----- Original Message -----
From: "Chuck Skinner" <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>
Sent: Friday, February 09, 2001 10:52 PM
Subject: RE: EOS 50 1.8
> <snip>
>
> > Jim, I can't speak to how often you would use the lens, but math tells
us
> > the 1.4 will let in 92% more light than the 1.8, so it's nearly twice as
> > bright.
>
> How do you come up with 92%? I've done the math too, and come up with 62%,
> or just under 2/3 stop.
>
> Chuck Skinner
>
In reading the subsequent posts, I think my error was not using the focal
length but the actual lens diameter in my calculations.
Plus, I think it's one of those problems that you get a different answer
every time you try it! The second try I got 106%.
Seriously, here's what I did, right or wrong.
I assumed the lenses to have the same diameter as the filters they used,
which is not pure science by any means :^)
So the radius of the 1.8 lens wide open is 52/1.8/2, or 14.44mm.
The radius of the 1.4 lens wide open is 58/1.8/2, or 20.71mm.
The areas of these openings then is 208.64 square millimeters and 429.08
square millimeters, respectively, or 106% larger for the 1.4.
I also assumed that an opening 106% larger would let in 106% more light,
which probably is pure science!
tomp
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