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






>
> If both lenses are f/2.8, won't the 100 mm lens have
>twice the aperture of the 50 mm?



Yes, indeed it would. My head is still hurting from the other part of this
original thread, trying to understand what's what. So I tried to answer
this question, instead. So I was thinking one thing, and typing something
else. I'm sorry. Thanks to those of you who pointed out my error so that I
can correct it. Here is what I meant to say in the second part of that
post.

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
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".
<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<


Whew, I hope that was less embarrassing,

John

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