[Leica] 1.8 vs. 1.4!?!?

[ken at iisaka.com (Ken Iisaka)]

Not exactly.

1/3 stop slower: 2^(1/6) = factor of 1.12246205
1/2 stop slower: 2^(1/4) = factor of 1.18920712
2/3 stop slower: 2^(2/6) = factor of 1.25992105


On Thu, Nov 3, 2011 at 12:11 PM, Matthew B. Filippini
<filippiniaia at hotmail.com> wrote:
>
> Time to beat a dead horse! ?If anyone is interested:
>
> It is all geometry, namely the area of a circle.
>
> For each f-stop, we have double the light. ?The f-stop is related to the 
> size of the aperture, which is approximated as a circle. ?The amount of 
> light coming is proportional to the circle?s area, which you may recall is 
> pi times the radius squared, pi*r^2. ?We use the f# for the equivalent 
> radius.
>
> Thus, starting with f1, and r = 1, the area is pi*r^2 = pi*1*1 = pi, as 
> the relative amount of light.
>
> For an amount of light 2*pi (next f-stop, double the light), pi*r^2 = 2 
> pi. ?Divide both sides by pi, and you get r^2 =2. ?r = the square root of 
> 2, or 1.414? ?f1.4 is the next stop.
>
> This is where the 1.4 factor George mentioned comes from; the square root 
> of 2 is 1.414.
>
> Next f-stop, double the light again: pi*r^2 = 2*2 pi. r^2 ?= 4, f2 is the 
> next stop.
>
> So, if you want fractional stops:
>
> 1/3 stop: ? Square root of 1.333 = 1.15456
> 1/2 stop: ? Square root of 1.5 ? ? = 1.22474
> 2/3 stop: ? Square root of 1.667 = 1.29112
>
> So going back to Mark?s f1.4 example:
> 1/3 stop slower = 1.414 * 1.15456 = f1.633 or f1.6
> 1/2 stop slower = 1.414 * 1.22474 = f1.732 or f 1.7
> 2/3 stop slower = 1.414 * 1.29112 = f1.828 or f1.8
> 1 stop slower = 1.414 * 1.414 = ?f2.0
>
>
> Matt
>
>
>
>
>
>
>
> On Nov 2, 2011, at 5:32 PM, Mark Rabiner wrote:
>
>> I looked up f 1.8 vs. 1.4 thinking it was between a half and a quarter of 
>> a
>> stop and they are saying its 2/3rds!?!?! Anybody know that that's true?
>>
>> Where is there a photo calculator that tells you these things?!?!?
>
> I always thought the basic math for 1 f stop revolved around a factor of 
> 1.4.
> 1.4 x 1.4 = 1.96
> 1.8 / 1.4 = 1.29
>
> so - yes - 2/3 would seem close enough for?
> what? I'm not sure.
>
> Regards,
> George Lottermoser
> george at imagist.com
> http://www.imagist.com
> http://www.imagist.com/blog
> http://www.linkedin.com/in/imagist
>
>
>
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>



-- 
Ken Iisaka
first name at last name dot org or com