Answer from Peter K. Burian:
Shutter Priority (abbreviated as S or as Tv depending on the camera) is semi-automatic. This one allows you to set a desired shutter speed for motion control. The camera then sets a suitable aperture. For example, you might want to set 1/500 sec. to render a cyclist as "frozen" or 1/4 sec. to render a waterfall with an effect of fluid motion. (Naturally, you will need to use a tripod at such a long shutter speed to prevent blurring caused by camera shake.) If you decide to change the shutter speed, the photo will not get darker or brighter; the camera will change the aperture to maintain the same exposure.
Aperture Priority (abbreviated as A or as Av depending on the camera) is also a semi-automatic mode. You set the desired aperture, such as f/22 for an extensive range of acceptably sharp focus (depth of field) or f/4 for a more blurred background. The camera then sets a suitable shutter speed. If you set a different aperture, the camera instantly changes the shutter speed to maintain the same exposure (image brightness).
PLANES DEPTH (photo at right): When depth of field -- the range of acceptably sharp focus from foreground to background is particularly important -- Aperture Priority mode is an ideal choice. For these photos, I used f/4 and f/22 in order to achieve entirely different effects. (c) 2012 Peter K. Burian
If you take a photo and it's too dark or too bright overall, you can modify the exposure using the camera's exposure compensation +/- control. Set +1 perhaps for a brighter photo and -1/2 (-0.5) when first experimenting with this feature. Then take the shot again. Later reset exposure compensation to zero when you no longer need it.
In other respects, both of the semi-automatic modes are easy to use and they allow you to control the most important creative aspects in serious image-making. Peter Burian teaches three interactive online online courses at BetterPhoto's school of digital photography:
In other respects, both of the semi-automatic modes are easy to use and they allow you to control the most important creative aspects in serious image-making.
Peter Burian teaches three interactive online online courses at BetterPhoto's school of digital photography:
Article by Peter K. Burian. To learn more about photography, explore the many online photography and Photoshop classes offered here at BetterPhoto.com.