From: "Nils Olof Carlin" 
To: "John Sherman" , 
Reply-To: "Nils Olof Carlin" 


John Sherman wrote:

> Now imagine two 25mm aperture lenses, one has a 50mm focal length
(f/2) and
> the other has a 100mm (f/4) focal length. Now, you want to make an
image of
> a person on a piece of film, say. If you use the 100mm lens the
person will
> be twice as tall as if you used the 50mm lens. Of course the
aperture is the
> same, and therefore the total amount of light gathered is the same.
But the
> 100mm lens spreads that light out over four times as much film (the
image is
> twice as high, and twice as wide). So each grain in the film gets
one-fourth
> the light per second as it would with the 50mm lens. The 50mm lens
makes a
> smaller image, using only one-fourth the number of pixels but the
same
> amount of light. So it records the image of the person in less time.
It is
> "faster".

But the image with the 100 mm f/l is twice the size, so without sacrificing
relative graininess, you can use a film with 4 times the sensitivity, and
(approximately at least) twice the grain size, and after printing to the
same image scale you get much the same result. Just as with eyepieces -
using the same eyepiece, the image is brighter with low f/ ratio - but if
you choose eyepieces to get the same exit pupil, and magnification, the
image brightness will be the same.

Nils Olof

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