Focal plane shutter speed adjustment?

[bjel]

Hi all,
While this topic has been addressed many times in the forums, I have not found this particular variation on the question, so I appeal to the collective wisdom.
I have a Canon vi-t and my DIY shutter tester revealed a large variation in shutter speed across the frame. Viewed from the back of the camera the curtains travel right to left and I was getting underexposure on the left side of the frame and also a very non-linear shutter speed progression.

I lack the confidence to do a complete stripdown of the camera so I lubricated the accessible bearings and gears and saw no marked improvement. Given that this is an older camera it seems to me unlikely that the 2nd curtain is running too fast, and more likely that the 1st curtain is running too slow. I tweaked the tension on the 1st curtain and got a great improvement in the exposure in the centre and left side of the frame. The right side is still overexposing, by more than half a stop at the highest speed. (see attached graphs) The data does not include speeds below 1/60 as I am concerned with curtain speed, not the slow speed escapement.

So I wonder what would be a reasonable next move. This could include just living with it, because I don't need 1/500 and 1/1000 very often anyways. I have only been measuring the exposure at three locations, so I do not know how wide the overexposed area on the right side is. I look forward to advice.

 

[Steve M.]

I've never heard of a shutter exposing differently on separate places in the frame. It's either dragging or it's not. If the negative exposures are giving you signs of shutter capping, then that needs to be remedied by a shutter CLA. If it's giving clean exposures on your negs across the frames, then yes, just live w/ it. The purpose of a shutter speed tester is to let you know at what speed the shutter is firing at. Then you can jot down the numbers and make your adjustments as you shoot to get proper exposures.

 

[bjel]

Hi Steve,
Here is what I thought was going on in the shutter before I started adjusting it. The exposures were getting shorter towards the left side of the frame. This suggested to me that the slit between the first and second curtains was narrowing as they crossed the frame. That seemed to likely be due to the curtains not running at the same speed, with either the first curtain too slow or the second curtain too fast. Whichever it is, or both, it would produce a slit that narrows and shorter exposures towards the left side of the frame. I am no expert, but it seems more likely in old mechanisms for something to run too slow than too fast, and so my suspicion falls on the first curtain.
Tightening the first curtain spring has helped the situation a lot, as the graphs show.

The overexposure at the right side of the frame might be due to the second curtain getting off to a slow start. I don't have a good notion of what might be causing that.

 

[mooge]

uneven exposure across the frame is caused by unequal curtain speed. But if the centre and left of the frame are okay I don't understand why there would be such a big difference on the right hand side.

Maybe it's an issue with your shutter tester? maybe try a test roll and see what the negatives look like.

 

[bjel]

Well, it has been no fun at all but I have managed to get the shutter within tolerable error.
+/- 1/3 stop at most speeds of interest.
During the course of all this measuring I found that there are some intermittent timing errors at 1/8 and 1/4, so I may have to take a look at the slow speed mechanism sometime, but that can wait. One day I hope to have a relatively new and reliable rangefinder camera.
Graph of my efforts attached.

 

[retinax]

Congrats! Did you get there only adjusting the tension? Could you share some info about your DIY shutter tester?

 

[bjel]

Hi retinax,
I first tried lubricating things but to no effect. I just hope the oil stays more or less where I put it and doesn't start moving around inside the camera. Apart from making a mess, I know from other instruments that I have worked on that oil can trap and hold particles, creating an abrasive slurry. But by the time this camera wears out I really think I will be able to afford something new like a ZM that simply works and has years to go before needing maintenance.
My initial shutter tension adjustments were sort of systematic, although I did have a couple of screwdriver slips where the spring unwound an unknown number of turns. So after that it became pretty much random.
My measuring setup is pretty simple but uses decent equipment. Photos attached, but since I have only two hands I had to place the camera body on a stand for photo purposes when I actually just held it by hand. It is much more convenient than a stand and allows rapid surveying of various areas of the shutter.
On the left is a small tripod with a laser pointer mounted on it. In the middle is the body being tested. On the right is a photo detector connected to a scope. The red laser spot can be seen on the shutter curtain, but it appears larger than it actually is due to sensor saturation on my DSLR. (Notice that when I am serious about taking a documentary picture the last thing I would use is film!) The picture of the scope shows a typical result at 1/60.

 

[Robert Lai]

Uneven travel means that friction is building up in the shutter train, due to old grease and dirt. You've overcome it temporarily with increasing the shutter spring tension. However, all that does is increase the strain on the shutter curtains. The real remedy is to have it stripped down, cleaned, and relubricated.

 

[bjel]

Thanks for the advice. I consider this camera body as a sunk cost and I am not inclined to spend anything on it, considering the state of the rangefinder optics etc. It is just not worth it to me. When it dies, it dies. With my finite resources I have other and better bodies that might be worth paying for a repair when they need it, not some random old canon.

 

[retinax]

Thanks for the advice. I consider this camera body as a sunk cost and I am not inclined to spend anything on it, considering the state of the rangefinder optics etc. It is just not worth it to me. When it dies, it dies. With my finite resources I have other and better bodies that might be worth paying for a repair when they need it, not some random old canon.

Agree with your thinking. Although these old things deserve being kept in good condition, I can't always spend the resources. I like to think I'd do the same, but the thought that there's dirt in there and things could go out of whack if it moves would haunt me.
Impressive shutter tester!