NavList:

   

Reply

Bill wrote:
|
| 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.
|
| 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.   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

   

Reply