Depth of Field - DOF

Depth of field, or DOF, in photography defines how much of a scene in a photograph is in focus, relating to the distance from the camera.

Sounds complicated, but it’s actually not that bad.
Let’s assume, in a landscape, you want to take a picture of a tree, the tree is, say 10 meters away from your position.

If you want the foreground, tree and background all in focus, this would be a deep DOF.
If you want to <isolate> the tree from foreground and background, it’s a shallow DOF.
There are of course lots of variables in between.

So, how do you generally control DOF?
For once, the aperture in your camera does that, similar to the iris in your eye (one of the few actual similarities).
The further you open the aperture, the shallower the DOF. 
The settings for this are a bit confusing though. Low <F-number> means wide open, high <F-number> means tighter – and with that a deeper DOF.

Another factor to control DOF is the distance to your subject.
A wide open aperture has less effect the greater the distance between you and the subject is.
For example, you shoot a portrait, your model is 1 meter from your position. You have a <fast> lens, which means the aperture can open very wide, somewhere around F 1.4-3.
If you open it say to F 2.8 (like my lenses), the DOF will be extremely shallow. This means, the tip of your model’s nose is in focus, all the rest fades into the out-of-focus area. For such short distances you would need to close the aperture more, say F 5 or so.
If your model is some 6-10 meters away, this effect is not so severe. You can open the aperture wider. The model would still be in focus, but anything closer or further would be out of focus. This <isolation> effect is often used in portrait photography and some landscape setups.
It’s also an interesting effect in macro/close-up photography. In fact, for macro and close-up, DOF can be a serious issue. Due to the physical closeness to the subject, the depth can shrink to a matter of millimeters, which makes focusing rather difficult. There are techniques to overcome such issues, and I’ll write about those later on.

Here an example of a portrait. Using a long lens, the real physical distance was some 4 meters, so I could open up the aperture to F 2.8 without losing any focus in the model's face.

In the second image, my subject was about one meter away. Opening the aperture to F 2.8, you see the strong effect. The tip of the beak in full focus, everything else out of focus. In this image it works well enough.

The third image is a typical landscape. This setting needs a deep DOF, so the aperture was much tighter, at F 8. All parts of the image are pretty much in focus.