M4-P: Need help diagnosing a light leak

[EarlJam]

My M4-P has developed an odd light leak.

- It's intermittent, sometimes occurring on adjacent frames, sometimes not at all. For example, the first time it appeared, the leak affected three adjacent frames, 7 8 9, then nothing else on that roll. On the next roll I shot, frame 2 only. The next two rolls, no leaks. Next two rolls, on about 50% of the shots.

- When it does occur, it's always in the same location, but to varying degrees; sometimes faint, sometimes affecting much of the frame along a vertical axis.

- The leak in the image area is always accompanied by a distinctive pattern in the film rebate. Based on location, it appears that the leak is in the upper right of the film aperture, viewed from the back. As in the attached photos, I marked a strip of film and laid it in the camera as it would normally be exposed. The pattern in the rebate always occurs between perfs 1 and 2, counted from the right side, when the film is oriented as exposed.

I've run the common tests for this type of leak - covering the small RF window (tests the light baffle below the window); covering the back and hinge areas; and taping off all of the top, then selectively uncovering sections - but the leak occurs regardless. The shutter curtains appear OK, at least to my untrained eye. That the leak affects the rebate are seems to imply that the leak is coming from the back of the camera but, as noted, taping off the back didn't resolve the leak.

Any thoughts on other places to look, tests to run, or anyone with prior, similar experience who can comment on possible causes, are greatly appreciated. Thanks, all.

 

[maddoc]

Which lens did you use? It might be internal flare in the lens. I had this happen using a 7-element 35mm Summicron lens and it looked identical to what you have observed.

 

[Huss]

Which lens did you use? It might be internal flare in the lens. I had this happen using a 7-element 35mm Summicron lens and it looked identical to what you have observed.

It is cutting across the sprocket line, not just the image area.
To me that rules out the lens or shutter.

 

[Dan]

Are you bulk loading your film?

 

[EarlJam]

Thanks, all.

Re lens, I primarily use a 50mm Summicron, alternately a 35mm Summi. I haven't had a flare issue with my M6, so hadn't considered that it might be related to the lens.

Re bulk loading, no, I use a variety of commercially-loaded film types. I have found that Foma cassettes tend to have a bit of edge leak on the first few frames, but that's balanced by the rolls being a few inches longer than those from Kodak and Ilford.

 

[monopix]

Just bear in mind that the leak that is adjacent to frame 1 (for instance) may not have occurred when frame 1 was at the shutter. The leak may be further on past the shutter.

My test for leaks is to load a new film. Wind on to frame 1 then a couple more, not exposing the film. Then expose one frame but under expose it so any leaks within the frame show up. Then, without winding on, expose the camera to strong light all round. I fire a big flash at it from all angles at close range but you could leave it in the daylight for a day. Then wind on a few more frames without exposing, then, if you want, use up the rest of the film. Once processed, assuming the leak shows up, you'll know exactly where it is in relation to the shutter because you can use the single exposed frame as a reference.

 

[EarlJam]

Just bear in mind that the leak that is adjacent to frame 1 (for instance) may not have occurred when frame 1 was at the shutter. The leak may be further on past the shutter.

My test for leaks is to load a new film. Wind on to frame 1 then a couple more, not exposing the film. Then expose one frame but under expose it so any leaks within the frame show up. Then, without winding on, expose the camera to strong light all round. I fire a big flash at it from all angles at close range but you could leave it in the daylight for a day. Then wind on a few more frames without exposing, then, if you want, use up the rest of the film. Once processed, assuming the leak shows up, you'll know exactly where it is in relation to the shutter because you can use the single exposed frame as a reference.

Thanks, that's an interesting process. I'll thaw a roll of film and try it when the weather improves, later this week.

 

[Bill Blackwell]

My M4-P has developed an odd light leak.

- It's intermittent, sometimes occurring on adjacent frames, sometimes not at all. For example, the first time it appeared, the leak affected three adjacent frames, 7 8 9, then nothing else on that roll. ...

This information alone informs me that the light leaks are most likely occurring while changing lenses in bright light. There is no way I am aware of where light can get through from the back door in this manner. If the light is bright enough is can reach the sprocket holes from the lens chamber side.

