A telephoto lens is not what you think it is.
There are three major types of lenses:
Likewise, there are three classic lengths of lenses:
- wideangle (< about 50mm for 35mm format)
- normal (about 50mm for 35mm format)
- long (> about 85mm for 35mm format)
Not all long lenses are telephoto.
A telephoto lens is a lens with an opposite design of a retro focus lens. In about 1750 Barlow discovered the principle of a telephoto design. In telephoto design, a diverging lens is placed behind the prime lens. This projects the light to a rear nodal point (where the light converges) that is in front of the main lens (rather than behind the lens with a simple lens). A teleconverter is an example of a telephoto lens attachment.
This design has some advantages and disadvantages. The most obvious disadvantage is the loss of light. There are some issues with the sharpness of the image and a reduced depth of field compared to a non-telephoto lens of the same length. In telephoto design, there is some pincushion distortion. Telephoto lenses have been embraced by the 35mm format providing for more compact lenses (the Canon 400mm f/5.6 lens is only 256mm long). This is especially true in the area of zoom lenses. The telephoto lens has a larger barrel and is heavier (more glass) than a simple lens of the same focal length, but is often easier to work with and hold when it comes to long lenses.
The displacement of the nodal point for the lens causes some issues with various types of photography. When trying to take a panoramic photograph by taking multiple photographs and stitching them together, the rotation should be done about the nodal point of the lens. With symmetrically designed lenses that are focused at infinity, the rear nodal point is always at the true focal length of the lens (may not be the same as what is marked) while telephoto lenses may actual have the rear nodal point in front of the lens.
Large format photography offers cameras that have the ability to shift and tilt the film plane and the lens board.
The Scheimpflug principle and the hinge rule work with the lens plane and assume that the the lens is a symmetric design. Placing telephoto lenses or retro focus lenses on a large format camera moves the location of the center of the lens and nodal points making adjustments of the lens plane much more difficult than with a symmetric lens design. They are still useful in cases where the bellow draw is limited (most useful for field cameras). Otherwise, telephoto lenses are large and heavy compared to their standard cousins.
Compare two large format lenses (TS is the telephoto lens):
brand: model focal max f image lens to price
len. circle film
Fujinon TS 400mm f/8.0 220 260mm $1264
Fujinon FS 420mm f/5.6 500 413mm $642
The telephoto lens of almost the same focal length is:
- casts a smaller image circle - means smaller film
- requires less distance between lens and film for
focusing at infinity
- is twice as expensive
while the other lens
- is a full stop faster
- twice as large of an image is cast
- has a lens to film distance the same as its focal length
- is quite reasonably priced (compared to other long large format lenses)