# Shutter curtain tension - measured as "CMP"

Discussion in 'Classic Manual Cameras' started by andyfalsetta, Jun 15, 2018.

1. ### andyfalsetta

I am attempting to resurrect a Pentacon Six TL and the documentation indicates the shutter curtains have an initial spring tension measured in "CMP" For example, the 1st curtain value is 110 cmp. I can use some guidance on what "CMP" actually is so if there is an SAE equivalent I stand a better chance of measuring it with the tools I have or can hunt for. Do any forum members have insight on what this abbreviation stands for? I've searched the web and found nothing relevant. Any help would be much appreciated.

2. ### Julio Fernandez

I have never seen the abbreviation CMP for force units. Could you tell us more about the context, origin of the manual, or rmaybe reproduce part of it here? Is the manual using other units of length, for instance? (this could help in tracking your CMP unit).

If it helps, Maizenberg mentions in his well-known book a tension of 160 grams-force for a curtain in a Leica-type Russian RF camera. Maybe your tension is just 110 grams? In a German manual grams-force would be a very likely unit for tension.

110 g would be about 1 Newton in the international system of units (i.e. metric).

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3. ### winfried_buechsenschuetz|1

Actually I never stumbled across the unit designator "cmp" although I was raised in a metric country where pond was used as a physical unit. But I can imagine that "cmp" stands for "centimeters x pond". Usually the designator for torque units had the unit of power first and then the designator for distance from center, like kpm (kilopond * meters). Maybe the usage in the 1950s and 1960s was different. You can find conversion tables for torque unit on Einheitenumrechnung für Drehmoment - Lorenz Messtechnik GmbH . I entered the value 160 in the "pond zentimeter" field and the result was 0,14 inch-pound. Hope this helps. PS. On Convert units - Torque (τ, M - Moment of Force, Moment) ... CALCULAND I found the unit "centi-pond * centimetre" (cpm), this conversion table yields the same result.

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4. ### rodeo_joe|1

When I last setup a P6 shutter, I just did it by trial and error. The first curtain tension was slacked off, and then adjusted upwards until the curtain snapped across smartly in about 12ms. The second curtain tension was then adjusted until the slit-width remained constant(ish) at the faster speeds. The only instrument needed was a light sensor at the focal plane and a pulse-width meter.

You have to bear in mind that the P6 isn't a precision made camera, and that aiming for perfection is pointless. There'll probably be some wear in the blind roller bearings that'll need a clean and lube before attempting to adjust the shutter too. And good luck with setting the slow speeds. The only adjustment is the amount of mesh of the slow timing train with the main shutter regulator. That's all held in place with a single screw head!

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5. ### Julio Fernandez

As Winfired wrote above, centimeter-pond would be a very likely interpretation of CMP. It is a unit of torque, not of tension (force). It would be logical to use torque units measured on a screw or nut head turned with a torque-controlled instrument.

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6. ### Gus Lazzari

The common standard for camera manufactures and therefore the majority of service manuals, all have the Curtain Travel Time (CTT or Run Time) specifications listed.
This is measured in milliseconds for each individual shutter curtain. This spec is important not only for performance reasons (accuracy of Slow & Higher speeds), but also for longevity of the curtain & ribbon materials.

I find that many folks who have attempted to remedy a camera that is experiencing flawed or malfunctioning shutter speeds, CRANK up the tensions so as to power through congealed/aged lubricants. This addition tension causes shutter curtains and ribbons to become separated from their hold ribs or detached from the spinning roll-up drums. Add to that brake malfunctions (Bouncing, light-leaks etc.).

Finally, in order to precisely measure this critical CTT measurement, a THREE sensor Shutter Speed Tester is a required piece of equipment.
Sensor #1 is the Beginning, #2 the Center and #3 the End of the film plane. The #1 & #3 sensors are used to calculate the CTT. - See the seventeenth entry in this Pnet thread - A 3 sensor tester <<< click

Last edited: Jun 17, 2018
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7. ### andyfalsetta

Thanks all. You have given me some needed homework. I fully understand that its pointless or foolish to try and set up curtains or timing with a dirty device. I have cleaned and lubed this body at this point and it should be ready for some adjustments. Thanks again.

8. ### andyfalsetta

Update. I have put aside the concept of measuring the initial starting tension for the curtains because this is just the starting point and cranking in much more tension on both curtains is needed to achieve the initial shutter speed required to then adjust all the other speeds (starting point is 1/1000th according to the manual). In order to prevent capping, the shutter curtains should move at the same speed (right?). I have a shutter tester that measures the speeds in milliseconds. My question is how much deviation from one curtain to the other would be acceptable if I can't get them both to be exactly identical at the combination of tensions that enables 1/1000th of a second? For example is a one millisecond speed differential acceptable? Is half a millisecond deviation acceptable or do I need to get much closer?

9. ### rodeo_joe|1

Use of a 3 sensor tester can be circumvented to an extent by using the flash synch socket. At least for the 1st blind travel. Positioning a single light sensor at the beginning of travel and using the X-synch as a signal for end-of-travel gives you the transit time of the curtain. Once the 1st curtain travel is set up, then checking the slit width (effective exposure time) across the frame automatically gives you a similar or identical timing for the trailing curtain.

If the camera has FP synch, then that can be used to signal the start of 1st curtain travel. (A dual trace oscilloscope allows flash-synch and optical signals to be synchronously timed.)

