Depth of field is a relatively simple concept that can get fairly complicated. Simply put, it’s the area of a picture from front to back that is sharply focused. What’s so complicated about that, you may ask. Here goes: depth of field is generally controlled by three factors. The first is the aperture opening (see my blog “It’s all about light”). The higher the f-stop number (meaning more light is stopped), the greater the depth of field. In gauging the depth of field, keep in mind that the rule of thumb is that one third of that depth is in front of the subject and the other two thirds are behind it. This will get clearer (no pun intended) later on. The second factor is the focal length of the lens. The shorter the focal length, the deeper the depth of field. What this means is, if you’re shooting scenery and want to get the foreground and the distant mountains all in focus, you should be using a wide angle lens rather than a telephoto. Of course, if you were trying to get the foreground and the distant mountains in the same picture, you’re probably already using a wide angle lens – duh! The third factor is the shooting distance – how far you and your camera are from the subject. The closer you are, the shallower the depth of field. There are some other contributing factors, but these are the big three.
All right; now that you know this, how do you use it and why would you bother? In viewing a photograph, the eye is always drawn to a clear image rather than a blurred image. If you’re taking a portrait, the subject’s face is usually what you want the viewer to concentrate on. Having a sharp background can be distracting. Keeping in mind the three factors above then, the optimum results would come with using a longer lens (at least 75mm), a low f-stop (around f4 or f5.6) and standing as close to the subject as you can get to have the picture properly framed. Doing this will normally result in the face being sharply in focus and the background blurred.
One of the other contributing factors that will get the desired results would be to have the subject distant from the background. If the subject is standing up against a wall, it won’t matter how low the f-stop, how long the lens or how close you are – the background won’t be blurred. Another example of how depth of field can be used is in photographing a long line of subjects, for example, a picket fence, a long walkway, or a long line of soldiers. Let’s use the line of soldiers as an example. Imagine 50 soldiers all standing at attention with one end of the line at your immediate right and you’re looking down the line at their overlapping profiles. If the goal is to show how alike each soldier is, then you probably want as many in focus as possible. To do this, first select a short lens: 50mm or less. Then select a high f-stop: f11 or higher. Finally, stand as far from the closest soldier as you can (remember, you can crop the picture later on to get rid of extraneous background). If you go to my photo of “lobster traps” under the Nova Scotia/Canada tab, you’ll see what this end result would look like. As you can see, there are 11or 12 traps in a long line. The depth of field in this photo extends from the first trap to about the sixth or seventh. The same thing occurs in picture 5 of 5 under the Quebec/Canada tab. The walkway at the foreground is in focus, and the lone walker at the other end is still at an acceptable level of clarity.
Using the same example of the soldiers, if you wanted to demonstrate the individuality in such a long line of similarity, you’d probably want to focus on only one or two of the soldiers, while keeping the extended line in the picture. To do this, use a long lens (at least 100mm), a low f-stop (f4 or lower, if possible), and get as close as allowable to the soldiers. What you’ll end up with is a very short depth of field and the soldiers before and after the subject soldier will quickly begin to blur. In most of the photos I take, I look for extensive depth of field. However, if you look at images 17 and 18 under the Virginia/USA tab, you will see examples of very shallow depth of field. Note how the eye is drawn to the sharpest berries or the sharpest grapes in those pictures, and that the background, although relatively close, is blurred.
I mentioned earlier that I’d “clear up” the concept of one third in front and two thirds in back. Depth of field doesn’t change abruptly from sharp to blurred. It’s a gradual transition, although the gradation of the transition will be different depending on the extent that the three factors above come into play. At some point the different subject layers in a photograph both in the foreground and in the background, reach a point where they are no longer visually sharp. Let’s use the line of soldiers again as an example. Regardless of the depth of field of your final shot, once you identify the soldier who is most in focus, the area behind this soldier that remains at an acceptable level of focus will be twice as far back as the area forward of that soldier. Clear?