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08-22-2015, 10:57 AM   #16
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The actual shutter speed of an image capture determined by the camera's onboard computer is available in the exif data.

08-22-2015, 02:03 PM   #17
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QuoteOriginally posted by DragonLord Quote

In my experience, accuracy of very high shutter speeds is subject to sample variation, and can degrade with use. My first K-3 II (returned due to service advisory) slightly overexposed (by about 1/2 stop) at 1/8000s, but I don't see any difference between 1/2000s and 1/8000s on my current body. My K-5 would overexpose by about 2/3 of a stop at 1/8000s as well.
A repair guy (for all brands) said he'd seen many well-used cameras where the highest shutter speeds weren't right.
08-23-2015, 07:59 PM   #18
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On the medium format cameras I use a photodiode in the photoconductive ( reverse bias) mode to measure.
This mode is faster responding than the "pixel " mode which is photovoltaic.
The photodiode , I expect, is responding at sub microsecond rates so it is more than 1000 times faster than the shutters.

The photo diode measures photons back scattering off a ground glass plate located on the focal plane, via a 1.2mm dia by 10mm deep hole.
I just checked a lens that I have never used, as I am taking it on a trip.

It is the wide angle Mamiya Sekor C 1:4.5 50mm.

The old lens is within about 2 millisec of specification all the way from 1 second to 1/250 second. ..Good!

At 1/400th ( should be 2500 microsecond) I measured at f/4.5 and then at f/32.

At f/4.5, the rise and fall times (penumbra) to full intensity were both approximately 1000 microsecond
The average on time, (half way up the penumbras) was 3500 microsecond.

At f/32, the intensity is obviously much lower, but the rise and fall times are faster.
I measured the rise time at 400 microsecond and the fall time at 200 microsecond.
The average on time ( half way up the penumbras), was about 4600 microsec.

At the peak intensity, the on time was about 3800 microsecond.

So the error is worse at f/32 compared to that at f/4.5. I did not check at f/11,... f/16 where the lens will be mostly used, but expect it will be about half a stop over exposed

I should put on my list to make a permanent jig for this, so I can check all the shutters here more efficiently.
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08-23-2015, 08:20 PM   #19
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QuoteOriginally posted by wombat2go Quote
So the error is worse at f/32 compared to that at f/4.5.
this is to be expected, as the blades are near the end of their travel.

08-23-2015, 08:58 PM   #20
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QuoteOriginally posted by Digitalis Quote
this is to be expected, as the blades are near the end of their travel.
Yes,
I have not tried to measure variation of a focal plane shutter with f/- yet
However I have messed around a bit with the Speed Graphic focal plane shutter ( up to 1/1000th) using this method.

I question whether my junction at the end of a long black tube pressed against the ground glass accurately measures the equivalent exposure time of film on the focal plane at various f/- and even focal lengths.
Probably not. A curtain shutter is a 3_D+T+motion problem I think with different exposure horizontally compared to vertically, whereas a leaf shutter might approximately be a radial+T problem up to a certain limit.

Also penumbra effects might depend on the illumination level that film/sensor starts to irreversibly record the information.
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