A question about Exposure Value (EV)

[tduell]

Hello All,
On a number of occasions I have seen references to exposure value (EV) worded such that a larger EV implies "more" exposure. I recently read (in a forum here) "...low light needs more exposure values".
Now this appears to be back to front to my understanding of EV, and maybe I have been misunderstanding what some have meant when they refer to EV, or maybe there are different definitions.
Wikipedia (http://en.wikipedia.org/wiki/Exposure_value) gives a good definition of EV, which accords with my understanding, and provides a table of exposure times for various exposure-values and f-numbers.
Here's a couple of examples...

f-2.8 at 1/1000 sec = EV 13
f-2.8 at 1/500 sec = EV 12
f-2.8 at 1/2000 sec = EV 14

which implies that higher EV means less exposure, lower EV means more exposure.

So, as they would say in school..."please discuss".

Cheers,
Terry

[WhoDo]

What is often being discussed here in regard to Ev (Exposure Value) is the facility in DSLR's to temporarily change the exposure settings (combination of shutter speed, ISO and aperture) by adjusting the Ev in camera either + or - from the physical settings being used. The point is that what is being discussed is not the total Ev (as per your example) but the amount of variance from that total by making Ev adjustments in camera on the fly. Hope that makes sense. If it doesn't I may need a drink! :beer_mug:

[Kym]

We have a couple of library items that will help...

http://www.ausphotography.net.au/forum/showlibrary.php?title=New_To_Photography:What_is_Exposure_Value

http://www.ausphotography.net.au/forum/showlibrary.php?title=New_To_Photography:Exposure_Compensation_Ev

[Mark L]

...... I recently read (in a forum here) "...low light needs more exposure values".

I wonder if the poster had confused definitions and was referring to exposure compensation?
Without knowing the context of the thread, maybe, maybe not.

[Kym]

I wonder if the poster had confused definitions and was referring to exposure compensation?
Without knowing the context of the thread, maybe, maybe not.

Probably! See my links above. Ev as an absolute value and +/-Ev as a compensations value are different.
Understanding Ev is very useful.

[tduell]

I have read the material in the library and some other material on the subject, and it does seem that there is more 'convention' than logic in the way EV is used, both in the definition as per Wikipedia, and in relation to exposure compensation, but perhaps that's got something to do with the way my brain's wired up.
Firstly, the basic definition is counter intuitive. If you didn't know anything about the subject, I suspect the assumption would be that a higher EV would mean more exposure, i.e. more light onto the film or the sensor, but in fact it is the reverse.
OK, that can accepted as 'by definition', but then the convention with exposure compensation seems to be that +0.3 EV means increasing the exposure by 0.3 EV, but the end result is an EV 0.3 lower!
To me, it does seem a bit 'back-asswards' as some might say!
I guess one just has to learn the conventions and live with it.

Cheers,
Terry

[dtmateojr]

Exposure values are fixed values. For example what we know of as sunny f/16 rule refers to EV15. To properly expose a subject that is under EV15 you use f16 at 1/ISO. EV14 is less intense so you would normally open up or decrease the shutter speed. You can go f11 at 1/ISO or f16 at 2/ISO.

Some photographers (myself included) refer to a camera's meter reading as EV0 and any compensation as - or + EV. This is different. A camera's EV0 could mean real EV7 or anything.

So if an AF system says it can focus at -3EV (Pentax K5II) that is equivalent to almost complete darkness. They are practically saying that a camera can focus even if there is no more light.

[arthurking83]

Hello All,
On a number of occasions I have seen references to exposure value (EV) worded such that a larger EV implies "more" exposure. I recently read (in a forum here) "...low light needs more exposure values"......

With respect to this specific comment, it could be a miscommunication/misunderstanding thing.

As already said, Exposure Values have predefined meanings, as already listed in the tables you have already read.

if some has commented somewhere that low light needs more exposure value, then this is simply wrong, you may have misread the comment or something along those lines.

That is, if the comment was that low light needs more exposure, then this is technically correct.

the comments, "more Exposure Value" and "more exposure" are not the same thing.

In fact a comment such as "more exposure" value is technically incorrect in that it's a misuse of grammar!
it can be used like that, but a better expression is something like "higher Exposure Value"

As you know (from the table), a higher Exposure Value will refer to a brighter scene.

A comment such as "more exposure"(eg. as in "you needed more exposure") simply means that the image may have been underexposed according to that viewer's perspective.


OK, that can accepted as 'by definition', but then the convention with exposure compensation seems to be that +0.3 EV means increasing the exposure by 0.3 EV, but the end result is an EV 0.3 lower!
To me, it does seem a bit 'back-asswards' as some might say!
I guess one just has to learn the conventions and live with it .......

On the topic of Exposure Compensation, which it looks like you have a fairly good grasp of the idea, you should note that there are two ways to look at the concept.

One point of view of exp-comp is the way you're reading it.
But an alternative perspective of it is that it's way to tell your camera that the scene is NOT as bright as it thinks it is, or that you want it brighter/darker and not as thinks it is.
The simple way to look at it is that you're recalibrating the meter in the camera by this exp-comp amount.
This can be important because different sensor materials expose differently and various lenses pass light through to the sensor differently too.
So the world needs a way to tell the tools to do it differently.

The way you're reading the notion of exp-comp is that you're dealing with both the absolute idea of Exposure Value and exposure as the same thing(which they aren't).

Remember Exposure Value is saying that the scene has a specified brightness(according to some redefined grey value).

Exposure(more or less of it) is saying that the resultant image needs more brightness, or less brightness.
The scene(as seen) and the resultant image are not the same thing(but sometimes the idea is to achieve some type of equality of the two).

