Sometimes you want the subject to stand out from the background and other times you want the entire scene to be sharp. The way a photographer controls how deep the focus is in the final image is by using Depth of Field. Depth of Field is how deep the focus is in the image. In other words it is how much depth of focus you have in the field of view.
You may remember from my previous article on Reciprocity that you can change the f/stop AND the shutter speed settings to control how deep the focus is in the image. A smaller aperture, for example f/16, will cause most of the scene to be in focus while a large aperture, f/2 (or near), will cause the background to go soft. But aperture is only one of the controls of Depth of Field. The other two are even more important.
The first is distance. Being far away from the subject creates more depth of field in the image. If you were to focus your camera on a mountain far away, the stars in the sky are in focus too, and they are trillions of miles away. You will also get miles of sharp focus in front of the mountain. When photographing something real close, your maximum depth of field, even when stopped down all the way to f/16 or f/22, may only be a few centimeters or less. When photographing at middle distances depth of field can be measured in feet. To get more depth of field you only need to back away from the subject or to decrease it you can move in closer.
The other control of depth of field is the focal length of the lens on the camera. A wide angle lens has much more depth of field than a telephoto lens. If you have a zoom lens, it is also true that you get more depth of field at the
zoomed out (wide) setting than you do when you zoom in (telephoto) setting. So if you want the entire scene to be sharp, you can shoot it with a 28mm or 35mm lens or the wide setting on the zoom. If you were shooting a portrait, you may want to use a longer lens, like an 85mm, or the telephoto end of your zoom lens, to lower the depth of field and throw the background out of focus.
Go out and shoot a few things close up and far away. You will see for yourself how distance affects the depth of field in the images. Also, if you have a few lenses or a zoom, take pictures of the same subject using wide angle (zoomed out) and telephoto (zoomed in). And if you haven’t done so already, go back and read the article on Reciprocity to learn how to take the same picture with different f/stop and shutter speed settings. It takes reading and shooting to really grasp the concepts of photography.
By Matthew L. Kees