While not claiming to be an expert, and I know there are other websites devoted to a complete description of shutters used in folders, the following is what I have discovered and experienced working on folders. The shutters that you will encounter on useable folders… meaning 120 film and frame sizes of 6×4.5, 6×6, & 6×9 are as follows.
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Pronto
Usually 4 speeds + B (1/25, 1/50, 1/100, and 1/200)
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Prontor
Same as Pronto except it also has a self-timer
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Prontor-S
Has a modestly complete range of speeds + B (1sec, 1/2, 1/5, 1/10, 1/20, 1/50, 1/100, 1/300 - on 6x9 shutters) and self-timer. Top speed is usually 1/250 or 1/200
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Prontor-SV or SVS
Same as Prontor S but often has a switch for M / X flash synch. Full compliment of speeds +B (1sec, 1/2, 1/5, 1/10, 1/2, 1/10, 1/20, 1/50, 1/100, 1200, 1/300) and self-timer
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Compur
This is usually found on pre 1945 folders and is the precursor of the Compur-Rapid, which of course preceded the Synchro-Compur. Has a full complement of speeds +B, (1sec, 1/2, 1/5, 1/10, 1/20, 1/50, 1100, 1/250)
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Compur-Rapid
Really the same shutter as the Compur, with the addition of the 1/400 speed... thus the term "Rapid!" And self-timer.
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Synchro-Compur
A continuation of the Compur line. Aside from some minor internal changes, not all for the better, it is the same as the Compur-Rapid. Full compliment of speeds +B (1sec, 1/2, 1/4, 1/8,1/15,1/30, 1/60, 1/125, 1/250, 1/500) Aside from using the "modern" shutter speeds, which vary from the "old style" by such a minute amount it is not worth the effort to discuss, the main difference from the Compur and Compur-Rapid is an M / X switch. Why is the Synchro-Compur thus named? Its for the M/X switch that ironically is no longer needed... EXCEPT for the infinitesimally few who still shoot with bare bulb flash guns, the M setting.
People looking for a folder often ask me for a Synchro-Compur shutter for their folder. I think this is because over the years the Synchro-Compur has earned the reputation of having been fitted to some rather prestigious cameras. I think it is also in part that the name “Synchro-Compur” just sounds so “high tech,” so sophisticated! Which is actually quite true, compared to the Prontors, it is much more complicated. And therein “lies the rub” as a 16th century English playwright once mused. A good Synchro-Compur is a beautiful shutter works great! BUT, my experience in CLAing over 300 folders during the past 5 years has not been very supportive of the superiority of the Synchro-Compur shutter. My experience has been that I have had to disassemble / repair far more Synchro-Compurs than Prontors by a 8:1 margin!!!!!!!!!!!!!!! In general then, based on my experience, is the obvious conclusion that the Prontors are more reliable than Synchro-Compurs, AND, if a Prontor needs service it does not a complete stripdown as a Synchro-Compur does. In general, the Compur is the most reliable of the trio and the Compur-Rapid is second.
One last point about the Compur-Rapid and the Synchro-Compur. Customers often write me after receiving their folder that they have difficulty moving the shutter speed ring to the 1/400 or 1/500 setting…it just won’t go they say. Well, both shutters achieve the hypothetical highest speed with the addition of a booster spring. When you set the 1/400 or 1/500 speed, the shutter speed dial bears against this spring. Think of it as drawing a bow. You are literally pushing that spring into position that will propel the shutter blades to 1/500 of a sec. Now, having said that, let me also add that most Compur-Rapids and Synchro-Compurs even on a “good day” or factory fresh, never reach 1/500 of a second…. maybe 1/350 or so (that’s Prontor territory!), but no way have I ever electronically been able to see one of these shutters hit the 1/400 or 1/500 speed.
Care of shutters
Just follow these simple rules from a renowned shutter guru:
“Shutters benefit from being fired every few weeks. In storage, shutters should be left uncocked, and on one second speed. It is best not to leave the lenses in areas of high heat, which can cause lubricants to run. In use, shutters work best in near room temperatures. During cold weather, try to keep your equipment warm. It is best not to change speeds AFTER cocking, as a general practice, on mechanical cameras. Remember to test your equipment all functions before you plan to use it. Be diligent about keeping your equipment away from dust, sand, water, and oils.”