Here’s a subject that will fry your brain. This subject is a dangerous weapon for those with a little knowledge. It’s easy to get into the mojo magic world of higher subjective math when people try to justify why their images come out the way they do. Not only that, this subject gets down right personal when the conversation migrates itself to the various sized digital sensors and therefore the associated camera manufactures. Talking about D.O.F is like waving meat in front of a rapid dog. Most sensible people wouldn’t touch this with a ten foot pole. So, here I am jumping in with both feet.

For those that don’t know, the concept of D.O.F is about the apparent sharpness of a scene based on the physics of optics and photography. First I’ll say the most obvious yet overlooked fact; there’s only one spot that’s in-focus for any given focus setting and everything else is out of focus--that’s a fact and irrefutable. D.O.F. is about the concept of why more than the thing you’re focus’d on appears to be sharp. Through the magic of physics, more stuff appears to be in focus because the things that are out of focus are not out of focus’d enough for you to notice. If you use a little logic and apply a couple of assumptions, you can actually quantify how much your “thing” can be off focus before you can see the affect it when viewing the final product.

BTW, if you haven’t guessed we aren’t going to be doing any math on this subject. You can do some at home in the comfort of your own study. I won’t tell anyone. For this article, we aren’t doing any math, thank gawd.

The size of your D.O.F. is affected by a bunch of things. Clearly some factors are more important that others. For practical purposes the most obvious factors are focus distance, focal length of your lens, and aperture setting. Between the three, these are the most obvious and less controversial.

Note: when I say focal length, I’m talking the real focal length. I’m not talking about the multiplied focal length of something because its sensor is less than whatever the standard may be. The focal length of a 200mm lens on a Nikon 2/3rd’s less than 35mm film sensor is still a 200mm lens even though it’s field of view is about a 300mm. If you remember this you’ll never get confused when trying to match apples to apples and oranges to oranges.

Just so you know, D.O.F. increases when you focus further away. That’s pretty easy to remember. Eventually, you get to a focus distance when everything is in focus out to infinity. You’re at what’s called the “hyper-focal” distance. D.O.F. also increases when you make the aperture smaller--choose a larger f-stop. Finally, D.O.F. increases when you pick a smaller focal length. A 50mm lens with the same focus and aperture settings has a larger D.O.F. than say a 1000mm lens. I say these assumptions presuming everything else is the same and not changing.

Let’s talk about some other factors that are important but don’t nearly get the press like these first three do. I’m talking about 1) how much you’re going to expand your image from capture to final size. 2) How big your subject is.

The first one is pretty easy to understand. Without getting into the math, let me just say when you blow an image up more and more, the D.O.F of the final print gets smaller, and smaller, and smaller. Why is that? When you view and image from the same distance--you don’t move your head but the size of the print gets bigger and bigger--you begin to be able to make out more detail. There becomes a limit when you can easily see what you have is in focus or not. The more you blow things up the more you’re able to see the details until the details stop being there because they’re... what? Out of focus. Blowing up an image tends to make you have less and less D.O.F.

Well, then the opposite must be true. The less you blow things up the greater the apparent D.O.F. of the print. That makes sense as well. Remember viewing your first image made from a large format camera? The detail was incredible. Why is that? Because the larger the sensor, the less you have to blow it up, the less detail you’re able to see. Obviously, the D.O.F. is very large for very large sensors.

Let’s take this idea a little bit further. If you blow up an image from a smaller sensor to the same size from a print made from a larger sensor, the D.O.F. of the smaller sensor will appear to be less than the other. Why is that? That’s because the information you’re using is being expanded more from the smaller sensor than the larger sensor. There’s only so much with what you can do with that smaller sensor.

From a practical point of view, if you have a very high resolution sensor--this means lots of pixels--and other factors aren’t important, this expansion thing becomes less of an issue because you’re taking advantage of the sensor’s high resolution.