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12-10-2009, 12:42 PM   #1
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What is exposure compensation doing?

So when you do the exposure compensation option.. what is it really doing?

It's not changing the aperture or shutter speed or ISO I don't thing... so what is it doing?

Or am I wrong? If it does change those, which is it changing?

Thanks,

Ken

12-10-2009, 12:54 PM   #2
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It might change all three parameters, depending what exposure mode you are using. If using Av, it will likely change the shutter speed, but it can also change the ISO if you have auto ISO enabled. In the above example, if auto ISO is disabled and you set an exposure compensation of -1 and the exposure recommended by the camera was 1/250 at f:8, it will change to 1/125 at f:8.
12-10-2009, 02:24 PM   #3
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Hey Ken.
It increases or decreases the *overall* light the camera allows to pass through the lens onto the sensor by the number of stops you specify.

To do this it calculates the combination of Av, Tv and ISO according to the parameters given (depending on mode and auto settings) as Yves has said.
12-10-2009, 02:38 PM   #4
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I guess what was confusing me is why you can't just alter those settings yourself? Now I realize that in automatic modes the camera would just alter other settings to bring you back to the 'correct' exposure....

12-10-2009, 02:54 PM   #5
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It is an excellent question. Something (aperture, shutter speed, ISO, possibly flash power) always changes, The answer depends mostly on what mode you're in, what camera you have, whether you're and whether you're using auto ISO. The more automatic your mode, the more variables the camera might change.

Exposure compensation is easiest to understand with a fixed ISO and Av or Tv. In Av (Aperture Priority), you choose the aperture, so exposure compensation adjusts the shutter speed. In Tv (Shutter Priority), you choose shutter speed, so exposure compensation adjusts the aperture. On my *ist DS, exposure compensation doesn't do anything in M mode, the logic being that I get to choose all the settings myself. But I think some cameras are different here. In P mode, the camera will adjust both, trying to keep each parameter within a certain range - shutter speed above the 1/focal length guide, for example. Different cameras have more options of programs, etc.
12-10-2009, 03:24 PM   #6
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QuoteOriginally posted by Javaslinger Quote
I guess what was confusing me is why you can't just alter those settings yourself? Now I realize that in automatic modes the camera would just alter other settings to bring you back to the 'correct' exposure....
You *can* alter any or all of the settings yourself. That's what M mode is for. And if on M mode, you want to just meter the scene according to the camera's set metering mode (matrix, centre-weighted or spot), then just hit the green button when composing.
12-10-2009, 04:44 PM   #7
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QuoteOriginally posted by Javaslinger Quote
So when you do the exposure compensation option.. what is it really doing?

It's not changing the aperture or shutter speed or ISO I don't thing... so what is it doing?

Or am I wrong? If it does change those, which is it changing?

EC is a way of "biasing" the meter. It's useful in P, Av and Tv modes (and perhaps elsewhere).

When you shoot in M mode, if you want to deliberately overexpose the shot - for example, so you can properly expose a subject that is backlit - you can SEE the amount of exposure, because in M, you can see the meter as a graph:


- . . . : . | . +

The red colon there in the center is a technically balanced exposure, and the vertical bar shows how from from technically balanced the exposure will be with the current settings.

The problem is, in P, Av and Tv modes, you DO NOT get an exposure graph, at least not initially. Why not? Because in those modes, the camera - by definition - is always going to give you a technically balanced exposure. That's what those modes are for. You set one parameter, like aperture, and the camera figures the other (or others, if you're using auto-ISO as well).

So what do you do if you're shooting in P, Av or Tv mode, and you want to shoot a subject that's backlit? You bias the meter, that is, you dial in +1 EC or something like that.

Now the interesting thing is, while on an older camera like the *ist DS, the EC is shown in the Finder simply as a number ("-0.5", say), on a newer model like the K10D/K20D and I presume the K-7, when you use the +/- button to dial in some exposure compensation, an exposure graph appears. The exposure indicator is now displayed where you want it to be - say, 1 stop to the right of center. But when you're in P, Av or Tv mode, the exposure indicator stays put while you adjust the settings.

This is rather clever, I think. It means you can visualize what you're doing pretty much the same way whether you shoot in M or one of the other modes.

Will
12-10-2009, 05:22 PM   #8
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QuoteOriginally posted by Javaslinger Quote
I guess what was confusing me is why you can't just alter those settings yourself? Now I realize that in automatic modes the camera would just alter other settings to bring you back to the 'correct' exposure....
Precisely. Compensation is a way of telling the camera *not* to bring you back to "correct" exposure. Applying compensation tells the camera to alter whichever parameters is might normally be free alter in the mode you are in in order to make your exposure "incorrect". So, for example, if you're in Av mode, you set the aperture and the camera choose the shutter speed. If you apply exposure compensation, then the camera still chooses the shutter - but it simply chooses a *different* shutter speed than it otherwise would have. Similarly, in Tv mode, the camera picks aperture based on your selected shutter speed; with exposure compensation, it picks a *different* aperture than it otherwise would have, In P mode, the camera picks both aperture and shutter speed, and if you apply compensation that remains true.

