From: "John Sherman" 
To: 
Reply-To: "John Sherman" 






Hi Steve,

> This must be why they are called "fast" but doesn't make sense to me.

The term "fast" comes from photography.

Imagine two 50mm focal length lenses, one a f/1.8 and the other a f/2.8.
The f/1.8 has more aperture, so it collects more light, so it can record an
image on the film in less time. It is "faster".

Now imagine two f/2.8 lenses, one has a 50mm focal length and the other has
a 100mm 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".

Got it?

John

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