Generally, the trailing curtain tension and freedom of movement needs to be checked at slow speeds as well as high, since there can be a drag effect from the 1st curtain at high speeds; i.e. the first curtain tends to pull the 2nd curtain behind it.

"My question is how much deviation from one curtain to the other would be acceptable if I can't get them both to be exactly identical at the combination of tensions that enables 1/1000th of a second? For example is a one millisecond speed differential acceptable? Is half amillisecond deviation acceptable or do I need to getmuch closer?"

- Well, since 1/1000th S speed is 1 millisecond, measuring only to the nearest millisecond is pretty useless. You need to measure to at least 0.1 millisecond accuracy. Having said that, a variation across the frame of one or two milliseconds at 1/30th or slower is a negligable error.

Last edited: Jul 7, 2018
10. ### andyfalsetta

Thanks Rodeo_Joe; I have a lot to learn. I read Gus's comments on his Contax IIa vs the Leica and the speed difference he observes. It sounds like the best I can do is get as close to the ideal as possible because I am not so confident the Pentacon TL is any more sophisticated than the Contax set up he used in his example. The tester I am using does measure down to thousandths of a millisecond. If I am setting up for 1/1000th of a second am I then looking for a delta between the two curtain speeds of .1 or less (with the 1st curtain traveling fastest)? Is that what you are saying? Or am I chasing something unattainable? Lots of questions and not much experience.

Last edited: Jul 7, 2018
11. ### rodeo_joe|1

I seem to remember I got something close to 1.1 mS at the start of travel and 0.9 mS at the end with the P6. Most blinds will tend to speed up as they cross the frame - acceleration under spring tension is the natural order of things.

I thought I'd done quite well to get only a 20% variation at the top speed. At slower speeds a 10% variation is what I'd settle for. The P6 has a big old shutter curtain to move; it's never going to be perfect.

As I said first off; it's adjustment of the slow speeds (>1/15th) that'll likely give you the most grief. The whole shutter assembly tends to get shifted about as you re-build the camera, and the speeds you get testing the camera in 'skeletal' form aren't the ones you get when all the casing is in place. Aim for 'good enough' with this camera, otherwise you'll just drive yourself nuts trying to get it perfect.

12. ### andyfalsetta

I'll let you know how I make out. Totally agree on how much things can change in and out of the camera body. Only two flimsy screws each in sloppy slots holding the shutter and the timing plates to the mirror housing. I have only been testing with the entire shutter/mirror/timer in the body for this reason.

13. ### Julio Fernandez

Just a comment on this, maybe I have it wrong and let the experts correct me.

I thought that both curtains travel for more than 1 millisecond. Exposure is determined by the gap between curtains at higher speeds (above 1/60), and CTT is about 1/60 or 1/100 second at high speeds, i.e. 10 to 17 ms. At speeds above 1/60, what varies is the gap between curtains. If CTT were different for both curtains, the gap between curtains is not constant but varies over the frame, possibly leading to uneven exposure. I also understand that you need more than 10% uneven exposure to be noticed in typical film.

If this concept is right, then timing of CTT could be measured with 1 ms precision and still be quite OK for the purpose.

Please correct me if I am wrong.

14. ### rodeo_joe|1

Yes, you could get away with an accuracy of 1mS for the transit time, but you need greater accuracy to time the slit-width/effective exposure time.

Also, a higher accuracy gives you better guidance of the consistency of the shutter, and of how your adjustment of tension is affecting the shutter.

A stuttering, or partly sticky shutter indicates a need for cleaning/lubrication (or maybe a magnetised spring), and if you can only measure to 1mS, then such stuttering might go unnoticed.

You can only measure the transit time of the blinds individually at the T or B setting. The fact that both blinds have the same transit time doesn't necessarily mean that the slit-width will stay constant across the frame, or that the marked shutter speeds will be accurate - you need 0.1mS or higher measurement accuracy for that.

Consider the case where the first curtain is over-tensioned, but drags toward the end of travel and measures 30 mS from start to finish. Then you measure the 2nd curtain, which is correctly tensioned, but speeds up linearly across the frame, and yet still transits at 30 mS. Will the slit-width and exposure remain constant? No, certainly not.

Last edited: Jul 8, 2018
15. ### Julio Fernandez

OK, I understand. It is not only transit time, but constant speed of movement (or equal speeds in both curtains) what determines results. Thanks for the explanation.

16. ### andyfalsetta

Well the Pentacon Six is off the bench. Thanks to Rodeo_Joe, Gus, and Winfried for your posts and replies; they were very helpful. Thanks too to Rick Oleson who sent me his P6 notes. Although I had a pdf of a partial service manual, the parts nomenclature was a real challenge for me. Rick's notes clarified this greatly and neutralized what might have been translation errors when the pdf had been created.

Last edited: Jul 22, 2018
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17. ### rodeo_joe|1

I quite miss the sharp and nicely-toned shots I used to get with the Jena 80mm Biotar. Not so impressed with the bendy straight lines of the Arsat.

Anyway, congratulations and well done on resurrecting that P6!

That's one down.... and how many to go?

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18. ### rick_drawbridge

Congratulations, Andy, you deserve an award for your motivation and perseverance, and I hope that comes in the shape of some fine photographs. It looks as if you're off to a good start; you'll find a use for the Arsat but you just have to be selective with your subject matter. I have a 35mm lens for the Pentax 67 kit and it took me ages before I felt at home with it.

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