Auto Exposure Lock

[NNN]

I'm a noobie.

Can someone clarify this specific question about AE lock to me? I know that you would use ae lock so that you could expose to one thing and focus on another, so is this only used in Tv and Av, and not in manual mode? Since exposure is controlled by the shutter speed and aperture, can't you just point at what you want to expose at, set the aperture and shutter speed, focus on the subject, recompose, then shoot?

It would help out a lot if someone expained a scenario and how they would use ae lock.

Thanks in advance.

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[Slowboat]

Your understanding of AE lock is correct, used only for aperature and shutter priority camera modes.

A good example would be a scene that is mostly snow and there is a lonley evergreen tree standing all alone and snow has begun to drift around the tree. If for example you where is aperature priority so you could have control over DOF. You stop down to f/16 so you get detail in the snow and tree.

If you let the camera meter the scene it will most likely come out underexposed because the camera will try to make all that bright snow 18% grey. Instead you could spot meter on the green tree, which is close to 18% grey, press AE lock, recompose, focus and snap.

[NNN]

Thanks slowboat for explaining it. You explained it just right.

I have ANOTHER question now concerning focus and exposure locking together.

On my camera, I notice that when I focus on a subject, my exposure locks onto the point of focus so that when I recompose my scene, it doesn't change. This make sense to me since I would want my subject, say a person, to be focused AND exposed properly. So in a way I still have the option of locking exposure on something else before taking the picture, OR having it lock automatically with the point of focus. However, on my brother's Canon 10D, I noticed that exposure does not lock with the focus and that I would have to LOCK exposure, or else after recomposing, the exposure would change to whatever the center of the frame was directed at.

Which is the standard on most cameras? Locking with the focus? or having to lock exposure separately?

[Slowboat]

Normal operation for a 10D is to lock focus and exposure when the shutter release button is pressed halfway. This behavior can be changed using the camera's custom functions. These are well documented in the users manual.

[drlynn]

On my Canons the AE lock also changes the metering mode from matrix to partial.