Not just expensive but size of the lens, and thus ease of use. As . . . in
the field. A 300mm f1.2 should be theoretically feasible; it would also
amount to lens so large, so unwieldy as to render it useless to a
photographer handling an SLR. Hell, you'd need to be Superman just to get
it up onto a tripod. Take at look at the fastest, longest telephoto lenses.
They're huge. They're heavy. They're awkward to use already. Now imagine
telephoto lenses larger yet, heavier still and even more awkward to use.
That's why.
The expense would be high, though I'm sure someone out there (for instance,
the military) would pay any sum for something it needed. As this would
likely not be a "production" item the cost could be anything at all. Pick a
figure.
Anyway, you're reasoning strikes me as backward. Think of glass dictating
lens dimensions as opposed to the other way around and you're on the right
track.
Tris
At 10:08 AM 7/31/01 -0600, you wrote:
>Not sure what you mean by "least" amount of glass. Other things
> being equal the faster the lens is the more glass it will have
>in order to allow more light to get in. There's no reason the
>40mm f2 could not be an f1.2, though with cost in terms of its
>size and weight--if that lens were an f1.2 it would come to us
> looking an awful lot like the 50/2.
Okay, so I'm obviously confused about something. I was assuming that the
shorter the lens, the less glass there would be between the outside world
and the film, so there would be less light reduction because the light is
passing through a smaller amount of stuff. Though that, of course, is
forgetting that faster lenses tend to be bigger at the front, which
obviously lets more light in..
But I'm still not sure why it is that the 50mm lens is the fastest -- is it
just that there's not the demand for a $100,000 300mm f1.2 lens? Is it just
the case that 50mm lenses happen to match up to 35mm film in a particularly
handy way to allow making fast lenses at that size without it being too
expensive?
-- dan (perhaps I should just get a book on optics and this would all
become clear..)
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