Tested results about Sinar Behind lens shutter.

[Harvey King]

Hi, Guys:

I have a brand new Sinar Behind lens shutter but I recently tested it by using a stopwatch, I found it always took about 80% of the marked time for the exposure. For example, if labeled for 8 second, the actual exposure time, on average for double click sound is 6.63 seconds. So are 1/2s,1s, 2s, 3s, 4s, 6s. Can someone explain to me if my shutter is not accurate and what can I do with it? How can I correct this problem? Thank you guys very much.

Harvey

[Emmanuel BIGLER]

Hi
OK, so 6.6 seconds instead of 8 means that your shutter is slightly faster by 20%
Not kidding, that is not too bad ;-), mechanical shutters when ageing tend to be on the slow side.
Mechanically timing a 8 second delay is a real challenge, the Sinar (Copal) shutter does it quite well actually.
So, your shutter is wrong by 20% which is the actual limit of the acceptable tolerance for a mechanical shutter, but surprisingly this error is not a real issue for photographic use.
20% means 1/4 f-stop in the exposure, you won't probably notice anything on your images.
Of course, 10% would be more satisfying, 10% means less than 1/6 f-stop.
If you show the shutter to a competent repair technician, I'm not sure that he will actually suggest you to have it repaired.

You should try and make a decision afterwards. The real issue for long timing is about reciprocity failure of films.

For example, if you look at this chart published by Ralph W. Lambrecht for B&W films,

http://www.darkroomagic.com/Library/Reciprocity.pdf

you'll see that for Kodak T-max, a theoretical 2 sec. exposure requires actually 3 seconds (+1/2 f-stop), and that a 8 sec exposure should actually be extended to 12 seconds ; conventional films instead of 8 seconds would require 19 seconds.

So, do not worry about the calibration of your shutter, even it is off by 20%, compensation of reciprocity failure usually demands a much bigger adjustment.

[Capocheny]

Harvey,

How did you trigger the stop-watch and shutter? The discrepancy may be due, in part, to a lag in response/reaction time. Was the stop-watch electronic or mechanical? A measure of 6.63 seconds versus 8 seconds...

But, I agree with Emmanuel in that I certainly wouldn't worry about this small difference. Besides, depending on what you're shooting... the film could well have the latitude, for the most part, to compensate for the inaccuracy in the shutter.

Lastly, if you're doing tests to ascertain the proper ISO/ASA rating for your film and equipment... the 20% difference will be adequately compensated for.

So, I certainly wouldn't worry about the slight discrepancy. :)

Cheers

[Paul Ewins]

Harvey,
rather than spend money getting a tech to "repair" the shutter you would be much better off just getting them to work out the actual times you are getting for all speeds. If these are consistent then you can make a little table somewhere and adjust accordingly. I've seen quite a few conventional shutters that have a piece of paper stuck to them showing the actual speeds at each setting.

[Rob Vinnedge]

Paul is right. It is no big deal to set the f-stop at 1/6, 1/8, or the more common 1/3 which is already marked on the Copal shutter. I bought an inexpensive, but accurate, Calumet shutter tester and periodically check my ten copal shutters. All of them, new or old, have some speeds which are slightly off. I just make a list for each lens and tweak the appropriate f-stop setting a little bit to compensate for the "inaccuracy".