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Steve_s
Tinkerer
Username: Steve_s
Post Number: 163
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, July 15, 2009 - 10:33 am: | 
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I've been changing curtains in my Pentacon F. Now I come to tensioning them I'm having problems achieving a setting that will give acceptable results tapering-wise at the intermediate speeds (1/50, 1/100, 1/200) as well as at 1/1000 and 1/500, however it seems as though the higher I set the tensions, the less the differential between them becomes.
When I let down the original tensions there were just over 3 turns on the 1st curtain and just under 3½ turns on the 2nd, and I'm always reluctant to go much above original settings. Does anyone know what would be a safe maximum for these cameras? |
Rick_oleson
Tinkerer
Username: Rick_oleson
Post Number: 945
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, July 15, 2009 - 03:51 pm: |
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Do you think the 3 turns might have been when the shutter was already wound? I start with about 7 turns and adjust from there. If you get the tapering to go away at 1/1000, I can't imagine how you can see it come back at a slower speed.
Curtain replacement/tension notes: http://rick_oleson.tripod.com/index-87.html
shutter test notes: http://rick_oleson.tripod.com/index-135.html |
Steve_s
Tinkerer
Username: Steve_s
Post Number: 164
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, July 16, 2009 - 12:00 am: |
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The 3 turns was with the shutter un-cocked. I'm confused by your figures, though. The spring spindles turn just over 2 turns when the shutter is cocked, so if the 3 turns had been with the shutter cocked, this would suggest an un-cocked tension of around one turn. 7 turns seems like a very high setting compared to my originals. As far as I could see, the lacquer on the screws hadn't been broken. You've used this tension without any problems?
The tapering problem is one I seem to have whenever I attempt a cloth focal-plane shutter. The tapering at the faster speeds is in the opposite direction to that at the slower speeds. Basically, the curtain speeds vary depending how much of the frame the 2 curtains travel together. I assume this is caused by interaction between the curtains due to friction where they run together on the shutter spindle. So, in this Pentacon for example, if I adjust the tensions for even exposure at 1/1000 where the curtains are released simultaneously, at 1/100, where the 1st curtain is about half way across before the 2nd is released, the first curtain is slower, and even though the exposure is correct at the start of frame, the gap will be closing across the frame.
The difference between the tapering at the fast speeds and the slower speeds becomes less as the tensions increase, presumably because the effect of any friction becomes proportionately less.
I have your CD notes. The tip on removing the gate was alone worth the price of the CD! |
Rick_oleson
Tinkerer
Username: Rick_oleson
Post Number: 946
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, July 16, 2009 - 07:33 pm: |
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3 turns from slack with the shutter uncocked shouldn't even open the curtain. My procedure is to wind until the curtain fully travels, and then just add enough tension to make sure it will run consistently... basically the lowest amount of tension that will operate the shutter. That, in the case of the Pentacon, has been about 7 turns total.
The curtains and their springs don't know what speed the shutter is set at... the speed setting only controls when each curtain starts, the velocity of each curtain is solely a function of its spring tension. That tension does not change from one speed setting to another. The reason for checking for fading at 1/1000 is first because it's most critical at that speed and second because it's easiest to see at that speed so you can adjust it precisely. A shutter that fades 50% at 1/1000 has a travel-time difference of .0005 seconds between one curtain and the other. The difference between .0005 and .0010 seconds is quite visible on film and on a TV monitor. At 1/100 second, an error of .0005 is only 5% of the exposure time (.0100 vs .0095 seconds). This amount of error would be very difficult to detect on a shutter tester and impossible to detect on film.
The curtains should not be touching each other at any time during their travel, so friction between them should not become a factor. They touch, of course, as the second curtain seats into the first as they cap at the end of travel.
I've recurtained quite a few Contax S's and Pentacons, and I have never experienced what you're describing. |
Glenn
Tinkerer
Username: Glenn
Post Number: 628
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, July 17, 2009 - 04:29 am: |
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I agree with all of Rick's comments - my experience with the Pentacon shutter also shows that around 7 turns gives very good run consistency. |
Steve_s
Tinkerer
Username: Steve_s
Post Number: 165
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, July 17, 2009 - 08:21 am: |
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Running together, the curtains will actually run 100% reliably with no tension at all at the uncocked position. The 2 turns it takes to cock them gives more than enough momentum to close them fully. Running separately, it takes about 1 turn of tension on the 1st (resistance near the end of travel - flash contact?), and ½ turn on the 2nd.
The only interaction between the curtains is where the 2nd curtain drum and the 1st curtain pulleys run at the main shutter spindle (and where the 1st curtain runs over its roller on the 2nd curtain spring-spindle), and there isn't much friction, but the curtain speeds definitely are affected by whether the curtains are running together or whether one or the other is stationary.
Anyway, I'm outnumbered as usual, so I guess I'll just have to go for the higher tensions! As I said earlier, higher tensions would probably mask any friction effect. First though I'll dig out my Pentacon FM (also needing curtains) and open that up to see what the tensions are like. I think fear of spring fatigue is probably my major problem when it comes to focal-plane shutters.
I'll post any result here, but it won't be in the immediate future. I've been working on this one far too long, and meanwhile have acquired some other (hopefully, much easier!) projects that I really need to look at.
Many thanks for all your help, Rick and Glenn. |
Rick_oleson
Tinkerer
Username: Rick_oleson
Post Number: 947
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, July 17, 2009 - 06:26 pm: |
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Okay, whatever. |
Steve_s
Tinkerer
Username: Steve_s
Post Number: 169
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, November 28, 2009 - 08:44 am: |
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About time I finished off this thread! I've had next to no time available the last 3 or 4 months for camera stuff, but have finally managed to find a few hours.
With the tensions set for a roughly correct result at 1/1000 sec I checked the curtain speeds at 2 mm intervals across the frame at 1/1000 sec and at 1/50 sec. It was clear from this that at the slower speed the 1st curtain was accelerating more slowly up to the point where the 2nd curtain ran, then accelerating at the same rate as at 1/1000, but due to the slower start never getting close to the same speed as the 2nd; i.e. there is interaction between the curtains causing the stationary 2nd curtain to act as a brake on the 1st.
There doesn't appear to be any significant contact between the curtains/ribbons/cords. The only other way I can see that the curtains can interact is at the main shutter spindle, where the 2nd curtain drum runs on the same spindle as the 1st curtain pulleys, and to a lesser extent where the 1st curtain runs over a roller on the 2nd curtain spring-spindle.
I flush-cleaned both areas thoroughly and lubricated the main shutter spindle with a synthetic watch oil. There seems to be some improvement, though it's still very far from perfect. I'm running at curtain tensions of 2½ turns on the 1st curtain and 3½ turns on the 2nd, and results are now quite usable from 1/500 sec down. There is about .8 of a stop tapering at 1/1000, but bearing in mind that I've never used 1/1000 on any camera except as a test just to see if it works, I guess I'll have to live with that!
For the record, I checked the curtain tensions on my Pentacon FM, which still had its blue lacquer on the lock-screws and had apparently never been touched. 1st curtain tension was 2.6 turns and 2nd curtain 3.1 turns. I let the tensions down very carefully and would guarantee the figures accurate to within plus or minus 1/10 of a turn. |
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