Hi
Curtain velocity is helpful information to have—but it has to be interpreted in conjunction with the actual amount of exposure at the various parts of the gate.
As I think I may have mentioned in a previous comment, some manufacturers, particularly some of the Japanese ones, would actually advise a curtain velocity that should achieve good shutter accuracy when the camera involved was new, in their service manuals. This information isn’t universally provided, (and you should always make a note of the number of turns or clicks etc when de-tensioning the springs for reference, anyway). But if, for instance you’ve just fitted new curtains to a shutter and have to adjust the tension from scratch, if the amount of adjustment the springs previously had, as noted, does not produce good accuracy to start with, then—dialling in the required tension to get each curtain travelling at the recommended velocities, should see the actual exposure times looking fairly close (assuming, of course, that all else is well with the mechanism).
That said—the curtain velocities and relationship between them (timing of their release, which establishes the slit width at the beginning of the gate) together combine to determine the actual exposure on the film. Achieving the correct actual amount of light hitting the film at the various settings is always your ultimate goal, follow? The velocity of each curtain is helpful information but is a means to an end, the end being accurately calibrated shutter speeds.
If you take a look back over my last 50–100 posts on this forum you will find another one that mentions the Leica screwmount shutters, and how the dimensions of their drums are designed to give the first curtain a bit of an edge as both curtains run across the film gate, thus, offsetting the effects of curtain acceleration.
In your shoes, if you do not have an absolute number for the curtain velocity of your camera (and whilst that can be helpful it’s not absolutely critical, usually), I would dial in a couple of turns of initial tension for each curtain, and then start checking the relevant exposure times according to the accepted wisdom for adjusting Barnack shutters. With many cameras this may be the fastest speed Ie 1/1000, however some designs have individual timing adjustments available for various speed ranges and in some instances, the best speed for initially fine tuning the curtain tensions might be Eg 1/60. Whilst the information I’ve relayed is accurate overall, as far as how focal plane shutters function, I prefer Contax rangefinder to Leica and have successfully avoided having to work in Leica cameras to date.
So if the repair manuals dictate a certain procedure for adjusting the shutter accuracy or other experienced repairers have informed comments about how to dial your shutter in, I would defer to their wisdom about the order in which any adjustment(s) ought to me made.Cheers
Brett
sreed2006
Well-known
Thank you, Brett. I have never seen curtain velocity data before, which makes me a newbie to understanding the information (and prone to asking simple questions).
It appears that the curtain velocity data is valuable if one has the tools to make adjustments - and has data as to what the velocities and accelerations should be. I don't have either. So right now, I am just trying to understand what the numbers mean as far as what to expect from the camera's ability to expose properly.
If curtain velocity data are not normally published then I will regard it as information that is nice to know, but not totally needed. It's something that can be useful once I learn more.
It appears that the curtain velocity data is valuable if one has the tools to make adjustments - and has data as to what the velocities and accelerations should be. I don't have either. So right now, I am just trying to understand what the numbers mean as far as what to expect from the camera's ability to expose properly.
If curtain velocity data are not normally published then I will regard it as information that is nice to know, but not totally needed. It's something that can be useful once I learn more.