Lens designs: Telephoto


Unlike short focal length lenses, longer focal length lenses don't have a problem with retaining infinity focus on an SLR. They suffer from having too much focal length to deal with. A simple 300 mm lens will need to have the detector 300 mm behind the rear principal plane at infinity focus and even farther away for closer focusing. 300 mm is about a foot. Most people don't want their lens to be quite that long.

long focal length non-telephoto

Telephoto lens design allows the lens to be considerably more compact by moving the principal planes out in front of the lens. This is accomplished in the opposite way from retrofocus lenses. Telephoto lenses have positive lenses in the front of the lens and negative lenses in the rear of the lens. Retrofocus lenses are sometimes called reverse telephoto lenses because of this similar but opposite design.

telephoto design

Since the principal planes are out in front of the lens, telephoto lenses will have additional working distance when compared to their focal length. Their minimum working distance will be their focal length plus the distance form the front principal plane to the front of the lens.

telephoto lens working distance