The theory is pretty simple. A teleconverter moves the lens farther from
the film and inserts a negative lens between the lens and film. This
enlarges the image at the film plane and moves the focus back to the
film plane as well. The result is that the image circle originally
designed to cover about 44mm, the diagonal of a 35mm frame, is made
larger (but, of course, the part off the film is lost). This has two
primary effects. First, it enlarges the image in the center that fits on
the film frame. Second, it spreads the light out over a larger area,
thus decreasing the brightness on the film.
The results are easily calculated. A 2x converter that doubles the
effective focal length doubles the image size, thus spreading the same
amount of light over 4 times the area (pi*R^2), diminishing the light
density 4 times, which equals a speed loss of 2 stops. A 1.4x converter
loses 1 stop. In the practical world of complex lenses, multiple element
converters, etc. none of these numbers work out quite that simply.
Moose
John Cwiklinski wrote:
Does someone have a technical reference to teleconverter theory. (I am
more interested in speed and light passage effects rather than the
magnification). Thanks.
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