Retrofocus
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| M42 Angénieux Retrofocus lens image by Woradate Pattayawan (Image rights) |
Retrofocus refers to a type of lens design, pioneered (and originally trademarked) by Angénieux, which permits a short lens optical focal length while providing a greater back focus distance. This is typically required in an SLR camera, to allow space for the swinging reflex mirror; and also in some cine cameras using small image formats, to permit space for the shutter mechanism.
Sometimes known as a "reverse telephoto" design, retrofocus designs typically have outermost lens groups with negative power, with convergent elements behind them. It can be difficult to design a retrofocus lens that is fully free from barrel distortion.
Since the rear element is further away from the film / sensor plane, retrofocus designs also have much lower vignetting (i.e. more even illumination across the image).
