Understanding Aperture Priority
When you first use an SLR camera, you are inclined to spend a great deal of time shooting in Automatic, or at least one of the program modes (if your camera supports them). For many of you, this might be sufficient. But if you desire to work towards ultimate control and artistic freedom, you'll need to step outside of that comfort zone. The next stop along your path would be one of the semi-automatic modes. You have a choice of Shutter Priority or Aperture Priority. Today, we're going to focus on Aperture Priority.
With the camera in Aperture Priority (or AP) mode (check your manual to see how to set it up), you will have full control over the aperture – the opening inside your lens. The aperture controls the depth of field – the amount of area that is in focus. The wider the opening, the narrower your depth of field. With a small aperture, the depth of field expands. The direct correlation is light. A small aperture will yield a large depth of field, but the shutter speed will be significantly longer to compensate for the limited amount of light getting through the lens.
What the f-stop?
To compare the relative size of these openings, we use a unit of measurement called a focal ratio, which is also known as an f-stop. The proper way to note the aperture would be either 'f/4′ or 'f:4′, but it is also commonly noted as 'f4′ or simply '4′. I prefer to use the fraction notation simply because it makes things easier to remember. I actually think in terms of fractions when comparing apertures. Assuming the value for 'f' is constant (1, for example), it makes comparing sizes as simple as elementary math. In our example f/4 (1/4) is clearly a much larger opening than f/22 (1/22).
As for how aperture relates to depth of field, the larger the opening, the narrower the depth of field. If you have trouble remembering this, simply think only of the number on the bottom. The smaller the bottom number, the smaller the depth of field. So a f/4 has a significantly shallower depth of field than an f/22.
Pulling All the Stops
Now there are many f-stops on your camera lens. The standard f-stops are as follows:
f/1 – f/1.4 – f/2 – f/2.8 – f/4 – f/5.6 – f/8 – f/11 – f/16 – f/22 – f/32
In this case, f/1 is the largest aperture whereas f/32 is the smallest – though your lens's abilities will differ. The numbers may not make a lot of sense. The full-stops are derived from each step up where the amount of light is doubled. In other words, f/4 permits twice as much light as f/5.6, and so on. Depending on your lens, you may not have all of these stops, especially at the low end. Most likely, your lens will also have half stops or third stops between some of these stops. My 50mm lens, for example, starts at f/1.8 and ends at f/22.
With all these stops, half stops and third stops, it's enough to make your head spin. Worry not, there are only a few that truly matter. These are what I refer to as the favorite stops. Here are the favorite stops:
f/2.8 – This is the isolation aperture. This is the aperture that you will use if you want to completely isolate the subject from the foreground or background. The depth of field will be very shallow, so watch your focus carefully. If your lens doesn't have f/2.8, use the largest aperture on your camera.
f/4 - For a fairly narrow depth of field with a little more flexibility, use f/4. This is a good setting to use with larger (deeper) subjects to get more in focus.
f/11 – This the aperture that I prefer when I am less concerned about depth of field. It is a great aperture to use when you're posing the subject in front of a surface (lets call it your backdrop) parallel to the camera. The depth of field is deep enough to pick up almost everything on your subject. But it's shallow enough that if you position your subject well, you can soften the backdrop, if that's your intent. Focus on a point on the subject that is closest to the camera, and everything else will fall into place nicely. But with such a setup, there's no reason to to use a smaller aperture.
f/22 - If you want to see everything in the shot clearly, use this setting (or the smallest your lens supports). You will still need to focus, of course, but the depth of field is quite incredible.
Everything Else - Since I noted the four favorites, you might wonder if the rest of the f-stops matter at all. These are for fine-tuning. If you want slightly more or less depth of field, you can try a half-stop larger or smaller to make sure everything you need (but nothing more) is in focus.
Pulling it all together…
I like to think of Aperture Priority mode as the artistic mode. It's the mode where you have full control over the depth of field. It's great for planned shots, still life, portraits or detail work. Use it when speed doesn't matter, but the presentation of the subject does. Simply determine the intent of your photo, select an aperture that is appropriate and click the shutter. It's as simple as that. With digital, I would suggest that you review your photos to check your depth of field. Use the zoom feature to verify the edges of your subject. If the depth of field is anything but perfect, adjust and shoot again.
There is of course a formula to figure out exactly how deep the depth of field will be. But for the sake of simplicity, there's no reason why we can't take advantage of the instant review capabilities of your camera. We'll save the complex math for another day. In the mean time, go experiment with Aperture Priority mode. With enough practice and comfort, it will become your favorite mode to shoot. And then you'll wonder why you ever used full-auto in the first place.