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DPChallenge Forums >> Hardware and Software >> Need Help with Exposure Compensation...
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04/14/2007 01:22:35 PM · #1
Hi. Can someone explain exposure compensation to me? I've looked around, and my understanding is that moving it up (to say +2.0) might compensate by opening up the aperture. So I took a few tripod shots where the only thing I changed was the EC. But in the EXIF data the aperture and shutter speed (even ISO) are the same, but one picture is clearly brighter than the other. Can someone how the picture is brighter when EC didn't seem to change any settings? Thanks!
04/14/2007 01:35:04 PM · #2
Exposure compensation makes an adjustment to the meter reading. +1 will overexpose by one stop, and -1 will underexpose by one shot.

It's very weird that the EXIF data stays the same. Maybe the camera records the meter reading, before any exposure compensation...
04/14/2007 02:09:23 PM · #3
Huh. When I change my EC the EXIF has a line in it EC +1 or whatever.

I'd think it'd also depend on HOW you are shooting. I tend to shoot Aperture Priority which means to expose +1 EC it would have to take a longer exposure.

If you're shooting in Shutter Priority it would have to adjust your aperture.

If you're in Manual, you've got me.
04/14/2007 02:11:32 PM · #4
Originally posted by bvy:

Hi. Can someone explain exposure compensation to me? I've ...

What that means in terms you and I can understand is that it will look like the exposure is set correctly when you look at the meter even though it may, in "reality", be over or underexposed by 1 or 2 f/stops.

Sounds silly, but there are times you MIGHT want that, but not often unless your meter is truly "off".

Message edited by author 2007-04-14 14:12:50.
04/14/2007 02:15:24 PM · #5
Originally posted by stdavidson:

Originally posted by bvy:

Hi. Can someone explain exposure compensation to me? I've ...

What that means in terms you and I can understand is that it will look like the exposure is set correctly when you look at the meter even though it may, in "reality", be over or underexposed by 1 or 2 f/stops.

Sounds silly, but there are times you MIGHT want that, but not often unless your meter is truly "off".


You will want to overexpose a bit if you are shooting a high key image for example.
04/14/2007 02:36:29 PM · #6
Exposure is adjusted by three settings; aperture, shutter speed and sensitivity (ISO). Exposure compensation does not directly adjust the exposure, it adjusts the way the meter reads the scene. That is, EC is a meter adjustment.

To understand what it does, you must first know what the meter is doing. The meter will try to expose the scene so it matches neutral grey (18% grey). That is the grey that is midway between white and black. This is true no matter how bright the scene is; take a picture of a black square and then a white one, and the images will look to be the same gray tone (or very near). This is why exposing on a grey card will produce technically perfect exposure -- it is exposing the card to the grey level it actually is.

This is where EC comes in. What it does is adjusts the grey tone the meter will match the exposure to. So, for instance, when shooting in the snow it is a good idea to adjust the exposure up a stop. This causes the meter to expose the scene at one stop lighter than middle grey and keeps the snow white.

The EC really comes into its own when used with spot metering. Using a spot meter a person can meter the subject (say a face) and adjust the EC to put the face at the exposure level needed to match the actual tone of the face. For instance my tanned face is best at -2/3 stop, but my red-headed wife's face needs a +1 EC to be exposed to match natural tones.

That is the mechanics of what EC does, but how it affects your shooting depend on your camera settings. As has been mentioned, in aperture priority the shutter will be adjusted, in shutter priority the aperture will be adjusted. In manual mode, the exposure isn't adjusted, but the meter reading is (the graph seen in the view-finder), so the photographer can adjust exposure as desired. It's a matter of what the photographer wants to be in control of.

However, things get complicated when using anything other than spot (or center-weighted) metering. On the full scene metering modes the camera makes adjustments based on how it reads the scene. How the camera reads the scene varies widely with the different cameras and metering modes. Some evaluate the scene based on the tones of different areas and some try to match the general pattern of tones to a database of exposure templates (such as backlit subject and so on).

You didn't mention what exposure mode you had the camera in, but it wouldn't suprise me if one of the more automatic modes ignored the EC completely.

David
04/15/2007 12:43:29 AM · #7
Good replies. Thanks so much. Should have mentioned that I was in aperture priority mode. I think I understand better now, but I'm still baffled as to why aperture/shutter/ISO stayed the same (or is recorded in EXIF as staying the same). One picture is much brighter than the other, but EXIF shows only EC as different (+2 vs. 0). And it sounds like EC works by making adjustments to one of these variables.
04/15/2007 12:58:56 AM · #8
Originally posted by bvy:

Good replies. Thanks so much. Should have mentioned that I was in aperture priority mode. I think I understand better now, but I'm still baffled as to why aperture/shutter/ISO stayed the same (or is recorded in EXIF as staying the same). One picture is much brighter than the other, but EXIF shows only EC as different (+2 vs. 0). And it sounds like EC works by making adjustments to one of these variables.


It does (or at least, should) - depending on which mode you're in, it should either change the Aperture (in Shutter Priority) or Shutter (in Aperture Priority) to correctly expose. In Program mode, it probably depends on the camera for which is changed.
04/15/2007 03:33:42 AM · #9
Originally posted by bvy:

Good replies. Thanks so much. Should have mentioned that I was in aperture priority mode. I think I understand better now, but I'm still baffled as to why aperture/shutter/ISO stayed the same (or is recorded in EXIF as staying the same). One picture is much brighter than the other, but EXIF shows only EC as different (+2 vs. 0). And it sounds like EC works by making adjustments to one of these variables.

Keep in mind the difference may be what the camera was exposing. That is, even while on a tripod the scene it is exposing may not have been exactly the same. With spot metering for instance, it moving the spot from a light object to a dark object (at some distance) could be done with no noticable movement of the camera. With the more automatic metering modes there is no telling what the camera used to decide what the correct exposure should be.

The only way to check if it is working properly is to fill the frame with a single solid shade, making the metering mode irrelevant. Then adjust the EC and see the changes made.

David
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