Getting Your Priorities Straight
By Mike Pasini, Editor
Imaging Resource Newsletter
Correct manual exposure is a two-step dance. Without the music. The shutter
speed and the lens aperture must both be adjusted to allow just the right amount
of light to reach the CCD (or film, for that matter).
While some automatic cameras let you choose whether to control the shutter speed or aperture setting (and then selected the appropriate corresponding setting), rare was the camera that offered shutter speed priority. (Konica long distinguished itself with a shutter speed priority system in the 35mm world.)
Shutter speed priority simply means you pick the shutter speed and the camera picks the aperture setting for the light it meters. Aperture priority means you pick the aperture and the camera sets the shutter speed.
Unfortunately the two systems are not equivalent. Very different images may be captured using one or the other under identical circumstances.
Shutter speed priority gives you the chance to freeze fast-moving subjects, but takes away your depth-of-field control.
Aperture priority gives you depth-of-field control, but risks blurring fast-moving subjects.
So your subject -- and nothing else -- is the key to choosing an automated exposure mode. Action shots do best with shutter priority while anything that requires depth-of-field control needs aperture priority.
How about an indelible example of each?
If you never want to forget how aperture priority works, you only have to remember
the noted photographer Eadweard Muybridge, who made a very famous panorama of
San Francisco one bright day in 1878 from the middle of the city. The image
had one disturbing feature -- obvious to even the casual observer. Despite the
beautiful weather, it showed hardly anyone on the streets. Anywhere in the city!
This miracle of timing occurred because Muybridge used such a long exposure that nobody who was in motion stood still long enough to register on the film. Six second exposures, in fact.
That's the trouble with aperture priority. Moving subjects blur. And sometimes entirely disappear.
The problem with shutter priority has to do with sacrificing depth-of-field control. When you (or your camera) focus your lens, you do so as if focus exists in only one plane -- about an inch deep. Not quite. While that plane is where the sharpest focus resides, there is an area just in front of and behind the plane that is also in focus, if not quite as sharp. Depending on the aperture setting, the depth of field can be, well, quite deep. The further down you stop your lens (the larger the f/stop number) the deeper the depth of field. In fact, these ranges were routinely engraved on 35mm film camera lenses.
A common technique used to "enhance" the most important part of your image is to shoot with a relatively wide open lens so that it narrows the focus to just the subject, consequently altering the shutter speed to compensate.
After some practice, you can even start using depth of field in the field to estimate focus, stopping down your lens to bring into focus a subject that otherwise wouldn't sit still for you.
Getting the picture, after all, is your highest priority.
This article is reprinted from The Imaging Resource Digital Photography Newsletter,
Advanced Mode Column, published June 16, 2000