Exposure variations across the film plane
Old 1 Week Ago   #1
jcb4718
Registered User
 
jcb4718 is offline
Join Date: Jan 2008
Posts: 129
Exposure variations across the film plane

I have been checking the shutter speeds on my Leica IIc. I use a phototransistor for detection but the problem is interrupting a light beam with the shutter if you don't want to remove the casing (I don't). I therefore slipped in a white card and used a He-Ne pointer. The beam diameter is about 2.5mm but I used tape to make it about 1 x 2.5mm, taking care to keep the 'slit' vertical. There is enough light reflected off the white card as the curtains pass to allow the shutter speed to be measured. Interestingly, the shutter speed variation can be measured at any point as the curtains pass. I therefore measured the speed left-centre-right. My IIc was recently serviced and I compared it with my OM4 which, as far as I know, has never been serviced. The results are attached. The centre IIc shutter speeds are accurate but show a consistent variation right to left amounting to about 1 stop at 1/500 sec. Even at 1/100 sec its nearly 1/3 stop. In comparison the centre shutter speeds of my OM4 are very roughly the same as the IIc but there is little variation right to left. I'm inclined to think the right to left variation of my IIc is a feature of the camera and the service technician has made the best compromise by getting the centre setting accurate. Can anyone confirm this? It seems that one curtain must accelerate more than the other one and the curtain slit width varies across the frame. Is this correct? I should say that I have never noticed the effect in an actual picture (about 95% of which are taken with a shutter speed of 1/100 sec)...but I would like to know if this performance is typical.
Attached Images
File Type: jpg Camera shutter speeds.jpg (66.6 KB, 28 views)

Last edited by jcb4718 : 1 Week Ago at 10:44. Reason: typo
  Reply With Quote

Old 1 Week Ago   #2
Chriscrawfordphoto
Real Men Shoot Film.
 
Chriscrawfordphoto's Avatar
 
Chriscrawfordphoto is offline
Join Date: Apr 2007
Location: Fort Wayne, Indiana
Age: 42
Posts: 8,155
The exposure should be the same across the frame. The problem you're having is caused by the two shutter curtains not moving at the same speed across the film gate. The spring tension is adjustable on each shutter curtain individually so that it can be made to give even exposure. Whoever serviced it didn't really service it. Probably never even opened up the body; setting curtain tensions is a basic part of a CLA.
__________________
Christopher Crawford
Fine Art Photography
Fort Wayne, Indiana

My Portfolio

Learn from Me! Free tutorials for film and digital and my tested film developing times.
  Reply With Quote

Old 1 Week Ago   #3
charjohncarter
Registered User
 
charjohncarter's Avatar
 
charjohncarter is offline
Join Date: Dec 2006
Location: Danville, CA, USA
Posts: 7,913
Someone told me that the Leica shutters are accelerating shutters which contribute to this problem. Apparently, there are good things about an accelerating shutter and some good thing about the constant speed shutters. There is an easy way to adjust this with only removing the bottom piece like when you load film. I did it on my IIIf.
  Reply With Quote

Old 1 Week Ago   #4
Chriscrawfordphoto
Real Men Shoot Film.
 
Chriscrawfordphoto's Avatar
 
Chriscrawfordphoto is offline
Join Date: Apr 2007
Location: Fort Wayne, Indiana
Age: 42
Posts: 8,155
Quote:
Originally Posted by charjohncarter View Post
Someone told me that the Leica shutters are accelerating shutters which contribute to this problem. Apparently, there are good things about an accelerating shutter and some good thing about the constant speed shutters. There is an easy way to adjust this with only removing the bottom piece like when you load film. I did it on my IIIf.

I'm certain this isn't true. The curtains in a Leica (and every other 35mm camera) always move at the same speed. The slit width varies to change exposure.
__________________
Christopher Crawford
Fine Art Photography
Fort Wayne, Indiana

My Portfolio

Learn from Me! Free tutorials for film and digital and my tested film developing times.
  Reply With Quote

Old 1 Week Ago   #5
charjohncarter
Registered User
 
charjohncarter's Avatar
 
charjohncarter is offline
Join Date: Dec 2006
Location: Danville, CA, USA
Posts: 7,913
https://books.google.com/books?id=xj...hutter&f=false

Page 108, but it doesn't really matter as it is easy to adjust. You are right that this type of shutter was improved upon.

