Retrofocus and Tele Lenses

I mention retrofocus on this site and have not offered any explanation. Retrofocus lenses are basically a regular focal length lens with a wide-angle attachment built on the front to decrease the focal length of the combined lenses. They are also called "inverted telephoto" designs.

Retrofocus lenses are necessary because the rear element of a simple wide angle lens would be too close to the mirror of a single lens reflex still camera, the spinning mirror of a motion picture camera or the prism block of a 3 chip video camera. Also the design of faster lenses results in more elements and mirror clearance problems. "Back Focus" is the distance between the lens rear element and the image plane (film or video chip).
25mm Simple Lens on 35mm Camera
25mm Retrofocus Lens on 35mm Camera

A simple 25-mm lens on a 35-mm SLR or movie camera would hit the reflex mirror because it doesn't have enough back focus. Lens designers start with about a 35-mm lens and add retrofocus elements in front of that lens to make the combined lens wider-angle. It's not quite that simple because many design compromises must be made for good optical performance with the combined lenses. Today computers can optimize a new lens in a short time that in pre-computer days took years of calculations by hand.

Most zoom lenses are a retrofocus design and include a prime lens group in the rear, a middle zoom group and a retrofocus group on the front. Each group is optimized to work with each other for good optical performance.

Wide Angle Front Attachments

More expensive wide-angle attachments allow for a zoom lens to zoom all or most of the zoom range, hold focus while zooming and maintain distance focus calibrations.

Cheaper video attachments require a back focus adjustment of the zoom lens on professional video cameras to obtain focus and zooming is no longer possible. The back focus adjustment is usually a ring on the lens next to the lens mounting ring on the camera body. If you change this adjustment the zoom will not track and it must be readjusted for proper zooming.) Focusing all wide-angle lenses is difficult because the resolution of viewfinders is usually less than the recording capability of the film or video chips. Once focus is adjusted, don't change it while shooting. You will have a great depth of field and focus should hold for most distances. See Adjusting Back Focus of Video Zoom Lenses and Focus Pattern

For cheaper camcorders with built-in lenses, some attachments rely on the camcorder's auto focus feature to adjust focus automatically. This works well but only at the wider zoom lens angles. As you zoom in, focus will suddenly jump out when the auto focus feature can no longer correct focus. I suggest using the Seimans Star for checking focus. Let the auto focus do the focus and then check with manual focus if the auto focus is working correctly. See Focus Pattern.

Telephoto Attachments

There is often confusion with the term "telephoto". A telephoto design is the opposite of a retrofocus design. A negative lens attachment behind a prime lens increases the effective focal length of that lens. This allows for a more compact design.

In use, the term "telephoto" usually means a longer focal length lens or position of a zoom lens and usually not a lens of telephoto design.

Any wide angle or tele attachment, not included in the original lens design, is a compromise and will not produce as sharp an image as a lens designed for that focal length. We have to live with compromise and budgets that don't allow for the cost of wide-angle zooms. In most cases when using an attachment, stopping the lens will increases sharpness and can reduce vignetting.

Front tele attachments often limit wider-angle zoom lens angles. They do not reduce exposure. With built in lenses these are the only way to increase focal length, excluding "digital zoom" which uses a smaller part of a video recording chip with reduced resolution.

To increase the focal length of removable lenses, rear negative lens attachments called "Barlow lenses", "tele extenders", "range extenders", and "teleconverters" can result in better performance than most front tele attachments. Rear attachments decrease the speed (light transmission / higher f-stop) of the combination and front attachments do not. Attachments made specifically for a lens by the same manufacturer often give better resolution than "generic" ones. It is possible to convert the rear camera end of a tele extender to another mount and allow still and video lenses to be used with the converted extender. The Canon XL1 does this. Check out Century Precision Optics for professional adapters and good quality lenses.

Extension tubes are not tele extenders. They are used for focusing closer.

As mentioned in other sections. Retrofocus lenses need less exposure composition when extended out of the mount for macro photography. For a zoom lens extended out of the mount, exposure increase is calculated for the maximum focal length of the zoom and is often negligible.

Note: Most video and newer zoom lenses for still cameras become slower at longer focal lengths resulting in different f-stops. The auto exposure feature of the video camera helps correct this with an iris or gain increase.

Many of newer still lenses also depend on camera auto exposure features to compensate for different F-stops at different zoom lens focal lengths. This can affect exposure when these lenses are used on movie cameras with no auto exposure ability.

For more information on this subject and lenses in general read Applied Photographic Optics by Sidney Ray and Focal Press. The book is about $60.00, but well worth it if you really want to understand working lens systems. A new updated 3rd addition is due Summer 2001 at $150, also worth the price.

© Copyright 1999-2004 Ron Dexter. All Rights Reserved.