nikon users will have to wait a while for them to develop a Nikon compatible system.

[jev]

My camera says it syncs with the flash at 1/200th. Can someone please explain what happens when my shutter speed symbol on the LCD flashes, how is the photo effected when I use a shutter speed faster than 1/200th?
Chances are your image will show a black bar where the shutter is already closing before the flash fires.

Think of the shutter as consisting of two curtains. When it's closed, there's one curtain completely covering the sensor. When you trigger the shutter, the first curtain opens, thus exposing your sensor to the image. After a while, the second curtain closes, shielding the sensor from the image again.

Now, the shutter curtains are fast but opening and closing still takes time; in your 5D, it takes them 1/200 second to go from completely open to completely close. Faster shutter times are realized by shutting the second curtain before the first one is completely open. Thus, the shutter is never completely open anymore; there's always a part of the sensor covered by a shutter curtain. The minimum shutter time you can set for both curtains to be completely open is known as the minimum synchronization time (often referred to as "X-Sync").

Now imagine what happens if you use flash when this happens. The flash is very fast, 1/10000 sec maybe or even faster. Thus, the light of the flash is shielded where the curtain is covering the sensor.

The solution to this is called "focal plane" or "High speed sync" flash. In that mode, the flash starts firing before the 1st curtain is opening and keeps firing in short bursts until the 2nd curtain is completely closed. The result is comparable to having a continuous light behind you, shining on the subject. If the ratio ambient light versus flash is more than, say, 2 stops, your image will show a dark background (provided that background is more or less far away compared to the subject).

Usually, you can "repair" such a background by taking a longer exposure. Not so with HSS since the flash fires during the complete exposure cycle. You can hoewever choose a higher sensitivity level (increase ISO), which make the ratio flash / ambient shift towards ambient light. Now, the reason to use flash at all might be that the ambient light doesn't suffice to get an exposure that is reasonably short. If you have to increase the ISO to help the HSS, you'll get closer to the point where you could do without flash at all. The trick is to find that threshold where flash and ambient are balanced while still helping to nail the exposure.

Hope this helps a bit to understand what is happening and why it's happening with exposure time and flash.

[wideangle]

Thanks so much for your reply Lani and jev, really much appreciated. Glad I saw those photos of yours Lani with the black line going through, would have freaked out if I saw them for the first time without knowing what it was.