My shutter only exhibited the OP's problem when using 1/500 and 1/1000. At least I couldn't see it at lower speeds.
  Reply With Quote

Old 1 Week Ago   #6
jcb4718
Registered User
 
jcb4718 is offline
Join Date: Jan 2008
Posts: 129
Ah! Thanks for the insight, guys. Its a difference in curtain speed that matters. If one curtain travels across the film plane (36mm) in 1/30 sec, its speed is 1080 mm/sec. At 1/500 sec shutter speed, the curtain gap should be about 2mm. A 1 stop error at the 'far' end means that the gap has widened by 2mm (to become 4mm). The second curtain travels only 34mm in 1/30 sec so its speed is only 1020 mm/sec. It all makes sense e.g. why the error always increases in the same direction. Also why the error is less at the slower speeds (at 1/30 sec the curtain width is 36mm so a 2mm increase has little effect). Does anyone know where I can get instructions on making the adjustment, particularly by removing only the base plate?
  Reply With Quote

Old 1 Week Ago   #7
Chriscrawfordphoto
Real Men Shoot Film.
 
Chriscrawfordphoto's Avatar
 
Chriscrawfordphoto is offline
Join Date: Apr 2007
Location: Fort Wayne, Indiana
Age: 42
Posts: 8,155
Quote:
Originally Posted by jcb4718 View Post
Does anyone know where I can get instructions on making the adjustment, particularly by removing only the base plate?
The instructions are "Don't." Get the tech who serviced it to fix it, that's what you paid him for. If you don't trust him to do it right, find another tech. You can do serious damage to the mechanism in one of those shutters if you overtighten the springs. Just let someone who is qualified to do it repair the camera.
__________________
Christopher Crawford
Fine Art Photography
Fort Wayne, Indiana

My Portfolio

Learn from Me! Free tutorials for film and digital and my tested film developing times.
  Reply With Quote

Old 1 Week Ago   #8
Sarcophilus Harrisii
Registered User
 
Sarcophilus Harrisii is offline
Join Date: Jun 2009
Posts: 2,217
I'd written a reply discussing the way focal plane curtains have to accelerate across the gate in some detail, but the site discarded it when it forced me to log in (again). So I'll simply say that as the curtains must begin moving from a standstill under the power of their springs, they have to accelerate and this process continues as they traverse the gate.

Maintaining a constant slit width is fine, in theory, if the velocity of the curtains also remains constant.
Same slit width x same curtain velocity = same effective exposure.

Unfortunately because the curtain velocity increases, it's not that simple. Same slit width x increasing curtain velocity = decreasing exposure.

Hence, quality fp shutter designs must have a means of compensating for the curtain acceleration.

Designers know physics dictate that the curtains cannot instantly achieve ideal nominal velocity: mass, inertia etc. make this impossible. What they try to do, then, is to keep the rate of acceleration more or less constant, because this makes compensating for it easier.

Compensation is achieved by setting up a commensurate increase in the slit width the closer the curtains get to the end of the gate. As a result, the product of the increasing curtain velocity and increasing slit width achieves consistent exposure across the gate, to within acceptable tolerances.

From the above it will be understood that, whilst the oft-used description of a typical 35mm fp shutter is that it operates at a constant curtain speed at constant slit width for each time setting, actually, this is a myth, since, in reality, both speed and slit vary unavoidably throughout the course of the exposure if acceptable accuracy is to be achieved.

There are several texts that delve into the principles of camera design but I think it is still hard to improve on J Lipinski's definitive discussion, Miniature and Precision Cameras (Iliffe & Sons Ltd, 1955, 1956). Despite being long out of print, itís highly recommended.

