Common photography myths (14)

Myth #13: Using a TC on a shorter base lens results in different DOF compared to using a longer base lens

September 5th, 2009 - 08:50:50 PM:

When you mount a TC between the lens and the camera, the TC changes both the true focal length and the true relative aperture of the lens (the physical aperture does not change, but since the true focal length becomes longer, the relative aperture becomes smaller). With a 1.4× TC the focal length becomes 1.4× longer, and the aperture becomes 1 full stop smaller. With a 2× TC the focal length becomes 2× longer, and the aperture becomes 2 full stops smaller.

DOF only depends on true focal length, true relative aperture and focus distance. When you use a TC to make a lens longer, you get all the characteristics of the longer lens, including DOF. For example, when you use a 2× TC on a 200/4 lens, you get the DOF of a 400/8 lens.

The following two pictures are taken from the same distance. With both shots the focus was on the middle of the ruler. One picture was taken with a 400/4.5 lens at f/8. The other was taken with a 200/4 lens + 2× TC (=400/8) at f/8. As you can see, DOF is the same for both shots.

Shot 1: 400/4.5 at f/8Shot 2: 200/4 + 2x TC at f/8

There are a number of general statements about photography passed off as “the truth”. They are repeated again and again in introductory texts about photography and on the Internet. Repetition, however, doesn't make a false statement true. Here are the most common myths I've encountered: