I want to talk about depth of field (DOF) and why it matters. First, a definition:
In optics, particularly as it relates to film and photography, the depth of field (DOF) is the portion of a scene that appears sharp in the image. Although a lens can precisely focus at only one distance, the decrease in sharpness is gradual on either side of the focused distance, so that within the DOF, the unsharpness is imperceptible under normal viewing conditions. –Wikipedia
Huh? Well, the easiest way to say this is, depth of field determines, in any given photograph, what is sharply focused and what isn’t. How do you set depth of field? With your choice of f/stops. What is an f/stop? It is how big or small your aperture opening is. What is an aperture? Inside each lens is a shutter, shaped like a wheel composed of individual, curved, overlapping blades. These blades can open wide or close tight, and that, in a nutshell, is an aperture. The larger the f/stop number, the smaller the aperture opening, and vice versa. F/1.4 is a very big opening in your aperture, and creates very shallow DOF. F/16 is a tiny opening in your aperture and creates great DOF. See image:
So why do f/stops, apertures and depth of field matter? Because they will affect how your photographs look. They will determine what the FOCUS of your images is. How powerful is that? Let me illustrate. The three photographs below were all taken within seconds of each other, using the same camera (Nikon D70), the same lens (Nikon 50mm f/1.4-16), and the same point of focus (the rose.)
The first image was taken at f/16, which is the smallest aperture for this particular lens. Notice how much of the background is in focus. The rose was my point of focus, but it is overwhelmed by background clutter and just one more element in a busy image, with lots of different elements competing for your attention.
The second image was taken at f/5.6, which is effectively the ‘middle’ of the focal range for this particular lens. Notice the background has become slightly more blurred and less focused, thus making the rose stand out a bit more. However, there is still a lot of competition for your attention.
The third image was taken at f/1.4, which is wide open for this particular lens. Notice the absolute focus of the image is now on the rose, while all else falls out of focus. Your eye is forced to focus on the rose and nothing but the rose. A powerful point of focus has been created.
Shooting with a wide open aperture, i.e., shallow depth of field, is a very effective way to photograph people, even in the midst of cluttered, busy backgrounds. Below is another photograph shot last summer in the same alley the rose grows in. In this one, the focus in on the boy’s right eye, yet the depth of field is so shallow his left eye is slightly out of focus.
Shooting ‘wide open’ does have its challenges. With a fast moving target, such as a child at play, it is easy to come up with totally out-of-focus photographs. However, with practice, you can learn to anticipate the direction the action is going and you can improve your reflexes. Remember, if the subject’s eyes are in focus, all the rest can be a blur and all is well. So aim for the eyes, or at least one of them.
One of my very favorite lenses, the Nikon 50mm f/1.4D Auto Focus AF Nikkor Lens, is also surprisingly inexpensive.
If you would like to read more, check out these articles: