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Magnification At Prime Focus

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Posted by John Huggins on March 30, 2001 12:58:13 UTC

Magnification is an undefined concept at Prime focus.

You should think of this in just like the lenses for your camera.

In 35mm, the lens that most approaches a similar field of view as your eye has a focal length of 55mm. With 120 film, this "normal" lens is 80mm.

I suppose we can call this "normal" lens a power of 1x. A 110mm FL lens would have a power of 2x for 35mm and (110/80) = 1.375x for 120 film.

Figure out the effective focal length of your telescope in mm to provide a comparison to the above normal lens.

My 10inch f/6 telescope has a FL of 60 inches = 1524mm. So with 35mm film, I can get away with this:

(EFL Telescope)/(FL of normal lens for 35mm film),

(1524mm)/(55mm) = 27.71x

The larger 120 film takes in "more sky" from the same scope:

(1524mm)/(80mm) = 19.05x

Additinal references:
This site assumes you are using 35mm film and the normal lens is 50mm.

And if these links die with time try this:

I hope this helps.


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