Depth of field is not a wall, it's a gradient of focus - by Ligar
Understanding Depth of Field, or DoF is a topic that is actually more complicated that it seems on the surface. The theory is simple: When you focus on an object, there is a distance on either side that will be in focus. How much area is in focus depends on the f-stop of your lens.
Simple, right? That’s the easy part, after that it gets tricky. Turns out that DoF is more like a gradient than a wall. The fall off in clarity happens gradually and the actual DoF distance is more of an estimate.
The exact distance is based on observation. Specifically the point at which you will notice a difference of a point on a standard 8×10 photo viewed from a distance of one foot. The area of fuzziness around a particular point even has a name, it’s called the “circle of confusion” or the “blur spot”.
There are other factors that go into the exact DoF besides f-stop. The focal length of the lens (in mm), and the distance to the subject.
The practical application of all this is to remember that the actual DoF is a subjective observation based on your eyesight. So take the markings on the side of your lens with a grain of salt. Just because there are a couple marks that show the depth of field, doesn’t mean they’re accurate! The standard your lens manufacturer uses are well inside what you can see with your eyes. It’s just a guess, don’t trust it. The only way to be sure is to shoot a series of pictures and check it yourself.
A simple rule of thumb is to use f/8 as a starting point. If you want a wide depth of field, think about an f-stop larger than f/8, which translates into a smaller aperture. Use f/11, f/16, or, ideally, f/22. If you’re aiming for bokeh, deliberately throwing the background out of focus, select an f-stop below f/8 like f/5.6 or f/2.8.
Again, when it comes to your lenses, shoot some pictures and check the depth of field. When you get a new lens, that’s job one.
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