Chuck Norcutt wrote:
>No, you're not doing something wrong, I am. Your numbers are correct.
>I went the long way and computed the glass area and verified your
>numbers. I had been using what I thought was a shortcut to the
>calculations. Applying ratios to the focal ratios. Well, it's a
>shortcut all right but it's wrong. I'll have to rethink what I was doing.
>
>
The very simple formula I have in my spreadsheet, and which I probably
worked out from areas, is simply a column of decimal values with the
f-values being 1/sqrt('decimal value'). Here's the first part of the
half stop table:
Half Stops
f-stop area/f1.0 area
1.00 1
1.15 0.75
1.41 0.5
1.63 0.375
2.00 0.25
2.31 0.1875
2.83 0.125
3.27 0.09375
4.00 0.0625
>What's interesting out of all this is that, as you report, 1/3 stop down
>from 2.8 (2.828427) is 3.1. My camera, however, reports it as 3.2. 3.2
>is also a value you're somewhat likely to see printed on a lens as its
>maximum aperture. I think 3.1 there is an unlikely value. How come 3.2
>seems to be the commonly used value?
>
>
Entropy.
Moose
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