12-10-2009, 05:59 PM   #9
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It's also a way of correcting the exposure for when the camera's metering gets it wrong. For example, in snow scenes, the camera's metering will tend to underexpose because of all the white, so the snow comes out gray. By increasing the EV compensation, you're essentially correcting the incorrect exposure value that the camera's meter determined.

But as everyone else mentioned, it doesn't actually change the way the sensor sees things. It only changes the amount of light that the camera will allow to reach the sensor by adjusting aperture, shutter speed or ISO, depending on which one is locked and which one is not. Hence, "Exposure Value Compensation"

On the K7, I use Program mode a lot because it allows me to switch to Hyper Av and Tv modes on the fly with either dial. But I'm still somewhat restricted in my shutter speed range in Hyper Tv mode depending on the EV that the camera determined and the available aperture settings. The camera won't let me set a shutter speed that will result in an over- or under-exposed shot. I can adjust this range by adjusting the EV compensation, but then I end up purposely over- or under-exposing my shots.
12-11-2009, 12:11 PM   #10
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So in M mode where you have dialed in aperture, Shutter speed, & ISO, what is the camera changing since you have locked all 3 in?

George
12-11-2009, 12:43 PM   #11
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QuoteOriginally posted by gsrokmix Quote
So in M mode where you have dialed in aperture, Shutter speed, & ISO, what is the camera changing since you have locked all 3 in?
George,

What is the camera changing when?

In M mode, the camera doesn't change ANYTHING. If the exposure sucks, the camera can blame it entirely on you.

Will
12-11-2009, 02:24 PM   #12
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QuoteOriginally posted by gsrokmix Quote
So in M mode where you have dialed in aperture, Shutter speed, & ISO, what is the camera changing since you have locked all 3 in?
Even in Manual mode, the camera still calculates a "proper" exposure value. It can't change anything, but it can complain when you're out of range. For example, on my K7, if my settings put me out of the EV range, the screens (both the monitor and top LCD ones) blink at me furiously. Once I've adjusted the settings enough to be back in range, the blinking stops. I can still take the picture while the screens are blinking, but it may or may not look very good depending on the results I'm going for. In this case, the calculated EV from the camera acts as a guide for you to adjust your settings by, and EV compensation doesn't do squat except change when the screens will blink at you.
12-11-2009, 02:53 PM   #13
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QuoteOriginally posted by GoremanX Quote
Even in Manual mode, the camera still calculates a "proper" exposure value. It can't change anything, but it can complain when you're out of range. For example, on my K7, if my settings put me out of the EV range, the screens (both the monitor and top LCD ones) blink at me furiously. Once I've adjusted the settings enough to be back in range, the blinking stops. I can still take the picture while the screens are blinking, but it may or may not look very good depending on the results I'm going for. In this case, the calculated EV from the camera acts as a guide for you to adjust your settings by, and EV compensation doesn't do squat except change when the screens will blink at you.
Don't disagree with this. I'd just put it differently.

In M, the camera's meter doesn't "complain." It just tells you what it thinks. Depending on how you have your settings configured, you may see two to three stops +/- in the exposure graph in the Finder, when you're using M. Now if the meter shows you that you're at +3, the shot may be really badly overexposed. The meter only starts blinking at you when the meter's reading is literally off the chart, because it's more than +/- 2 or 3.

I say this because I'm worried that someone who doesn't quite understand this would think that blinking while "out of range" implies that, if the numbers in the finder are NOT blinking, then the exposure is "in range." Not so, if "in range" is understood to mean "generating an acceptable exposure." IN this case, "in range" should be understood simply to mean, "within the scope of the graph." It's not even out of the range of the meter. It's just out of range of the DISPLAY.

Will
12-11-2009, 03:51 PM   #14
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QuoteOriginally posted by WMBP Quote
In M, the camera's meter doesn't "complain." It just tells you what it thinks. Depending on how you have your settings configured, you may see two to three stops +/- in the exposure graph in the Finder, when you're using M. Now if the meter shows you that you're at +3, the shot may be really badly overexposed. The meter only starts blinking at you when the meter's reading is literally off the chart, because it's more than +/- 2 or 3.
Yeah, and my mother-in-law isn't complaining when her volume starts going off the scale, she's just making sure her alternative point of view gets across

Maybe the word "complain" isn't accurate, but it sure feels like the camera's bitching at me when all 3 settings start blinking just because I'm taking a picture of snow

In any case, the point is that changing the EV compensation does not change any picture-taking settings in M mode, but it does change the range at which the settings will start blinking.
12-11-2009, 09:05 PM   #15
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Ahhhh makes sense. I knew to make any kind of EV change in manual you had to actually change aperture, shutter, or ISO yourself. I was confused as to what the EV comp let you change in M mode. So it changes nothing with the exposure (because you have to) but changes the "scale" of the meter?

So if I dialed in a +3 EV, then while looking at the regular meter, "0" would than actually be +3?

Thanks!

George
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