A few of my friends are comfortable using their DSLRs in Auto or Program mode, but they're intimidated by Aperture Priority and other shooting options on their complex cameras. In this post, I'd like to send them a message: You have permission to shoot in Auto or Program for 80% of the images you capture. Program mode is preferable when you want to change the flash settings or the ISO, but in either of these modes, the camera will do a good job of selecting aperture and shutter speed settings that work well in most lighting situations.
But sometimes you have a vision for the photograph that the camera's processor can't discern. Perhaps you want a shallow depth of field to enhance a portrait, or you want to freeze the action in a sports scene. For these situations, I'd like to help you overcome your anxiety by offering the simplest explanation I can produce.
I'll begin by reminding you of an obvious analogy between your camera and the human eye. Because of the complex physiology of the eye, this analogy is limited, but I think it's useful here if we don't go too far. Think of your pupil as an aperture that controls the amount of light (per second) that is focused by your lenses onto your retina (the sensor). Your eyelid acts as a shutter; when it's closed, no light can enter; when you blink quickly, you simulate a fast shutter speed. It should be intuitive that you will need a wide aperture at low light levels -- and maybe a slow blink.
I don't think it's intuitive that a wide open aperture leads to a shallow depth of field and that a small aperture provides a large depth of field; let's just stipulate that for our purposes. So for that portrait effect you're seeking, set your camera to Aperture Priority and select a setting like f/2.8 or f/3.5 or even f/4. Set your ISO at 200 or 250 on a bright day or 400 or 800 or 1600 if the light is dim. In Aperture Priority, your camera will select the shutter speed necessary to give you a good exposure. (For simplicity, let's save Shutter Priority for another day.)
To get a feel for the relationships among aperture, focal length, and distance of the subject, try playing with a depth of field calculator. I just downloaded an app, Depth of Field & Exposure Calculator, that shows me that if I'm photographing a friend at a distance of ten feet with a 105 mm lens at f/2.8, the depth of field will be less than six inches. If I focus sharply on my subject's eyes, I should be able to keep her whole head in focus, but just barely. The depth of field is the range over which an acceptably sharp image will be produced. Try pressing the preview button on your camera just before taking the picture. This will allow you to look through the lens at the pre-selected aperture so you can see the depth of field you're going to get.
We've glossed over a few technical terms that must be dealt with, however briefly, to ensure your aperture anxiety does not recur. The focal length of your lens is the distance over which a collimated beam of light is brought to a focal point. You're familiar with this because you used to play with a magnifying glass to burn holes in paper. When we say that the aperture of a lens is set to f/4, we mean that the ratio of the focal length to aperture diameter is 4. Now that I've said that, you can stop worrying about it. The only thing you have to remember is that there is an inverse relationship between aperture diameter and f-stop. f/16 denotes a smaller effective aperture (or entrance pupil) than f/4.
There's a story about Arthur Fellig, a great New York street photographer of the 30s and 40s, whose pseudonym was "Weegie." He was usually the first press photographer on the scene of a grisly crime scene and his stark black and white images were almost always technically good. When asked about his successful technique, he said "f/8 and be there."