CENTURY MYSTERY @@ Goerz Wide Angle Aplanat Aperture Puzzle

[cwe001]

This experience makes me think deep in to effective aperture.

Recently I acquired a 150mm wide angle aplanat lens made by georz with wheel aperture stops reading 24,48, 192,384,768.

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According to above chart, they correspond to: f16 22 45 64 128 and the largest hole meares to be 10mm which is about 150mm divided by 15. So I take the result granted.

But I kept getting overexposed results when testshooting on polarod..and the comaprison from two lenses with standard copal shutter showing excellent results ,so the meter is ok. I started to look carefully at the holes on aperture wheel and found they seem to be one stop larger than coresponding opening on copal shutter.

This morning I did a test with a grey card. First I made a exposure of 1/2s at f11 with 150mm sironar and copal 0 shuuter. Next I set the aplanat lens at f16 (Maximum aperture ) but still gave 1/2s exposure instead of 1s. Here I assume the effective apeture at f16 position is one stop up to f11. And I obtained EXACTLY same results on two photos, looking at the grey card part.

And same results when I exposed the 150mm sironar at f/16 and the aplanat at f/22 .

It seems the effective apeture coressponging to 24 48 192 384 768 fon the wheel to be f/11 16 32 45 64.
I am in a total missing now: how come the result has one stop deviation from the conversion chart ? And based on definition of aperture , the size of the holes correspond to the chart but in fact the effective aperture is one stop up?. And why the openings on copal shutter do not correpond to aperture definition while proper exposure is obtained?

ANy body had similar experience or can cast some light?



I

 

[Ole]

Goerz used "the Stolzesche System", according to my literature. That was based on f:3.16 (square root of 10) as 1, with a doubling of the number for each stop. So Stolze 24 would be about f:15.

Using Polaroid to check exposure values at long "shutter" times is risky - Polaroid films have very strange reciprocity failure characteristics. And they're all different too, so I can't tell you if your result is as should be expected or not.

 

[cwe001]

The mystery is if I gave exposure time based on f/15 ,the print is over exposed about 1 stop. And normal result is obtained based on one stop open apeture(1/2 exposure time).

Also when I look at the opening, the 24 ( f/15 ) hole is the approx same size as
that of f/11 hole of a 150mm sironar. Where the diviation came from..>_<

 

[Ole]

Could it be a case of mismatched parts? The next bigger lens in the series was 180mm, so an aperture wheel intended for that lens would give f:12.5 as the largest opening.

Just a thought...

Next one up again was 210mm, for f:10.7; but I think the wheel would then have been too big for the barrel.