Depth of field is one of the foundation concepts in photography and yet it is one that many of my students still don’t quite understand. Well, they think they understand it and they mostly do but the practical implications of using depth of field are almost universally poorly executed.
Everybody knows that depth of field refers to the amount of the final image that is in focus. To simplify, a shallow depth of field means that there is very little of the final image that is in focus; a lot of depth of field means most of the image is in focus.
Everybody also knows that the physical size of the aperture -the f-stop- determines the resulting depth of field. A small-numbered f-stop – f2.8 or f4.0- means a large aperture and very little depth of field. A large-numbered f-stop –f16 or f22- means a small aperture and a lot of depth of field.
This is the basic stuff. Once you understand the seemingly reversed relationship that a small aperture equals a large amount of your image will be in focus and that a large aperture equals a small amount of your image will be in focus you pretty much understand the basics of depth of field.
Ah, but there is more! Understanding the concept and putting it to use are two different things. The f-stop you choose is not the sole determinate of how much of your image will be in focus. The other key component that effects the final appearance is the distance the subject is from the background. This is especially true when doing any kind of portraiture be it of a flower, an elk or a grandchild. If the background is too close it will always be annoyingly in focus no matter what f-stop you use.
I see this all the time while doing critiques in the workshops I teach. Someone will show an image of a portrait they tried to do. They wanted a nice sharp subject and a completely out of focus background (what is known as a poster board background). Almost always either the subject is fine but the background is not out of focus enough or the background is wonderfully out of focus but the subject is not in focus enough. Ideally, you want either everything in your image to be in focus or just your subject in focus and everything else way out of focus. If something obvious is not quite in focus it will likely be a major distraction. This is because the mind tries to resolve slightly blurry things thus making it distracting. If something is way out of focus the mind accepts it as unable to be resolved and ignores it.
So what is one to do? In order to get a beautiful poster board background for your wildlife, wildflower or wild-child portrait the f-stop you choose is not nearly as important as the subject you choose or the angle you choose to shoot that subject. Both of those things will determine how far away the background is going to be from your subject. Assuming you have both pretty light and a pretty subject if you want a dynamic portrait let the background be your guide as to both what and where to photograph.
How do you do this? Put your hands in your pockets and spend a little extra time looking for the best subject to photograph. There may be lots of pretty flowers in the field or elk grazing in the meadow but only a couple will have a good background. If you are photographing a person just place the person in a place where the background will enhance and not detract from the shot. If you can’t move your subject then pick an angle to shoot with the best background. A big part of becoming an accomplished photographer is always being aware of the background. It is as simple as that.