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.
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?
Chuck Norcutt
Moose wrote:
> Chuck Norcutt wrote:
>
>
>>Not that it's terribly significant but 3.5 is actually 2/3 stop slower
>>than 2.8.
>>
>
> You beat me to it! :-)
>
>
>>1/3 stop is 3.2, 1/2 stop is 3.4.
>>
>>
>
> Here, my numbers start to disagree with yours., 1/3 stop down from f2.8
> as f3.10 and 1/2 stop down as f3.27.
>
>
>>I'm showing off. :-) Now that I've got a digital read-out for shutter
>>speeds, aperture and ISO all in 1/3 stop increments I'm beginning to
>>memorize all the intermediate settings. But I did have to calculate the
>>1/2 stop increment. Actually, the precise value is 3.43.
>>
>>
>
> Again, I get the precise value of 3.27. Am I doing something wrong?
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