Exposure compensation is not referring to the scene directly, it's a proposition(made by the camera operator) directing the camera to 'darken'(-Ev or let less light onto the sensor) or 'brighten'(+Ev allow more light) the image it's about to capture.... ie. the meter is incorrectly seeing the scene(by this amount)

Just quickly, it should be noted that the camera(or more specifically, the camera's metering) is not quite as smart as you may think it is.

I've been wanting to do a quick/simple thread on what the camera meter actually does, but either forget, or don't have the time.(strangely I'm rushed at this moment too!)
I'll try to do it this coming weekend if I can and reply with a link to it here too.

Once you fully understand it, exposure compensation will make a bit more sense.

[Steve Axford]

Thanks Arthur. Some straight explanation. I actually thought I understood it - then I was totally confused by some of the posts - now it seems simple again.

[arthurking83]

..... Some straight explanation. ......

LOL!
I almost confused myself at one point. After posting it I tried to read it back but was rushed not to burn dinner, get the kids fed and then off to their taewkondo.


I actually don't think understanding the technicalities of it all is important.

That is, who really cares if a large room lit with a single 100W globe represents an Exposure Value of about 5 or 6 .. or that the Moon has an Ev of 15 .. but that a (full)moonlit night is 4 or 5?

The problem being that every camera sensor is probably different(slightly different sensitivities), I can't think of any camera that doesn't have an exposure meter .. so this is probably the best guide to Ev requirement anyhow(except for specific purposes when you're adding your own special lighting into the scene).


These exposure tables were created for film(obviously in the film era).

While some films are different to others, you still have the situation where two different cameras can have film of a similar brand/type .. and this would be where the Ev tables could help with consistent exposures between different gear.

But in general, my three cameras all produce slightly different exposure results using the exact same lens, and exposure values in camera.
If I remember correctly ... I think the D70s is darkest, D300 comes in at mid range, and I've noticed that the D800 comes in brighter for their resultant images.
It might not be all that much different, maybe 1/6-1/3 Ev difference, but it is different.

I expect different exposures from the D800 over it's entire frame as it's a different format, but even in Dx crop mode it shows a brighter exposure over the D300(maybe by only 1/6 Ev tho).


Also, for the OP .. the way to visualise how exp-comp is recalibrating your cameras meter, and nor directly referencing the Ev of the scene .. set your camera to manual mode.



In Manual mode, you don't actually have access to exp-comp per se .. (you do but it works differently to how it does in the Semi Manual modes)
That is, the definition of manual mode is that you have to do all the hard work yourself.
BUT! You can still use exp-comp(or at least you should), and the way it works is different to how it does if you're in Aperture priority or Shutter priority modes.

In the latter two modes you will see a visual indicator in the meter indicator(which could be what's causing the confusion). So it's showing you that it's going to be different to how it thinks it should be.

In Manual mode, with exp-comp set, you don't see this same indication in the vf. You may see that the meter shows a certain expected brightness or exposure, but you adjust the cameras exposure parameters to achieve a neutral point if you want too.
If you set exposure compensation whilst in Manual mode, you still see the same meter indication but the exposure parameters will be different.

An example of this could be something like you've set 1/10s, f/2.8 and ISO 100 on a plain evenly lit surface, and the meter reads 0Ev on the bar indicator.
If you set -1Ev(still in Manual mode) to get the meter to read 0Ev, you now have to change one of the exposure parameters with less sensitivity.

In effect what you've done is reset/recalibrated the meter by using exp-comp in Manual mode.

if you're wondering why you would want exposure compensation whilst in Manual mode .. again it comes back to the gear used(eg. lenses).

[Steve Axford]

I must admit that I've never used EV in anger. I just see it on the camera with +or- for fstop. I guessed what it meant and was more or less right. I first thought it meant electron Volt (or eV), but couldn't figure out what electrons had to do with exposure.
I'm one of those strange old timers who never used film, so I missed the reasons for many of these things. Digital makes exposure so easy. If you can't think of any other method, take a photo, see what it looks like and adjust the exposure to taste. That would be impossible with film, or at least, very slow.

[Kym]

If you can't think of any other method, take a photo, see what it looks like and adjust the exposure to taste. That would be impossible with film, or at least, very slow.

Two words... Light Meter :D

[Steve Axford]

Um - see what it looks like? Very slow with film, even with a light meter.

[Kym]

Um - see what it looks like? Very slow with film, even with a light meter.

The point is with a light meter you can sort of the exposure, you don't need to see the neg or print to get a good exposure.
Also: Think about Ansel Adams Zone System http://en.wikipedia.org/wiki/Zone_system

[Steve Axford]

Ok Kym, imagine you are taking a photograph of a tiny mushroom. It is about 1mm across the cap and it is a primary colour (eg yellow). In order to get a light meter small enough to measure that you would probably need to pay a lot. Also, when you have a primary colour, light meters usually read different from the what the film or sensor responds to. So, you would have to calibrate your light meter for each primary colour. The net result is - you would need a very expensive meter, plus a lot of work.

The alternative would be - you use the camera light meter to get an approximate value, you take a picture, you look at the result and correct the exposure before taking the final shot. You could even do without the camera light meter at all, though this would take several shots to get it right.

Both methods are possible, but I know which one I would choose. With film, you have to find a way to make light meters work, as in most cases you just can't look at the result and then take another shot. In the case of the mushroom, it has probably gone by the time the film is developed.

[arthurking83]

Two words... Light Meter :D

Note tho that the light meter can 'lie'.