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Re: [OM] Teleconverter Theory

Subject: Re: [OM] Teleconverter Theory
From: dreammoose <dreammoose@xxxxxxxxx>
Date: Sat, 18 May 2002 23:30:30 -0700
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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