Do telephoto lenses have a narrow depth of field?
A.: The belief that long focal length lenses have a narrow depth of field is a common misconception. The depth of field depends on the aperture but is independent of the focal length. The misconception has probably arrived from the fact that for the same f number long lenses have a larger aperture and consequently a narrower depth of field. The f number is the ratio of the focal length to the aperture. If we increase the focal length, let us say by a factor of 2, the aperture has to increase by a factor of 2 as well for the f number to stay the same. For those who are not allergic to algebra I give the formula for the depth of field W : L2 W = ————– (D/k) – L where L is the distance from the optical center of the lens, D is the aperture, and k is the required angular resolution (circle of confusion). The aperture D is the diameter of the opening in the lens that admits light to the film. The equation shows that the depth of field depends only on the aperture of the lens and th
