I shoot a majority of my photography using Aperture Priority, and my number one concern beyond how to frame the shot is what depth of field to use. Depth of field can be critical in telling a story. If I want to create a portrait with a blurry background, I use a large or open aperture, such as f/1.2–2.8, to create what we call a shallow depth of field, meaning an in-focus foreground with a blurry background (Figure 9.1). If I’m shooting a landscape and the whole scene needs to be sharp, then I’ll use a smaller or less open aperture, such as f/16, to ensure sharpness.
Occasionally a greater depth of field is required to maintain a sharp focus across a farther distance. This might be due to the sheer depth of your subject, where you have objects that are near the camera but sharpness is desired at a greater distance as well (Figure 9.2).
Or perhaps you are photographing a reflection. With a shallow depth of field, I could only get the reflected mountains and the mirror in focus. By making the aperture smaller, you will be able to maintain acceptable sharpness in both the subject and the reflection (Figure 9.3).
A mirror is a two-dimensional surface, so why do I have to focus at a different distance for the image in the mirror? The answer is pretty simple, and it has to do with light. When you focus your lens, you are focusing the light being reflected off a surface onto your camera sensor. So if you wanted to focus on the mirror itself, it would be at one distance, but if you wanted to focus on the subject being reflected, you would have to take into account the distance that the object is from the mirror and then to you. Remember that the light from the subject has to travel all the way to the mirror and then to your lens. This is why a smaller aperture can be required when shooting reflected subjects. Sit in your car and take a few shots of objects in the side view mirrors to see what I mean.