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From: Bill (no email)
Date: Thu Jan 26 2006 - 20:25:44 EST
Bill wrote:
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| As per your request, did a quick series of tests using a tripod mounted
| Minolta digital flash meter, pointed at a flat white interior wall
| illuminated by an incandescent bulb.
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| Baseline reading, no filter f5.6 .9
| With Vivitar 77mm filter f4.0 .4
| With Hoya 67mm filter f4.0 .6
| With Nikon 52mm filter f4.0 .5
| 77mm and 67mm combined f4.0 .0
Frank asked
"That last observation is bit of a surprise to me. And what does the second
figure represent, that follows the f-number?"
It is a digital meter, so reads out f-stops to the tenth. For clarity the
.X numbers on the display are in smaller-size fonts/digits than 5.6, for
example, to avoid confusion. Stops like 2.0 and 4.0, appear with the .0 for
like reason I suspect. 5.6.9 is just confusing. Imagine f-1 + 4 tenths
reading out as 1.4, or f2 + 8 tenths reading out as f2.8. Close but....
George added:
"I would have expected the two polaroids combined (and aligned) to let
through a bit less light than a single polaroid does, simply because of
surface reflections and imperfections in the transmission. But I thank Bill
for confirming the point that I was trying to make, in numerical terms."
My pleasure. You have been very generous with your time in mentoring me and
others. BTW, 2 filters stacked at 90d reduced transmission by approx. 10.5
stops.
Take it as one for the win column if you wish. Commons sense would suggest
if 2 identical filters were face to face and in register, you would have
beaten the point spread as well. <G> But to date practical experience and
models based on other filters have failed the common sense test relative to
polarizing filters for me, so ignore the man behind the curtain ;-)
Thanks to all for the many links with varying levels of explanations. The
grade-school model of the second fence realigning the spring motion was
about my speed ;-)
Off topic, I have always wondered about the order of the f-stops. The
relationship is clear enough: relative increase in the radius of the
aperture to double transmitted light. It has always struck me as strange
given aperture dimensions that 1, 1.4, 2 etc are the largest openings, while
45, 64, and 90 are much smaller. An inverse relationship.
Any history buffs out there than can explain that?
Bill
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