Photography, from a purely technical standpoint, is all about getting the right exposure, which, in turn, is all about light: how intense, how much, and how long. The controls you’ll be using most often on your camera (or, if you’re shooting in Auto, the controls the camera’s choosing for you) deal with these three things. This week, we’ll be taking up the role of the aperture, and shooting in Aperture Priority.
The aperture’s a diaphragm located on the end of the lens that connects to the camera body. It looks a bit like, and functions more than a little like, the iris in your eye; the more “dilated” it is, the more light it lets in. It does something else, as well, controlling depth of field, which, in plain English, is how much of your scene will be in focus.
Aperture Priority (usually denoted by an “A” on your control knob), regardless of the camera you’re using, means that you’ve chosen to control the aperture, and are leaving the rest of the guesswork up to the camera; it will “choose” your shutter speed based on how it meters the scene. If you’ve chosen Auto ISO, it may also adjust your camera’s ISO, especially in low-light situations.*
Why choose Aperture Priority over Shutter Priority or Manual? Well, for one thing, it’s useful in low light situations, since a larger/faster aperture is letting more light into the lens, and therefore onto the sensor. For another, it can also be useful as an artistic tool. If your subject is in an area where there’s a lot of background clutter (figure 1), changing your depth of field can blur that clutter, making it less recognizeable and keeping the focus on your subject. Likewise, when you want more of your shot in focus, you can set your aperture to f/11 or higher, giving you greater depth of field. Shooting in Automatic removes much of your ability to do that; often as not, you’ll get a nicely blurred background on something you wanted in focus, and vice versa.
Now, let’s look at what the camera’s doing when you shoot in Aperture Priority. Since it was a nice, bright day out, I kept my ISO at 200 for all of these shots. Figure 2 was shot at 21mm, with the aperture set at 3.8, which is the widest my f3.5-5.6 will let me go at that focal length.. The shutter speed is 1/400 (1/400th of a second). On Figure 3, I’m using f/8, and the camera’s set the shutter speed to 1/100. By the time we get to figure 4, I’m using f/18 (remember, same ISO and focal length) and the shutter speed defaults to 1/20. Notice what’s happening here: there’s very little difference in each exposure, because each time we stop down**, we’re letting in a bit less light, and the camera, in order to compensate, is making the shutter speed a little slower in order to make sure that the sensor’s “seeing” that light for a longer period of time.
Suppose you’ve chosen your aperture, and the picture’s either under- or overexposed. You have two possible fixes. One is to make note of what shutter speed the camera’s choosing for you, go to “M” and make the shutter either that much faster or that much slower to compensate (because in “A” you could technically stop the camera up or down, but then it’s just going to choose a shutter speed that gives you the same exposure issue). The other is to use your camera’s exposure compensation settings. Exposure compensation allows you to lighten or darken the picture by anywhere from three to five stops, in 1/3 stop increments. So your viewfinder, or LCD display, will show a value of +/- .03, .07, 1, etc.; some will preview the effect if you’re using the LCD and shooting in live view. You may need to take a few shots ’til you’ve got one with which you’re happy. If you notice your camera’s consistently under/overexposing in a certain situation, you may want to keep the exposure compensation at a given value. Just make sure to check your photos from time to time, since an area may not be as dark as it seems, or the sun can slip behind a cloud.
A side note on Aperture: you’ll sometimes hear people talk about a “bright” or “fast” versus a “dark” or “slow” lens. What they’re generally referring to is its maximum aperture. A “fast” lens (generally in the range of f1.4-2.8) lets in more light, allowing for a quicker shutter speed (the “fast” part). Because a lens will stay at its maximum aperture for its focal length until you press down the shutter (an f/2 lens stopped down to f/22 stays open at f/2, for example), it’s going to look brighter in your viewfinder than something with a maximum aperture of f/5.6 (the “bright” part). What difference does that make? Well, focusing — whether you’re doing it yourself, or using the camera’s autofocus — is a heck of a lot easier the “brighter” the lens. Autofocus tends to “hunt” in low light with a darker lens, and you, likewise, will find it harder to focus, especially if the lens doesn’t have a focusing scale on it.
*I’ll be discussing ISO later this week.
**”Stopping down” is going to a higher f/ number, while “stopping up” is going to a smaller f/number; if this sounds confusing, think of the aperture like a dimmer switch: turn it up, you get more light, whereas if you turn it down, you get less.
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