Auto Exposure Lock

Most DLSRs have an AEL button and if you use it at all, chances are that you probably use it lock the auto exposure, so that it uses the same exposure values from shot to shot even if the scene changes. So for instance you are shooting a parade and you have a good exposure, then a bunch of cheerleeders show up all dressed in black and the camera wants to turn them 18% grey. But having locked the exposure, they will be properly exposed.

If your camera allows it, there is another thing you can do when you have locked the exposure. You may be able to use your control wheels to change the aperture or shutter speed. With the exposure locked any change in aperture will cause a corresponding change in shutter speed so that the overall exposure stays the same. So imagine you are working with flash, you could expose for the ambient using your fancy multi-segment matrix metering with the flash switched off. Then, with the ambient exposure sorted hit the AEL button and turn on the flash. You can now adjust the power of the flash using the aperture wheel without changing the ambient exposure. Neat?

If like me, you can never remember your f-stop tables, the AEL button can tell you. Put the camera in manual mode, set your shutter to something you can multiply or divide by two then hit the AEL button. If you halve the shutter speed the aperture will close down one full stop. If you double the shutter speed the aperture will open up one full stop.