What does exposure compensation really do?

[evil4blue]

Okay, I've been thinking about this for a little while and am still have trouble understanding it completely.

As far as I can tell, exposure compensation is really only a gain setting on your camera. It doesn't affect your shutter, aperture, or ISO; so the only way it can adjust your exposure is apply a gain (+/-) to the data recorded to your sensor. Correct??

If the above is true, then you would always be better off adjusting one of you main 3 settings (shutter, aperture, ISO) to get proper exposure rather than exposure compensation. Now, I know that there are times when you need exposure compensation (need a faster shutter or artificially increase ISO), but aside from those instances, why would you ever use exposure compensation?

Now there is one interesting application, I guess you can take a shot in low light and dial in a +3 compensation, then in your raw software push the compensation another 3 or more stops. The result would probably show horrible noise, but you could take a shot (hand held) that you probably couldn't have otherwise.

The next confusing piece is flash compensation. Flash compensation actually makes more sense in my mind because it physically changes something (flash duration), but when you use the flash in ttl mode coupled with exposure compensation, things get kinda tricky (at least for fill flash).

Take this example, you have a back lit subject that needs some fill flash. You expose for the background and and set you camera (in manual mode), then you dial in -1 flash compensation to give you a more natural looking fill. As long as your flash is in ttl mode, this should work great (assuming your flash sync speed isn't surpassed).

Okay, next scenario, you took the above mentioned picture, but your background is too bright and is giving your subject a halo like effect you don't want. Common sense says you need to turn up your shutter speed, but you only have a 1/250 (or 1/500) sync speed. You realize that you're only about a stop off so you dial in an exposure compensation of -1 and take the same picture. Now the sky is correct, but your subject is underexposed by 1 or more stops. If you readjust you flash exposure to 0, then your photo has that flash picture look. What do you do, your flash exposure is based on your camera's exposure (in ttl mode). I guess you can shoot your flash in manual mode, and adjust until the settings look right, but then you lose all the nice ttl functionality. I'm totally confuse on how the 2 compensation values work together.

There are a few other answers to this scenario, move your subject or use high speed flash sync. Moving your subject may not be feasable, since you wanted a certain background. High speed fash sync confuses me since I don't understand how it works. If my camera has a 1/250 flash sync, why can I sync at higher speeds?

Sorry for all the rambling, but if someone can make heads or tails of the above mish-mash, thanks in advance.

[Gordon]

Your starting premise is wrong. Exposure compensation does adjust the shutter speed or aperture setting that the camera uses (depending on which mode you are shooting in ) In some cameras it will also shift the ISO.

It doesn't directly change the gain of the sensor - (actually that's what the ISO does)

Exposure compensation is a way to provide a hint to the camera meter.

Essentially your camera's meter expects that everything you'll ever point it at will be an average, 18% reflective grey value (or 11% but that's a different tangent to go on )

Exposure compensation is a way to indicate to the camera in any of the modes where it picks the
aperture or shutter speed that what you are pointing at is lighter or darker than that mid tonal
value. It then compensates the aperture or shutter speed that it selects accordingly.

Now, depending on the meter pattern you use, the area that is considered can vary - spot, average,
center weighted etc. It can even be made up of a whole lot of historical data and complex samples
from across the scene (matrix modes for example) But even there, you can bias the final result
with exposure compensation.

However, at least for the matrix modes, exposure compensation is a dangerous game to play -
as you don't know what the camera thinks it is pointing at. (I.e., it doesn't assume a mid tone
setting for the matrix modes, from what I understand)

You can test this easily btw: Put your camera in spot/center meter mode. Switch it to aperture
priority, point it at a white wall, in light that gives you a middling level shutter speed. Now play
with the compensation - what changes ?

Now switch to shutter priority and do the same again - what changes ?

[Gordon]

The second part of your question is about flash usage. Flash is easy, once you understand two concepts

1/ that every time you use flash, you are using fill flash.
2/ every time you use flash, there are two exposures going on.

Ambient light exposure: Aperture and shutter speed control the exposure (and ISO)
Flash light exposure: Aperture and flash duration control the exposure (and ISO)

Note that for the flash exposure, the shutter speed has no influence on the exposure
(ignoring the case where you are operating beyond the sync speed of your camera)
The flash pulse and the aperture are what matter, not the shutter speed.

Also confusing, at least with a Canon camera, is that in Av or Tv mode, when shooting with flash,
the camera always meters for the ambient light - never the flash. So if it is dark, in Av or Tv mode,
you'll get a long shutter speed, even with a high powered flash.

Basically you need to switch to manual, set an aperture and shutter for the ambient light and
then either set the flash to manual or use TTL for the flash duration control (along with the
aperture you set )

[Silvernic]

Thanks Gordon! You're a king, you know that?

[evil4blue]

Originally Posted by GordonMcGregor

Your starting premise is wrong. Exposure compensation does adjust the shutter speed or aperture setting that the camera uses (depending on which mode you are shooting in ) In some cameras it will also shift the ISO.

If this is true, then why doesn't my exif reflect that my aperture or shutter has changed? Also, compensation is really only practical in the different program modes, correct? Why would you use compensation in manual?

