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.
