From: "Steve Houlihan" 
To: 
Reply-To: "Steve Houlihan" 


> 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".


Thanks for the explanation, John.  This is the first response that makes
sense.  I understand the photo analogy well, although an analogy using film
speed might work better.  Anyway, now I understand.  A fast telescope makes
a smaller, brighter image.  Now I expand that thought to say that then if
you use a digital camera analogy that the pixels are bigger (actually, less
pixels across the width of the picture) and therefore the picture would be
grainier, just like in the photo analogy.  Makes sense to me.

Thanx

Steve Houlihan
Santa Cruz, California

sho{at}starband.net
http://sho.mystarband.net/
http://www.surfnetusa.com/sho

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