One of the benefits of working in Shutter Priority mode with fast shutter speeds is that, more often than not, you will be shooting with the largest aperture available on your lens. Shooting with a large aperture allows you to use faster shutter speeds, but it also narrows your depth of field.
To isolate your subject in order to focus your viewer’s attention on it, a larger aperture is required. The larger aperture reduces the foreground and background sharpness: the larger the aperture, the more blurred they will be.
The reason that I bring this up here is that when you are shooting most sporting events, the idea is to isolate your main subject by having it in focus while the rest of the image has some amount of blur. This sharp focus draws your viewer right to the subject. Studies have shown that the eye is drawn to sharp areas before moving on to the blurry ones. Also, depending on what your subject matter is, a busy background can be distracting if everything in the photo is equally sharp. Without a narrow depth of field, it might be difficult for the viewer to establish exactly what the main subject is in your picture.
Let’s look at how to use depth of field to bring focus to your subject. In the previous section, I told you that you should use Shutter Priority mode for getting those really fast shutter speeds to stop action. Generally speaking, Shutter Priority mode will be the mode you most often use for shooting sports and other action, but there will be times when you want to ensure that you are getting the narrowest depth of field possible in your image. The way to do this is by using Aperture Priority mode, or A.
So how do you know when you should use Aperture Priority mode as opposed to Shutter Priority mode? There’s no simple answer, but your LCD screen can help you make this determination. The best scenario for using Aperture Priority mode is a brightly lit scene where maximum apertures will still give you plenty of shutter speed to stop the action.
Let’s say that you are shooting a soccer game in the midday sun. If you have determined that you need something between 1/500 and 1/1250 of a second for stopping the action, you could just set your camera to a high shutter speed in Shutter Priority mode and start shooting. But you also want to be using an aperture of, say, f/4.5 to get that narrow depth of field. Here’s the problem: If you set your camera to Shutter Priority mode and select 1/1000 of a second as a nice compromise, you might get that desired f-stop—but you might not. As the meter is trained on your moving subject, the light levels could rise or fall, which might actually change that desired f-stop to something higher like f/5.6 or even f/8. Now the depth of field is extended, and you will no longer get that nice isolation and separation that you wanted.
To rectify this, switch the camera to Aperture Priority mode and select f/4.5 as your aperture. Now, as you begin shooting, the camera holds that aperture and makes exposure adjustments with the shutter speed. As I said before, this works well when you have lots of light—enough light so that you can have a high-enough shutter speed without introducing motion blur.