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09-23-2015, 09:25 PM   #31
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QuoteOriginally posted by SpecialK Quote
It's the Eisenstaedt Uncertainty Principle.



That sounds very serious and official.

Oh...that Eisenstaedt!


Steve

09-23-2015, 10:50 PM   #32
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QuoteOriginally posted by stevebrot Quote
Please interpret. I took a gander at the note sheet (image #5) and believe I know what they show, but don't want to jump to conclusions.


Steve
Hi Steve,
The tests were to see the difference in shutter open time at the f/- stops f/4.5 and f/32, and the difference in brightness levels between the frame axis and the frame edge.
This test is on a vintage Mamiya RB67 which has a much larger film "sensor" than a dslr. The lens was the wide angle RB67 Mamiya-Sekor C 1:4.5 50mm. So the optical angles are different to an ordinary dslr.
The test was done with a ground plastic plate on the focal plane.

Behind that, the detector was a photo diode in a tube so that it received photons from an angle restricted to about a cone of 3 degrees angular.
Referring for example to John B Williams "Image Clarity" page 89 Fig 11.2, for the "edge" results, the tube was rotated from the camera axis to an indicated peak value which was recorded in the results.

The photo diode was in the photo conductive ( reverse biased) mode to give photo current, a response more than 1000 times faster than the mechanical shutter, and faster than the integral photovoltaic mode of a dslr sensor

For the test results:
As you mentioned, the shutter tends to even out the integrated light fall off at the frame edge at open apertures. At normal apertures (f/11~16) , the exposure is not so much differentfrom axis to edge.
Also the film gamma tends to correct the difference from centre to edge.

Anyway, I have just run a few rolls of Ektar 100 and Portra 160 using the wide angle RB67 Mamiya-Sekor C 1:4.5 50mm, and will process soon.
Regards!
09-23-2015, 11:43 PM   #33
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QuoteOriginally posted by wombat2go Quote
This test is on a vintage Mamiya RB67
With a shutter that is in highly questionable condition. Leaf shutters are only rated for about 20,000 firings before they start to show exposure inconsistencies.
09-24-2015, 04:35 AM   #34
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QuoteOriginally posted by Digitalis Quote
With a shutter that is in highly questionable condition. Leaf shutters are only rated for about 20,000 firings before they start to show exposure inconsistencies.
Of course I measured it:
The lens speeds measured were as follows { setting, millisec}
{1,880}, {2,480}, {4,240}, {8,128}, {15,70}, {30,35}, {60,16}, (125,11}, {250,5}, {400,5}

I thought the speeds were quite good for such an old lens. (I don't know if it was ever serviced).

09-24-2015, 11:23 AM   #35
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QuoteOriginally posted by wombat2go Quote
Hi Steve,
The tests were to see the difference in shutter open time at the f/- stops f/4.5 and f/32, and the difference in brightness levels between the frame axis and the frame edge.
Thanks, your explanation is helpful. Your results seem to indicate a significant (~1 stop) native vignette for the 50mm wide angle (not unusual) as well as similar shutter performance for on vs. off axis (ramp values, "r" and "f" are the indicators here). I would expect that a normal lens (say 90mm or 100mm) would show almost identical curves both on and off axis.


Steve
09-24-2015, 11:50 AM   #36
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I have a film box sheet dated 3-1972 maybe, for "Kodak High Speed Ektachrome Film ( Daylight) ASA 160

There was a warning for those using leaf shutters with medium to small lens openings.
The warning was, when the shutter speed is 250th, stop down the lens by 1/2 extra stop.
When 500th, stop down the lens by an extra full stop.

I am not sure why.
09-24-2015, 12:00 PM   #37
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Old School

QuoteOriginally posted by wombat2go Quote
I have a film box sheet dated 3-1972 maybe, for "Kodak High Speed Ektachrome Film ( Daylight) ASA 160

There was a warning for those using leaf shutters with medium to small lens openings.
The warning was, when the shutter speed is 250th, stop down the lens by 1/2 extra stop.
When 500th, stop down the lens by an extra full stop.

I am not sure why.
Reciprocity failure - where the film exposure curve starts to flatten out. The toe and head of the curve. In film the exposure curve is logarithmic where in digital it is for all intents and purposes linear.

https://en.wikipedia.org/wiki/Reciprocity_(photography)

The Elitist - formerly known as PDL
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