As it was the basis from which many other makes of fabric curtain focal plane shutter evolved, it's not surprising, then, that Lipinski covers the Barnack Leicas in some detail in the course of investigating the problems and solutions involved in designing a reliable, accurate, fp shutter. I quote, in part, from page 164 of my second edition, his comments about the Leica shutter:

ďUniformity of exposure will depend on the two blinds travelling at the same speed. Variation in blind velocity on acceleration is compensated for by the different diameters of the two drums, so that the slot opens up as the velocity increases.Ē

Regarding the thread starterís dilemma of an increase in effective exposure towards the end of the film gate: The problem isn't that the first curtain is getting further away from the second as they cross the gate, because itís actually meant to. The problem is that the rate of the increase of the slit width must obviously be excessive.

There could be a few reasons for this. I'm generalising here, because I prefer the Contax (which has its own foibles, of course) and don't want much to do with Leica cameras, but some are as follows (this is not an exhaustive list).
  • Change in dimensions of curtains or ribbons due to age/wear which can alter the relationship between the timing.
  • Need for servicing to ensure curtains are running at the proper speed and proper rate of acceleration, consistently.
  • Spring fatigue necessitating adjustment of spindle tensions, to restore proper relationship between curtains.
  • Mechanical wear, maladjustment, lack of cleanliness or lubricant deterioration connected to the second curtain latch or associated components, resulting in inappropriate delays in second curtain release.
Personally, having recently paid for the camera to be serviced, if you're unhappy with the accuracy of the shutter I think Chris's suggestion to discuss it with the person who worked on it in the first instance, is a good one.
Regards,
Brett
__________________

  Reply With Quote

Old 1 Week Ago   #9
Pioneer
Registered User
 
Pioneer's Avatar
 
Pioneer is offline
Join Date: Dec 2011
Age: 63
Posts: 2,916
In reference to both Chris and Brett's suggestion of having the service technician check the camera again, I do agree. However, you may end up having to go to another technician as I did.

Many camera techs do a relatively competent job of cleaning and lubricating your camera, but may not be as successful in the adjustment part. The reason may not be that they are not capable but that the camera itself needs more than just the standard CLA as performed in most cases.

Case in point. I have a wonderful 1935 Leica III that I enjoy a lot, but it was not in the best shape when it arrived at my house. Though it had just received a CLA it still was not working properly. After checking it out I sent it back to the tech who did the original CLA and he gave it a second try at his cost. Unfortunately it did not help.

Finally I sent it to another person who actually does a complete overhaul, tearing the camera down, cleaning and replacing parts where needed, and re-assembling and adjusting per manufacturer instructions. The camera now works perfectly. Old cameras, like old cars, do not run forever without attention and must be overhauled at some point in their lives. Your camera, like mine, may have been used far more heavily than many other cameras and may need more attention from an expert.

Unfortunately a complete overhaul is much more expensive and takes much more time than a simple CLA. So you must decide for yourself whether the camera you have is worth the time and money you will invest. For me it was worth it though I would never realize any of that money back if I were to sell the camera. The pleasure of working with a smooth, accurate, 85+ year old Barnack Leica is worth the money.

Of course, your mileage may vary.
__________________
You gotta love a fast lens;

It is almost as good as a fast horse!
Dan
  Reply With Quote

Old 1 Week Ago   #10
jcb4718
Registered User
 
jcb4718 is offline
Join Date: Jan 2008
Posts: 129
OK, guys, many thanks for the insights. It does appear that the performance can be improved upon and I now understand in a general way what the issues are. I will take this further and if I find out anything interesting I'll report back (but don't hold your breath!).
  Reply With Quote
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off



All times are GMT -8. The time now is 22:50.


vBulletin skin developed by: eXtremepixels
Powered by vBulletin® Version 3.6.8
Copyright ©2000 - 2018, Jelsoft Enterprises Ltd.

All content on this site is Copyright Protected and owned by its respective owner. You may link to content on this site but you may not reproduce any of it in whole or part without written consent from its owner.