View Full Version : What does Telephoto mean?

Colin D
22-Mar-2013, 13:36
I've seen 300mm+ lenses described as normal for 8x10 and telephoto for 5x4. What does telephoto mean exactly. Is this a technical description ie. the image to subject ratio is greater that 1:1, or is it just a generic term used to say the image will be much closer on the ground glass than a shorter lens.

I have a couple of 300mm lenses to use on my 5x4 and wonder what it will mean when composing an image and what difference does it mean in terms of exposure time etc.

22-Mar-2013, 14:02
Telephoto is properly the description of one type of optical design, in which the lens mounting plane
is significantly behind the rear node.

The rear nodal point is a point in the optical design from which all outbound rays appear to emanate.
Outbound rays are those headed toward the film (or sensor or whatever).
The optical focal length of the lens is the distance from the film to the rear node when focused at infinity.

The flange focal length of a lens is the distance from the film to the mounting plane when focused at infinity.
In a telephoto design this distance is much less than the optical focal length.
To give a specific example, a 600mm telephoto lens might have a flange focal length of 300mm.

In a "normal" lens the rear node is located roughly at the diaphragm.
When the camera has a bellows and the lens can be moved, all is well.

Telephoto designs became important with cameras that have no bellows, so the lens cannot move.
The most common example of these are 35mm and MF SLRs, with a fixed distance from film to lens.

Use of a telephoto design does place restrictions on lens performance.
The available magnification is still a function of bellows draw and optical focal length.

To achieve 1:1 (full-size) magnification, the lens must move forward from its infinity focus by a distance equal to its optical focal length.

- Leigh

22-Mar-2013, 14:12
The tele 300 mm lenses will probably have smaller image circles. Also tilt can be less predictable with the tele

Bob Salomon
22-Mar-2013, 16:05
But what telephoto is referred to in your question is simply a lens whose focal length is longer then a normal lens.

The lens can be a telephoto design (Tele Arton for example) or a non-telephoto design (Apo Sironar-S for example). Both may have the same focal length, longer then the normal lens, but the Apo Sironar-S will require a longer bellows extension to reach infinity then the Tele-Arton will.

22-Mar-2013, 16:18
Simply put, a telephoto lens has its so-called nodal point before the front element. That is is outside of the lens, in front of the lens. This makes the functional length of the lens longer than its physical length.

David Lobato
22-Mar-2013, 16:33
I have a Fujinon-T 300mm and it only needs 200mm of extension to focus at infinity. But due to its telephoto optical design it covers up to 4x5. A typical 300mm lens would require 300mm of extension and will cover at least 8x10. Each will have the same magnification on 4x5 format when focused at infinity.

My 4x5 field camera has 300mm max extension and the telephoto lens allows closer focusing than infinity. Those are the trade offs with a camera's max extension and the longest focal length you can comfortably use.