Chuck Skinner wrote:
>
> > -----Original Message-----
> > From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of Nick Taylor
> > Sent: Saturday, February 10, 2001 12:56 AM
> > To: [EMAIL PROTECTED]
> > Subject: Re: EOS 50 1.8
> >
> >
> > Chuck Skinner wrote:
> >
> > > How do you come up with 92%? I've done the math too, and come
> > up with 62%,
> > > or just under 2/3 stop.
> >
> > My sixth grade son did the math for me:
> >
> > 3.14 x (1/1.4) x (1/1.4) = 1.60
> >
> > 3.14 x (1/1.8) x (1/1.8) = 0.97
> >
> > (1.60/0.97) X 100 = 165%
> >
> > He also said that if we use pi to more than two decimal places, and
> > use the square root of 2 instead of 1.4 we'll get more accurate results.
> >
> > He also told me to remember that "f-stop" is NOT aperture, but the
> > ratio of aperture to focal length.
>
> Exactly, which is why you can't use the ratio (1.4 or 1.8) in the formula.
> You have to use actual sizes. Light transmission is dependent on the area of
> the opening the light passes through.
Had to go to my seventh grade son for this one Chuck. He said that since
the focal length is a constant, it can be ignored and it is proper and
okay to use the focal ratio.
3.14 x ((50/1.4)/2) x ((50/1.4)/2) = 1001.78
3.14 x ((50/1.8)/2) x ((50/1.8)/2) = 606.02
1001.78/606.02 = 165%
Same solution either way!
BTW, he also said that you may also disregard pi, and just compare the
squares of the radii.
(1/1.4) x (1/1.4) = 0.5102
(1/1.8) x (1/1.8) = 0.3086
0.5102/0.3086 x 100 = 165%
To rub it in a little, the sixth grader pointed out that the 1.4 f-stop
on your lens is NOT the exact figure; it should be 1.414213562 at the
very least ... or more accurately, it should be the square root of 2.
It's tough when a twelve year old makes you feel dumb!
-Nick
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