I was working from my head and could well be wrong but let's just take
the 35mm GN118 example. At 10 feet GN118 should yield f/11.8. The
measured result was f/8 + 0.7 stops. Therefore the measured result is
0.3 stops short of reaching f/11. Yet the requirement is for 0.8 stops
above f/11. Adding the 0.3 stops short of f/11 + the 0.8 stops yet to
go sure sounds like 1.1 stops to me.
Me thinks your math is wrong if you equate f/11 + 0.8 stops (meaning
8/10 of the way between f/11 and f/16) to f/11.2. It's actually
somewhere between f/14 and f/16.
Chuck Norcutt
Ian Nichols wrote:
> 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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