Ian, you calculation should be right.
For the rest, I did an estimation without calculation, the errors at 35mm
are less than 0.1 stop except full power, the errors at 85mm are a little
over 0.2 stop except full power.
C.H.Ling
----- Original Message -----
From: "Ian Nichols"
2009/8/21 Chuck Norcutt <chucknorcutt@xxxxxxxxxxxxxxxx>:
> for zoom setting 35mm you got
>
> GN30 - F2.8 0.1
> GN42 - F4
> GN59 - F5.6 0.1
> GN84 - F8 0.1
> GN118 - F8 0.7 (full power)
>
> At 35mm all of these up to full power are less than 1/3 stop error. But
> at full power you should have gotten f/11 + 0.8. Instead, you got f/8 +
> 0.7 or *more than a full stop off*.
My gut feeling was that you were wrong there, Chuck, so I threw
together a quick spreadsheet, getting the f numbers for the fractional
stops by n*(2^0.05) - a half (i.e. square root) time a tenth- where n
is the next lower full stop number.
The resulting table says that f/8 +0.7 stops is f/10.2 and that f/11.8
is f/11 +0.2 stops - only half a stop out.
> for zoom setting 85mm you got
>
> GN29 - F2 0.9
> GN41 - F2.8 0.8
> GN58 - F4 0.8
> GN82 - F5.6 0.8
> GN116 - F8 0.8
> GN164 - F11 0.5 (full power)
>
> At 85mm GN29 is correct, GN41 is 1/3 stop low, GN58 and GN82 are 0.4
> stops low, GN116 is 0.8 stops low and GN164 is 0.9 stops low.
Similarly I make f/11.6 = F/11 + somewhere between 0.1 and 0.2 so
0.3-0.4 stops low, and f/16.4 = f/16 + less than 0.1, so only about
half a stop low
Didn't check the others because it was only those that looked wrong at
first glance. So one of us is working this out wrong. If it's me,
I'd like to get my head around what I'm doing wrong.
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