Matt Bango Photography

Depth of field is basically a range of distance around the focal plane (subject) that will produce an acceptably sharp image. It is important to understand what depth of field is and how to use it so that you can have further control over the final image you are trying to capture. It is a very powerful tool and concept to know how to use and can help you capture that “perfect” shot. In this article we’ll cover what depth of field is and how it works, the variables involved in controlling depth of field and finally how to use it to your advantage.

In order to be able to use depth of field to make our images better we need to understand what it is and how it works. Depth of field is basically the distance in front of and behind your focal plane (or subject) that is in acceptable focus. It is not a sharp abrupt change from sharp to unsharp. It is a gradual transition from the sharp, in focus area, to the soft out of focus area. This is better illustrated in the following image:

This image illustrates the transition from unsharp to sharp and back to unsharp again. You can see where the focal plane is located and the area before and after it that is acceptably sharp is known as the Depth of Field. You will also notice that there is something labeled the “Circle of Confusion”(CoC). The Circle of Confusion is basically how much something needs to be blurred to be seen as unsharp. When the circle of confusion is noticeable, that area outside the depth of field is no longer considered “acceptably sharp” and therefore is the circle of confusion. The following image shows the circle of confusion and depth of field:

A very good way to see depth of field is to look at macro images. Since the subjects are usually very close to the lens, the depth of field is very shallow. For example, take a look at the following image is of a salamander. You can see very clearly where the focal plane is (at the salamanders head) and how the unsharp areas gradually go from sharp to unsharp to the circle of confusion.

• Areas out of focus are referred to as “Bokeh”, pronounced bo-ké. Bokeh will appear differently in many lenses, this is because of the shape of the diaphragm.
• The circle of confusion is usually not rendered as a circle, but rather has a polygonal shape consisting of between 5-8 sides, depending on the aperture.

Now that we know that the depth of field is the area that is “acceptably sharp” and the circle of confusion is the area that is unsharp, we can look at the variables involved in controlling depth of field. There are really 2 main factors. The first is the aperture. Larger apertures (smaller F-stop numbers) will create a shallower depth of field. An image shot with a F-stop number of f/2 will have a much shallower depth of field than an image shot at f/8. The focal distance is the other big player in controlling depth of field. The closer the subject is, the shallower your depth of field is going to be. A subject at 3 meters will have a much shallower depth of field than a subject at 20 meters. There is another factor that seems to have an effect on depth of field, your focal length, but really it is just a matter of understanding what is going on. Telephoto lenses seem to create a shallower depth of field but if you think about what is happening, it is really a matter of focal distance. Telephoto lenses are used to “get closer” to the subject. For example, if you use a 100mm lens to fill the frame with a bird at 3 meters, then use a 300mm lens to shoot the same bird and fill the frame with the same amount of subject at 9 meters, you should have very close to the same depth of field. If you don’t believe me, try out the new Depth of Field Calculator. Enter 100mm and 3 meters, then try 300mm and 30m, you will get the same size depth of field. So remember, the distance to the subject and aperture are the main factors in controlling how large your depth of field is.

Let’s look at some real world examples on when you would adjust and control your depth of field. Take example 1 into consideration. You are a wildlife photographer and you want to shoot a bird that is 10 meters away from you and your camera. You have a Nikon D200 that has a 1.5x Crop Factor (the digital sensor isn’t full frame, or equivalent to 35mm film, so there is a crop factor involved) and a 500mm f/4 lens. After waiting for a half an hour, the bird finally moves to an isolated branch where you can get beautiful isolation of the bird and background. To nail this shot you are going to need to understand your depth of field. If you shoot the bird at 10 meters away, using a D200 and 500mm lens at f/4 you will find that your total depth of field is about 0.067 meters (I used my Depth of Field Calculator to figure that out). That is about 2.5 inches for your total depth of field. For our song bird, that is probably enough, but for a larger bird such as a raptor it wouldn’t be. We would first have to back up, since the raptor is many times the size of a songbird. We then may have to stop down our lens (depending on how much we increased the focal distance between us and the bird) to probably around f/8 or f/11 to retain full sharpness throughout the bird.

Another real world example would be that you are a landscape photographer. You want to capture some beautiful wild irises in the foreground and a dramatic golden sunset all in the same picture and all in focus. You are using your trusty 12-24mm wide angle with your D200 again. The irises are 1 meter in front of you and the sunset is a very great distance away. For this situation we would want to set the lens to infinity and use an aperture of f/16, f/22 or even larger. With this small of an aperture our depth of field is going to be very large, large enough to capture the entire scene in focus.

• The closer you are to your subject (shorter focal distance), the shallower your depth of field is going to be. The further away (longer focal distance) the larger your depth of field is.
• A larger aperture (smaller F-number) will give you a shallower depth of field. The smaller your aperture, the larger your depth of field.

*Please note that the images are for illustration purposes and may be exaggerated. They are not 100% accurate representations.

You should follow me on Twitter here.