 

[farlymac]

One thought I had was an eyelet for the neck strap, but that would move across the frame as the film built up on the take-up spool, and would also affect other frames in the wind sometimes causing two light strikes as it shines through the film. But it really looks like a back door issue since it happens in the same place. Both edges of the film are getting struck, plus there is the arc. Looking at mine, I would say it happens at the left side of the door as viewed from the back of the camera, so the film is getting struck before it enters the frame gate. You get varying degrees of strike depending on how much time lapses between windings, plus where and how much pressure you exert on the back of the camera when you grip it.



PF

 

[EarlJam]

One thought I had was an eyelet for the neck strap, but that would move across the frame as the film built up on the take-up spool, and would also affect other frames in the wind sometimes causing two light strikes as it shines through the film. But it really looks like a back door issue since it happens in the same place. Both edges of the film are getting struck, plus there is the arc. Looking at mine, I would say it happens at the left side of the door as viewed from the back of the camera, so the film is getting struck before it enters the frame gate. You get varying degrees of strike depending on how much time lapses between windings, plus where and how much pressure you exert on the back of the camera when you grip it. PF

I hadn't considered that, but laid a bit of film in the chamber and the leak area also lines up on the left side, just past the cassette. I'll run another test with the back taped off, after the test process "monopix" suggested.

 

[CarrotRoom]

I agree this looks likely to be a leak into the film chamber itself rather than through the lens or shutter due to it affecting off frame areas.
Take a roll and load it into the camera (ideally in a dark area). Take a bright light source such as a torch (US: Flashlight) and systematically shine it through specific points on the camera in the areas identified as suspect; rolling the film on two frames between each test point. Log each frame (or more specifically off-frame) against where the light was directed. Hopefully you would have a roll where one or two off-frames would have fogging and the rest not and would know exactly where the leak was, based upon the log.

 

[Bill Blackwell]

One thought I had was an eyelet for the neck strap, but that would move across the frame as the film built up on the take-up spool, and would also affect other frames in the wind sometimes causing two light strikes as it shines through the film. ...

That's a very good theory! Everything lines up.

 

[EarlJam]

This information alone informs me that the light leaks are most likely occurring while changing lenses in bright light. There is no way I am aware of where light can get through from the back door in this manner. If the light is bright enough is can reach the sprocket holes from the lens chamber side.

Thanks, Bill. I had read about this possibility when I started researching possible causes, so went with a "minimize the variables" approach for testing: one lens, 50mm only; remove the grip I normally use with the camera; etc.

Film-wise, I've been primarily shooting ISO 400 rated at 250, with one roll of ISO 100 @ 60 in an effort to test slower shutter speeds.

 

[Bill Blackwell]

Thanks, Bill. I had read about this possibility when I started researching possible causes, so went with a "minimize the variables" approach for testing: one lens, 50mm only; remove the grip I normally use with the camera; etc. ...

As farlymac theorized/suggested, you have a loose strap lug. It would be on the take-up side. The light streak lines up too perfectly.

 

[Bill Clark]

I guess it depends.

If it were my camera it would go to DAG.

I do own an M4-P film camera and it works just fine. My bad, a few years ago I dropped it while photographing a wedding, picked it up and it didn’t skip a beat!

I had Youxin rebuild a Barnack and it works like a new camera.

Just my thoughts.

 

[farlymac]

As farlymac theorized/suggested, you have a loose strap lug. It would be on the take-up side. The light streak lines up too perfectly.

Bill, I was discounting the possibility of a loose strap lug.

PF

 

[Bill Blackwell]

Bill, I was discounting the possibility of a loose strap lug. ...

Why? It's the perfect explanation.

 

[Rob-F]

Bill, I was discounting the possibility of a loose strap lug.

PF

Why? It's the perfect explanation.

Sounds like it's time to give serious consideration to the possibility of a leak around the strap lug. Maybe you could try firing a flash, or directing the beam of a flashlight, around the strap lug at various angles.

 

[EarlJam]

Sounds like it's time to give serious consideration to the possibility of a leak around the strap lug. Maybe you could try firing a flash, or directing the beam of a flashlight, around the strap lug at various angles.

I have a roll of Tri-X in the camera now, running the test protocol suggested by monopix, and the back test described by farlymac.

I've checked the strap lugs and they appear tight, in the sense that neither budges at all when I use the strap rings to apply a bit of torque. I'll add a sequence of shots wherein the lugs are covered with tape and see what happens.