----- Original Message -----
From: "Murray" <uptown@uptowngallery.org>
> Time for some geek-speak...
Don't consider myself a geek but I can entertain your post.
> I had read the discussion of f-stop increments and rounding pinhole
> calculated f-stops to the 'standard' ones in a chart, in part because
> pinhole exposures tend toward (and I can't remember over or under) being one
> side of exposed.
I believe UNDER exposed (in my practice at least)
> This intrigued and bothered me. It made me curious what was the origin of
> the f-stop increments and also wonder why one's pinhole f-stop number
> couldn't be exactly what it's supposed to be.
I never heard anybody saying it COULDN'T. Of course you can use precise numbers if you wanted.
> If focal length and aperture define f-stop, rounding to a standard f-number
> equates to rounding off one's f.l. or aperture diameter...they're all
> interrelated.
You can say so.
> if f 1.0 is the theoretical 100% light getting in, (although I did see a
> Canon rangefinder supposedly with an f 0.95 lens...must have had some kind
> of amplification),
Light amplification would be nice (I could say bye to reciprocity!), but that's not the reason of f/0.95 or bellow. A lens with an f/stop of bellow 1 means the maximum effective diameter of the lens aperture diaphragm is larger that the focal length of it. f/1 therefore, just means the diameter is equal to the lens' focal length, nothing to do with "the theoretical 100% light getting in".
> the f-stop increments appear to come from an exponential
> relationship based on powers of the square root of two. I haven't derived
> where that comes from,
It comes from the fact that the NEXT f/stop has to allow double (or half) the light, the aperture is (usually/considered) circular, therefore the area of the aperture has to be double (or half). Area is proportional to the diameter^2, to have double the area the diameter would have to increase by SQR(2)
> Since reciprocity failure has to be factored in and detemining exposure is
> pretty variable, all this boiling down to a time multiplier, I question the
> necessity of choosing an f-number that 'fits' the chart.
You are looking more at an algebraic exponential equation where you could plug the uncorrected time for a particular pinhole camera and a particular film emulsion, rather than just a simple multiplier. BTW, you don't have to choose an f/stop that "fits" the chart.
> These look familiar? I think f11 looks better on a lens than f11.3.
I agree, only it goes against your statements above: "also wonder why one's pinhole f-stop number couldn't be exactly what it's supposed to be" and also against "you don't have to choose an f/stop that "fits" the chart".
> What it figures out is, what number to multiply exposure time by when
> using a f-stop off the chart, with respect to another f-stop, say f-16, or
> any other.
> basically, take natural logarithm (call it "ln", apologies to followers of
> European notation) of the larger f-stop number and divide it by ln(2^0.5).
> Now do the same with the smaller f-number and subtract the result from the
> first resulting number,
First, any base logarithm would do it. You could reduce the above to:
log (fstop_larger / fstop_smaller ) / log (SQRT(2))
> and multiply by two.
Don't think so, this log (fstop_larger / fstop_smaller ) / log (SQRT(2))
gives you the stops separating the smaller and larger f/stops. To find the multiplier you'd have to do:
Multiplier = 2 ^ [log (fstop_larger / fstop_smaller ) / log (SQRT(2))]
> Oh, almost forgot my question -
>
> Does anyone have reciprocity failure multipliers for common 35 mm films?
For manufacturer maximum suggested exposure times reciprocity corrections just go to their web sites. I know Kodak, Ilford and Fuji have them there.
Guillermo
Received on Sat Jul 21 12:07:25 2001
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