Basic SLR lens designs - Retrofocus


Retrofocus lenses are generally confined to the short focal length SLR lenses (typically less than about 50 mm). The retro focus design allows a short focal length lens to accommodate the flange-focal distance of typical SLR's, retaining infinity focus.

A simple 20 mm lens will focus and image at infinity one focal length behind the rear principal plane of the lens. If my detector is 46.5 mm behind the lens, that lens won't focus at infinity or even at reasonable distances. With a simple lens you are stuck at this point.

non-retrofocus short focal length lens

Compound lenses have multiple elements and the choice of elements determines the placement of the principal planes. If you place negative lens elements on the front and positive elements on the back of the lens, the principal planes will move back toward the detector. The shorter the focal length of the lens, more disparity between the positive and negative elements will be required to move the planes back to the proper position behind the lens. The position of the rear principal plane will need to be the focal length in front of the detector.

retrofocus design
retrofocus principal planes

The front principal plane will also move similarly backward in the deal. If the front principal plane is say 2 focal lengths behind the front element of the lens, that lens will never focus closer than 1:1. In that instance the lens will be touching the object at 1:1 magnification. This will limit the close focus capabilities of retrofocus lenses when mounted normally. These limitations become an asset when the lens is mounted in reverse and the position of the principal planes is also reversed.

retrofocus working distance

The magnification of the above lens design is limited to less than 1:1 because of retrofocus design. At a little less than 1:1 magnification, the above lens will have zero working distance.