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Old May 14, 2010, 10:48 PM   #1
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Default What does Exposure Compensation do?

If I set a camera to manual mode, and set the aperture and shutter and ISO, what changes when I change the EV?

For example, in a low-light situation, I set the shutter to something that will knowingly under-expose but will freeze the scene, and I add some stops of EV to compensate. But what changed? ISO?

(In this example, I'm using a Nikon D40, but I assume the answer is the same for any digicam).

Thanks!
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Old May 14, 2010, 11:05 PM   #2
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EV is the easiest way to correct exposure. If you are shooting a very bright subject, camera's generally overcompensate for it. And underexpose the shot. To simple correct for it just EV to +1/3 or +2/3, the same is true for shooting a darker subject, the camera generally tries to over expose to brighten thing up. To correct for it use -1/3 or -2/3.
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Old May 15, 2010, 4:54 AM   #3
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When using one of the AE modes (i.e.: AUTO, P, A, S, scene modes) the Exposure Compensation setting tells the camera to alter the exposure settings it would normally use by the amount you selected.

When using M Mode the Exposure Compensation setting just alters the exposure settings shown by the camera, but won't actually change anything.

Using the Exposure Compensation setting in M Mode is like, instead of using a plain piece of paper, you use a sheet of letterhead stationery, and then cross out the letterhead.
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Old May 15, 2010, 5:46 AM   #4
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If the camera is metering the scene where you end up with a darker or brighter exposure than desired, you can use Exposure Compensation to change it's behavior.

If you use a -EV setting (needle to the left of center) with Exposure Compensation while shooting in Aperture Priority mode, you'll have a darker exposure than the camera would have used. It gives you a darker exposure by using a faster shutter speed with the selected Aperture. If you use a +EV setting (needle to the right of center), you'll have a brighter exposure than the camera would have used. It gives you a brighter exposure by using a slower shutter speed with the selected Aperture.

If you go to Shutter Priority (where you select the Shutter Speed and the camera selects the Aperture needed), it does the same thing (brighter or darker exposure with Exposure Compensation) by varying the Aperture instead.

If you shoot in Programmed Auto, it may vary either one (or both) to change the exposure.

If you're using manual exposure, there is no Exposure Compensation. You're controlling the variables by setting the Aperture and Shutter speed (using the meter as a guide, with the needle to the left of center giving you a darker exposure, and the needle to the right of center giving you a brighter exposure, as compared to the way the camera metered the scene).

Exposure compensation is just a tool that makes it easier to vary your exposure while still taking advantage of other auto type features, without the need to resort to setting both aperture and shutter speed manually in changing lighting.
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Old May 15, 2010, 10:42 AM   #5
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Some cameras will provide a preset exposure setting in Manual mode by pressing a button somewhere. Pentax, which I'm familiar with, uses the 'green' button for this, to easily come back to a setting which the camera thinks is correct. I can use -+ EV to alter this preset. It is like using auto exposure in manual mode, if that is what you want to do. It can also be used to give a suggested exposure when using a fully manual lens.
In manual exposure, though, the settings you select are what the camera uses.

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Old May 15, 2010, 2:33 PM   #6
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TCav has it right.

On my Pentax K20D, if I'm in Manual mode, if I adjust the EV, what changes is... nothing. The EV indicator moves around; the shutter and aperture stay the same. If I'm in manual mode and I've sighted a target and Green-buttoned it, the camera metering sets an exposure -- shutter and aperture. I can accept or adjust the shutter and aperture, but tweaking EV does nothing but distract me.
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Old May 15, 2010, 3:25 PM   #7
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This may help!
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Old May 16, 2010, 8:21 PM   #8
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Ed, thanks for link, I had read it before.

Thanks to everyone else for their responses. i should have been more specific with the question. I'll rephrase:

If I shoot with shutter priority and set the shutter to, say, 1/250, and because of the low light, the aperture is already wide open, does adding EV+ do anything?

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Old May 16, 2010, 8:33 PM   #9
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Probably not, as long as you're using a specific ISO speed, although some cameras may try to increase ISO speed with Auto ISO (to a point, depending on the camera model) to allow a given shutter speed so that you are not "running out of aperture" (resulting in underexposure).

Using Shutter Priority is usually not a good idea in low light, because of that kind of problem (camera not able to open up the aperture wide enough, resulting in underexposed images). Personally, I stick with Aperture Priority (or manual exposure) for low light shooting; unless I'm trying to use slower shutter speeds deliberately for effect. For example, I sometimes use Shutter Priority at a slower shutter speed while panning with a car to cause some blur in wheels, background, etc. (emphasizing motion). You'll see people shooting planes, helicopters and more using the same technique (slower shutter speeds than would normally be used for something moving fast, so that you're not "freezing" props on them -- panning with them so that they're still relatively sharp, since they're not moving across the frame much).
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Old May 16, 2010, 9:10 PM   #10
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In your scenario, setting a +EV will either bump your ISO setting higher, if possible, or the camera will give you some indication of an out of range setting.
Is this just a 'what if?' scenario, or do you have something specific in mind?

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