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Evgenys
Tinkerer
Username: Evgenys
Post Number: 2
Registered: 11-2007
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, November 08, 2012 - 10:08 am: | 
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Hello,
I've built the shutter speed tester, using the instructions I found in the Internet (e.g. http://www.baytan.org/prak/shutter.html)
But when I started use it, the Audacity shows strange results:
1. When I measured the rim-set Compur of my old Rolleicord, the tester shows the slower shutter speeds than original ones, e.g. 1/56 of 1/100, when I used diode flashlight (white), but when I used the laser pen (red), the results were much different: 1/110 of original 1/100. FYI the shutter is about 70 years old and I'm not sure that it was CLA'ed at all.
2. On the contrary, when I measured the speeds of my recently CLA'ed (!) Leica M2, the results with the laser pen were much slower speeds: 1/450 of original 1/1000. But with the flashlight I've got more reasonable speeds, e.g. 1/850 of 1/1000.
I'm surprised, because different "lights" show completely different results not on the same shutter but the opposite results on the leaf and focal plane ones.
Does anybody have any ideas of it?
What light shouls I use? |
Waynemel
Tinkerer
Username: Waynemel
Post Number: 208
Registered: 08-2009
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, November 09, 2012 - 08:51 am: |
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I think it must have to do with the way the two types of shutter operate. The leaf shutter opens and closes and as it is doing so the amount of light allowed to hit the film will increase gradually. Over the total exposure the total amount of light will add up to create the image. So, the actual time that the shutter is open may be longer than expected (someone may want to check this, but I think I'm correct).
With the focal plane shutter, the film is exposed by a travelling slit in the shutter. The exposure is not only affected by the speed that the slit travels, but also by it's width.
So, why you are getting different readings using a point light as opposed to a broad light is still a bit of a mystery to me.
Actually, after typing this I think I am more confused.
But, I have used the same tester and always used a flashlight. My measurements seem reasonable. I would just use a flashlight, as it would provide a more "real world" situation. Unless you are photographing incoming laser beams. |
Waynemel
Tinkerer
Username: Waynemel
Post Number: 209
Registered: 08-2009
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, November 09, 2012 - 02:24 pm: |
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What aperture wer you testing the leaf shutter with? If you were using a small aperture, try re-testing wide open and see if it makes a difference. I seem to recall reading that leaf shutter speeds were calibrated at full aperture. |
Evgenys
Tinkerer
Username: Evgenys
Post Number: 3
Registered: 11-2007
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, November 12, 2012 - 05:43 am: |
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I always use the full possble aperture (e.g. f/3.5 of the Triotar on my Rolleicord). Leica was tested without any lens. |
Mareklew
Tinkerer
Username: Mareklew
Post Number: 277
Registered: 03-2010
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, December 31, 2012 - 02:41 am: |
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A difference in readings between laser and a bulb as the illumination source is pretty much to be expected.
When a leaf shutter gets triggered, it does not open and close instantly, it takes a significant time to move the mechanics. During that time the exposure starts at f:verylargenumber (the moment the leaves crack open) through f:11, f:8... f:3.5, at which moment the delay gears in the shutter kick in and time the exposure time. Afterwards the shutter starts to close, again reducing the effective aperture opening from f:3.5 towards f:11 and finally closing.
When you use a bulb, it is an omnidirectional source of light, so some rays travel through the lens center and some through the lens rim.
As you can easily see, the rays travelling along the axis of the lens will reach the photodetector from the moment the leaves crack open till the moment the opening finally closes. The rays travelling through the rim of the lens will, however, remain cut off until the shutter completely opens and will get cut off again as soon as the shutter starts to close.
When using a bulb as light source you can observe a slope in photodetector reading, a rising and a falling one, proportional to the gradual opening and closing of the shutter. The reading will actually reflect the true film exposure. It is important to integrate the amount of light, as exposure time in case of a leaf shutter is an equivalent value - the time that some light gets past the shutter is actually somewhat longer.
If you used an ideal laser source with a very, very thin beam (which a laser pointer isn't), you could pick which path of light you want to time. The closer to the lens axis you'd get, the longer time you would measure. This readings could be more useful, than a reading with a bulb, if you did a series of them for different light paths, but as a single reading this light source is pretty much useless, especially for short shutter times (at 1 second the shutter opening and closing times are insignificant).
For the slot shutter there's no such effect, but it might be so, that the shutter is not running evenly, it's hard to comment without seeing your exact setup. However, readings for slot shutter taken without lens are going to be misleading in general anyway. In any case I'd expect laser pointer reading to be closer to true in this case, it should be the shorter time none the less (contrary to your observation).
Greets,
Marek |
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