In a previous post I said I would talk about this later. Here it is. I believe most photographers only vaguely understand what depth of field (DOF) means. That’s probably OK, as long as we can still use it to our advantage. You don’t have to know in detail how your car works to be able to drive it, but it helps.
What is it?
You have seen it. We focus on a subject, but when we look at the image we are disappointed that another subject or the background was out of focus. We have been bitten by too little DOF.
Depth of Field is the distance between the nearest and farthest points in the frame that are in acceptably sharp focus. OK, we can intuitively understand distance that is in focus. But what does “acceptably sharp” mean?
The reality is that when your focus your super expensive, multi-element, rare earth material lens, it technically only focuses at exactly one point. Everything else is to some degree out of focus. But like many things in life, the precise details do not matter. What matters is the result.
For most of us in most of our applications, it is OK for things to be a little bit out of theoretical sharp focus. We can’t really see that unless we magnify an image greatly.
Circle of Confusion
So for most of our work, we will accept a certain amount of out of focus as unnoticeable. The measure of this for Physicists and Optical Engineers is called the “circle of confusion“. If you focus on a point of light, you expect it to be imaged as a sharp point. But as one of these points gets in front of or behind the sharp focus plane it becomes a circle instead of a point. You have seen this as you adjust the focus point in a scene.
The overly technical term “circle of confusion” refers to how large these circles are. And what we care about is how large they can get before we perceive them as out of focus. This picture helps illustrate that. The center diagram is focused precisely. The top one is focused slightly behind the focus point. The bottom one is slightly in front of the focus point.
Physics
You probably don’t actually care about the math. But here is it:
The approximate depth of field can be given by: 
for a given maximum acceptable circle of confusion (c), focal length (f), f-number (N), and distance to subject (u).
This is precise, but not helpful. There are DOF calculators available, but most of us will not use them when we’re out shooting. I never would, and I even understand the math. 🙂
What does it really mean?
It means that if we want to maximize the apparent range of sharpness in a scene to our advantage, we need to understand some basic things about our cameras and how to adjust them for the results we want. None of it is magic and we do not need to become mathematicians.
Controlling the DOF becomes just another of the design choices we make more of less automatically as we set up a picture. When we become familiar with the concept it gets to be an easy thing to take in consideration.
How do you control it?
If you unpack the equation above you discover that there are are really only three things to juggle: focal length, aperture, and distance to the subject. I take circle of confusion as a relative constant, since we don’t usually think about it while we are shooting.
Normally we consider the aperture to be our main control of DOF. That is because it is the easiest one to adjust. You have discovered or been taught that a wide aperture (small f/ number) gives a shallow DOF and a small aperture (large f/ number) gives the most DOF. This is true, as far as it goes.
But there are 2 other components to the equation. We also affect DOF by our lens focal length choice and by our distance from the subject.
For any given lens, the closer we are to the subject, the shallower the DOF is. Telephoto lenses exaggerate the effect more than wide angles. Increasing the focal length reduces the DOF. And getting further from the subject greatly increases the DOF. Looking back to the formula briefly, notice that the focal length and distance to the subject are both squared. This means they are a much stronger influence than the aperture.
Aperture, focal length, and distance to the subject all work together to determine the DOF A smaller aperture (larger f/ number) increases it. Moving further from the subject increases it a lot. Using longer focal length lenses decreases it a lot.
It is said that wide angle lenses have greater DOF
Conventional wisdom is that wide angle lenses have more DOF than telephoto lenses. Actually, no. But practically, yes.
The discrepancy is how we tend to use them. We shoot with a long lens and decide there is not enough DOF. So we put on a wide lens and shoot the scene from the same position. DOF increases a lot. But the field of view has also expanded, so we have a much wider shot. If you were to walk up to the subject to make the image size field the same as before, the DOF would be the same. Thank you physics.
But in practice, yes, using a wide angle lens usually gives us a great DOF because we usually shoot from relatively far away.
Hyperfocal distance
One “trick” that has been used for a long time and that simplifies getting maximum DOF is to know about hyperfocal distance. The hyperfocal distance is an optimum point where everything from infinity to a point near the camera is in acceptable focus. Seems too good to be true, but it is just physics again.
The technique is getting harder to determine now and is probably falling into disuse. Way back we tended to shoot prime (non-zoom) lenses and they had focusing scales. For a given aperture, all you had to do was adjust the lens so the distant aperture number was at infinity. You were now focused at the hyperfocal distance. Everything from the near focus scale mark to infinity is in acceptable focus. It was easy and very useful.
Now, though, zoom lenses have gotten very good and most of us use them. The problem is that they are optically complex and do not focus the same. They cannot, by their design, provide us with focus scales.
What to do? A pretty good solution is called the double the distance method. There is some estimating (e.g. guessing) and approximations involved, but it is better than a lot of alternatives.
Say you want to have a flower about 5 feet away in focus and have everything in focus all the way to infinity. Focus at about 10 feet. Choose a “suitable” aperture, probably around f/11 to f/16. I told you there was guessing. But by doing this, the field from about 5 feet to about infinity will be sharp. Check it in your viewfinder. Adjust if necessary. The hyperfocal point is about 1/3 of the way from the closest point you want sharp to infinity. You have to estimate it.
Making some educated guesses based on knowledge of what’s going on is better than a random guess based on no knowledge.
Just do it
Photography and video production are probably the most technical of the arts. We are constrained by the physics of the sensors and materials, the properties of the optical systems and lens design, and the effects that can be created by these. Compounding these is the reality that we are typically imaging real subjects with all their flaws and constraints. It’s wonderful!
Don’t get caught up in the math or the technical details unless you are a nerd that really likes that. It is seldom necessary for making good photographs. I just put the DOF formula in to show you it is based on science, not some mystical mumbo jumbo. I would never use it in the field.
Learn that depth of field is a balance between the aperture, the lens focal length, and the distance to the subject. Experiment with them. Get a feel of how they relate and practice getting the results you want. It is harder to describe than it is to do.
Today’s image
This image demonstrates intentional shallow depth of field. I wanted the foreground and background definitely blurred, but still recognizable. The effect was achieved with a moderate aperture, f/11, and a short telephoto of 70mm close to the subject. Remember, increasing the focal length and decreasing the subject distance both strongly reduce DOF.
Experiment more. Make the use of your equipment to achieve your intentions automatic.