lhoney2 wrote:
Hopefully one of the experts here will chime in on this, but I'll give it a shot.
When you depress your shutter halfway to get focus, the camera also takes a light reading to determine the exposure. In pictures where there is a bright light source and darker areas, the darker areas will almost always be underexposed because of the compensation for the overly bright areas. It will pretty much ruin your picture.
Quick and dirty solution - aim your camera at a point where the overly bright light is not in the frame, then LOCK the exposure setting. (RTFM for this, I barely know how to do it on my Rebel XT). Then recompose your original shot and take the picture.
Here's another interpretation of "locking" a light meter reading ...
When metering a scene, you can configure your metering system to be a "spot" meter reading (more or less - it's not quite as small a spot as the true spotmeters use, but it's useful). While you are metering, you notice what shutter speed is called for in Av mode, so you now have an aperture and a shutter speed setting that has been taken with a specific ISO setting and a specific area of the scene being metered. Making a mental note of those settings, you can then shift to manual mode and set the aperture and shutter speed manually to those setting, and that is essentially the same as "locking" the reading. Now, if you take the picture of the scene, it will be exposed so as to render whatever you were aiming at as middle gray.
If middle gray is not what you wanted, then you can modify the shutter speed or aperture to place that part of the scene at a brighter or darker level.
The net result is that you transport yourself back into the dark ages when people used a hand held lightmeter and set their apertures and shutter speeds appropriately - but it's about the only way to learn exposure, and it works!