inconsistent exposures in aperture/shutter priority?

There are some sneaky little issues that can cause confusion in getting correct exposures. Here are a couple of explanations for this problem and some easy fixes. The first thing to remember is that in any shooting mode except Manual, the camera will automatically adjust the exposure by changing the aperture and/or shutter speed (and ISO if Auto ISO is selected) to give a "correct" exposure based on what the camera "sees" at the time of exposure.

Scenario 1.  Let's say you are hand-holding your camera and are looking down at the top LED screen and setting an aperture value to give you a fast shutter speed. The chances are in doing this, you are not pointing your camera precisely at your subject. Next, when you take the picture with the camera pointing directly at the subject, the shutter speed may suddenly be quite different (slower?!) than the first reading unless the camera happened to be pointing at the same scene (not likely with hand-holding). These two exposure readings can vary widely if you happen to have made the first reading with the camera pointed at a white sky (or frame-filling white water as in this photo) and your subject is on the ground against a dark background.  The solution is to always make your exposure adjustments with the camera pointed at the subject or at least in the same lighting conditions as the subject. Another solution is to shoot in Manual Mode which will force you to choose all the correct exposure values for your specific subject without any unhappy surprises being made by the camera.

  Scenario 2.  You have your camera on a tripod and have zeroed-in the exposure values you want for that subject while looking through the camera viewfinder. Assuming the camera and subject do not move, there should be no problem getting the right exposure, right?  Not necessarily!  If you have strong light (i.e., sun!) coming from behind you and you step slightly away from your camera to make the exposure, allowing direct sun light to enter the viewfinder, the exposure will be modified. Most likely, your image will be underexposed since the camera adjusted for that bright light from the viewfinder. The solution is to cover the viewfinder with something (your thumb, your eye?) during the exposure. Or, alternatively, like the previous scenario, shoot in Manual Mode to avoid the problem. I should point out that this is only a problem for DSLR cameras that use an optical/mirror viewfinder and should not effect mirrorless cameras.  In tests that I have done, this problem is most likely to occur only when the sun shining directly into the viewfinder, but something to keep in mind.

One final note on this subject, zooming in or out or changing from horizontal format to vertical format will likely change your exposure values since more, or less sky, might be included in the scene. Again, shooting in Manual Mode avoids this problem. I hope this helps you in getting the right exposure every time!