> -----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.
A 50mm lens with a 1.4 aperture has an opening with a diameter of approx.
36mm (50 / 1.4).
A 50mm lens with a 1.8 aperture has an opening with a diameter of approx.
28mm (50 / 1.8).
The area of a circle is pi time the square of the radius. In the cases
above:
A 36mm circle has a radius of 18mm. 18 squared is 324. Pi times 324 = 1018
square millimeters.
A 28mm circle has a radius of 14mm. 14 squared is 196. Pi times 196 = 616
square millimeters.
616 is approximately 60% of 1018, just under 2/3 of a stop.
Chuck Skinner
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