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View Full Version : Exposure Compensation - How Does it Work?



spoonbill1970
09-15-2005, 01:54 AM
Hello all,

Been having great fun with my A95 this past couple of months. I'm a newbie as far as photography is concerned, but I'm enjoying learning about things like Av, macro etc. I love taking "background blurred" close-ups.

I was wondering if anyone could shed any light on how EV + or - works?
I can see what it does, but what is it about the settings that it actually changes? I've read that it moves something a stop up or down. Is it the f. stop? If so, by how much does it move it? Why do we not just change it manually if this is the case? Or does it increase/decrease the shutter speed?
I notice that when I have the camera on "Manual" that this option is unavailable, so I presume it has something to do with the things above.

Thanks in advance for your help.

Spoonbill1970

Norm in Fujino
09-15-2005, 05:16 AM
I was wondering if anyone could shed any light on how EV + or - works? I can see what it does, but what is it about the settings that it actually changes?

It depends on what you have your camera set to. If you are set to programmed exposure, it changes the shutter speed and/or the aperture. If you have it set for aperture priority, it changes the shutter speed, and if set for shutter priority it changes the aperture. Basically it fools the camera into thinking that the amount of light present isn't enough, or is too much, so that the camera sets the shutter/aperture for a different value. When shooting a strongly backlit scene, the camera's light meter thinks there is too much light, so it stops down the aperture, or shortens the shutter speed, to prevent "too much" light from coming in. But when it does that, the foreground may become too dark. So you have to override the camera's automation by choosing + exposure compensation by one or more steps (or fractions of steps), to force the camera to let in more light. The same is true (except opposite) if you're shooting a very dark object against a dark background.


If so, by how much does it move it? Why do we not just change it manually if this is the case? Or does it increase/decrease the shutter speed?
I notice that when I have the camera on "Manual" that this option is unavailable, so I presume it has something to do with the things above.

By how much depends on how much you choose to compensate. Different cameras have different units of compensation, and they may vary from 1/3 step to 1 full step (or more). You have to choose how much compensation you think is necessary given the lighting situation (and that's the great thing about digital; you can take a picture, check the histogram or the appearance on the LCD, and then adjust the compensation if necessary for a second shot).
And you're right, since the function is basically a manual override of the automatic metering system, there is no need for it when shooting on manual; you're compensating manually anyway.

spoonbill1970
09-19-2005, 07:17 AM
Dear Norm in Fujino,

Thanks very much for your clear explanation.

Sorry I've been a bit slow getting back to you - been unable to get on to my PC all weekend.

Been experimenting with your suggestion:

When shooting a strongly backlit scene, the camera's light meter thinks there is too much light, so it stops down the aperture, or shortens the shutter speed, to prevent "too much" light from coming in. But when it does that, the foreground may become too dark. So you have to override the camera's automation by choosing + exposure compensation by one or more steps (or fractions of steps), to force the camera to let in more light.
In Scotland we have really wonderful sunsets at this time of year but when taking these shots the foreground can come out really dark - so adding a + compensation has resulted, not only in nice sunsets, but satisfactorily lit up foreground too! Very pleasing. :cool:

Thanks again for you expertise!

spoonbill 1970

Norm in Fujino
09-19-2005, 08:02 AM
Glad things worked out okay.