[Gordon]

Exposure compensation does nothing in manual mode (well, it might vary the meter values shown in display, but) it doesn't change the resulting exposure at all.

So EC changes the shutter in aperture priority mode, the aperture in shutter priority mode and does 'something' in the more automatic/ and program modes - probably dependant on keeping a fast enough shutter speed for hand holding.

In some cases/cameras you can set it to bump the ISO if it wants to as well. However, all
it does, fundamentally, is change how the exposure value is calculated, before selecting an appropriate shutter speed or aperture.

In which cases does your exif not reflect that your aperture/ shutter has changed ?
What metering and shooting mode are you in ? Are you pointing at the same scene, in the
same light in each case ?

You wouldn't use compensation in manual. You _might_ use flash exposure compensation
in manual mode, because the manual exposure settings are only for the ambient light. The flash
exposure could still be in automatic or manual mode.

[evil4blue]

Well if I dial in EC, the EXIF shows the EC dialed in, but doesn't show that the A or S has been changed.

Wait a minute, something just clicked...I maybe lying, I need to double check before I go spewing some fallacies.

Thanks for your help Gordon.

[Gordon]

The exif will show the new A or S if you aren't in manual mode. But it'll just show the values, it doesn't say 'aperture changed from f8 to f5.6 due to EC'

So if you pointed at a wall and the meter reading was f8 at 1/250s and you then added +1 stop of exposure compensation, the meter reading would now be f5.6 at 1/250s, assuming you were in shutter priority mode.


Your exif would show f8 at 1/250s for the first shot, with 0 EC
Your exif would show f5.6 at 1/250s for the second shot with +1 EC

[jeffkohn]

Gordon is correct with regard to what EC does in Aperture or Shutter priority; it definitely affects either the shutter speed of f/stop.

In the case of Nikon cameras, if you're in Manual Exposure mode EC will "shift" the meter display by how ever much you set the EC to. I think in most cases this would cause more confusion than anything else, but it can be useful in some situations.

[Nevyn]

Originally Posted by GordonMcGregor

Flash light exposure: Aperture and flash duration control the exposure (and ISO)

Don't some flash units modify the output power as well?

[d2creative]

Originally Posted by jeffkohn

In the case of Nikon cameras, if you're in Manual Exposure mode EC will "shift" the meter display by how ever much you set the EC to. I think in most cases this would cause more confusion than anything else, but it can be useful in some situations.

So going back to the original question, using EC (in most cases) is pretty pointless. Just adjust your shutterspeed or aperature accordingly. Correct?

[evil4blue]

Well after doing a little more external research I can say that Gordon is completely correct on all accounts. Thanks for clearing this stuff up for me. I rarely ever use EC anyway, but mostly because I was confused about its inner workings.

@Dennis,

On my D70, I used to have +1/3 EC all the time to help the underexposure I usually got with the D70. I did it, just because I didn't what to have to remember to add a 1/3rd stop on every exposure. On the D200, it actually has a meter calibration that you can set to get better exposures. I have that one set to +1/6 since I feel the D200 meters slightly low, not as bad as the D70, but low none-the-less.

[Patti Edens]

Here's how I use it... Feel free to correct me if I'm wrong.

Example 1: I'm shooting a dark colored bird that takes up about 1/5 of the FOV and the background is a very bright sky. I'm using a Canon 20D on center weighted average and the bird is not in the center.

The camera will "see" all that brightness and will underexpose the bird, so while in aperture priority, I adjust the EC so that the meter indicates around +1 or so. The sky will be blown but the bird will be exposed correctly.

Example 2: I'm shooting a swan that is in very dark water. Same as above. The camera will "see" all the darkness and will overexpose the swan. So, I adjust to -1 or so.

In both cases, I check the histogram and shoot again if I don't get the EC right the first time (if the birds cooperate and hang around.)

Of course there are many ways to do this differently: Spot metering *, Manual mode, shutter priority, hand held light meter, exposure bracketing + blending, etc.

I do it as described above except when my subject isn't going to move on me. Then I bracket the exposure with at least three shots and blend in Photoshop later.

* Spot metering isn't available on the 20D. It has a partial metering mode, but it comprises around 9% of the field of view instead of 1 or 2% as I would expect from a true spot meter.

Patti

[Gordon]

Originally Posted by Nevyn

Don't some flash units modify the output power as well?

Yup, they do, and some other things like pulse type and the like. My basic point here
though was the shutter speed doesn't have much to do with the flash exposure. It's
aperture and 'amount of light that comes from the flash'

Where 'amount of light that comes from the flash' is varied by time or intensity (power is a function of time really - energy over a period of time)

[Gordon]

Originally Posted by d2creative

So going back to the original question, using EC (in most cases) is pretty pointless. Just adjust your shutterspeed or aperature accordingly. Correct?

No, not really. Using EC in manual mode is pointless. Using EC in aperture priority or shutter priority mode is pretty much required if you are shooting in most metering modes and you are shooting scenes that aren't average grey

(, white wedding gowns, black dogs, snow scenes, beaches, night time etc are all good times to consider using positive or negative EC, if not shooting in manual mode)

I use EC all the time, but I mostly shoot in aperture priority mode.