What Shiner's written is absolutely right. However most people have difficulty accepting it. I know at my seminars there's always one prosumer who says "I don't care about the theory, I know that I have less depth of field at the telephoto end of my zoom" and then I have to spend ages telling him why he's wrong.

Firstly, a lens is only sharp at the precise point of focus. A few millimeters either side and the unsharpness begins. The question is: at what point does "a bit unsharp" become "out of focus?" Which is like asking "how long is a piece of string?". So we say that once the point becomes larger than "X" it's unsharp. We come up with this figure depending on the number of pixels, HD or SD transmission and so on. This is important. Look at a picture on a 3" screen and it all looks sharp, look at a 42" plasma and it all looks out of focus! In other words, Depth Of field is variable, depending on all sorts of circumstances.

What we're calling Depth Of Field is normally thought of as the distance in front of the focussed mark plus the distance behind the focussed mark which "appears" to be still in focus. The main three criteria which affect video DoF are the aperture, sensor size, and distance.
So, let's take an interview as an example. A 25mm lens, at F2.8 focussed on 10 feet has a theoretical Dof (for Video) of four feet. To get the same size image from a 50mm lens, you need to be 20 feet away and, at f2.8 it has a DoF of...4 feet.

However, due to the foreshortening effect of a lens at the long end of the zoom, the focus appears to "snap" in and out a lot quicker than at the short end which gives an impression of a smaller DoF. By having a "smaller" area of background, this too can give the impression of a smaller DoF. Also an unsharp object (such as a plant) which is far in the background with a wide lens is "bigger" due to the compression of a telephoto and, therefore, more noticeably present and unsharp.

Try a few tests and you'll see what we mean.