Probably one of the more confusing areas of photography is what is known as Depth of Field.

In this graphic, the black line at 15′ is critically sharp. This is where the lens is focused.
The yellow area would appear ‘acceptably sharp’…The yellow area represents the Depth Of Field.

Depth of field is that area of an image that is considered acceptably sharp … in other words it appears to be ‘in focus’.

There now, don’t you feel much clearer on this subject? NO! Well, not to worry. It is not as hard to grasp as you might fear.

To understand what the term acceptably sharp really means, it helps to know what actually happens when you focus your lens.

The first thing to understand is that the lens can only render one single plane at a time in critically sharp focus. This is important and has a great deal of bearing on depth of field.

When the lens is focused at say 15 feet, everything that is 15 feet from the lens will be in ‘critically’ sharp focus. Anything closer than 15 feet and anything beyond 15 feet becomes gradually less and less in focus until at some point this lack of sharpness will be quite noticeable.

So now you have a critically sharp plane (15 feet in this example) and an area of gradually decreasing sharpness. And finally an area that is plainly out of focus.

That area of your picture that still appears to be sharp to your eye is called the “depth of field”.

Naturally, this term ‘acceptable sharpness’ is somewhat subjective. The final intended use of the image as well as the individual’s own personal standards will determine what is acceptable and what is not.

For example, a professional with a highly trained and critical eye may require a much higher level of sharpness than someone who is just taking pictures they can email to friends and family.

The 3 Factors That Control DOF:

The three primary factors that affect DOF are:

• the distance from the lens to the subject
• the f-stop being used
• and the focal length of the lens

Let’s look at each of these separately.

DISTANCE:

The closer your subject is to the camera lens, the less depth of field you will have. As you increase the distance from your subject, you also increase the depth of field.

With close-up or macro photography, you’ll have a very shallow depth of field, so what you focus on becomes very important. If you are shooting a distant subject and nothing important to your picture is very close to the camera, then depth of field will usually render the entire scene as acceptably sharp.

F/STOP:

The next factor affecting DOF is the f-stop which you use. A large f-stop…bigger aperture opening… will give you a smaller depth of field, and a smaller f-stop will give you a larger depth of field. (More on f/stops HERE.)

Choosing a larger f-stop is known as “opening up the lens” and choosing a smaller f-stop is known as “stopping down the lens”. So for less depth of field you open up and for more depth of field you stop down.

FOCAL LENGTH:

Lens focal length is the third factor affecting DOF. (For more on focal length, go HERE.)
Shorter focal length lenses yield a greater DOF than longer focal lengths. So as a general rule of thumb, when using a wide angle lens you will increase the DOF and when using a telephoto you will decrease the DOF.

Since depth of field is relative to a single plane of focus, it is important to know exactly where to focus your lens to get the maximum DOF in your images. At normal shooting distances, DOF is divided roughly into thirds…1/3 in front and 2/3 behind your focus point, as illustrated in the graphic at the top of this page.

For example, we’ll say that you have focused on a point that is 15 feet from your camera. And we’ll assume that the total DOF in this case is 15 feet. In this example everything from 10 feet to 25 feet would be acceptably sharp.

Notice that the DOF falls off faster as it gets closer to the camera. You have only 5 feet of sharpness in front of the point of focus, but 10 feet beyond the focus point is sharp.

Why is this important for you to know? Sometimes you may have both near and far objects in your composition that you want to appear sharp. So by focusing at a point about 1/3 of the distance between these two objects you will get the maximum depth of field.

While understanding depth of field may seem daunting at first, it is well worth whatever time you spend in learning to apply it in your photography. A working knowledge of depth of field will give you a very powerful and creative tool that can make your photos far more interesting and effective.

Here’s to better photography…

Al Hannigan