Problem with 500mm f/6.3 MTO 3M-6A

[pdccameras]

HI all,


I just picked up a 500mm f/6.3 3M-6A mirror lens. This lens has an excellent reputation, but I have heard it sometimes requires adjustment. I am having problems with my copy. It will not focus to infinity. I have tried it on both a digital camera (Sony A7Mii) with an adapter and on several Pentax native M42 cameras, and cannot attain focus at infinity. Have any members who own this lens experienced/solved this issue?


Many thanks,

Paul

[Sjak]

I have this lens too. Focusing is challenging because my tripod is way too light for a 500mm, but infinity (and beyond) can be achieved. It is not exactly on the infinity-mark though.

I'm wondering what might be off on your copy.

[pdccameras]

Thanks! I am wondering as well. It isn't off by much. It seems OK at short and medium distances - any problems I see are obviously user-error; missed focus or motion blur, but the long distance images at or near infinity are focusing.

P.

[Sjak]

I just looked at my copy. Between the infinity-mark, and the actual end of the focus-range of the lens, there's still maybe 15-20 degrees of rotation.

If with that, you can't reach infinity, maybe also the adapter need a closer inspection? (even if only to rule out a potential cause for issues)

[pdccameras]

Yes, most long mirror lenses focus past infinity to take into account for thermal expansion and contraction. I have turned the focus ring all the way to end without achieving focus at infinity. Checking the adapter was my first thought as well, so I tried the lens on a Pentax M42 body (a KX) - same results as the adapter/Sony combination - no infinity focus.

P.

[kypfer]

My limited experience with this genre of lenses is that although the interchangeable mount "looks like" a T2 mount, it may not be exactly the same.

You need to ensure you're using a manufacturer's original mount, probably in M42, then adapt from there, rather than removing the original mount and trying to fit an alternative.

It may also be possible to machine some metal off the lens-side surface of the existing mount to allow the lens to mount closer to the camera.

Of course, it's also possible that the lens has been disassembled at some point and rebuilt incorrectly.

YMMV - good luck

[marcusBMG]

Judging from this pic



This lens has the swappable, t-mount like mount. Loosen the 3 screws and the bayonet can be removed, exposing a fitting that looks like the internal collar of a standard t-mount. There is no thread - don't try to unscrew it!
In any case you should have infinity focus with any of the standard t-rings that fit.

[pdccameras]

Thanks for posting, Marcus!

My copy came with an M42 fitting already attached. I can always remove the remove the fitting and machine the lens side as suggested by kypfer.

[marcusBMG]

I have an impression that M42 mount looks too big! I wonder if it's a specific fitting not a generic M42. Try fitting a standard M42 t-mount. FYI I just measured one of mine it's ~9.7mm thick.

[pdccameras]

Thanks for sending the dimension, Marcus.

I will check the dimension this evening. I did compare the size of the Russian fitting to a standard M42 T-mount adapter by eye last evening, and a standard T-mount adapter is actually visibly thicker.

The seller claims to have done a complete internal cleaning and inspect/test of the lens prior to shipping, so that would lead me to believe that the unit has been disassembled and re-assembled. I would assume such disassembly/reassembly could result in some kind of internal misalignment or mis-spacing of optical components which could cause the infinity focus problem.

Best,

Paul

[Lloydy]

I seem to remember reading that the mount is slightly different to the regular T Mount, it looks the same but has slight dimensional differences.

[pdccameras]

You gentlemen are both correct. The Russian fitting is 8.9mm thick, while the standard M42 t-mount fitting is 13.1mm thick. Unfortunately, since my lens came with the thinner, Russian fitting already attached, this isn't the problem.

I have a spare M42-Emount adapter, so I think I will put that on the lathe and turn it down a few mm at a time till I can get the lens to focus at infinity. If that works, I can than decide if I want to modify the Russian fitting. I can only assume that the focus distance was thrown off during re-assembly.

P.

[pdccameras]

ok, so I removed 2 mm off the lens side of a known-to-work M42-E-mount adapter and the lens still will not focus to infinity. I took material off in .5mm increments and checked focus at each stage. i am thinking there is some kind of internal issue with the lens.

Paul

[Gerald]

My suggestion is that you first 1)adjust the focus to the infinity mark and 2)determine the actual focus distance in this condition. Then it will be easy to calculate how much you should decrease the thickness of the adapter to achieve infinity focus.

[visualopsins]

Focus helical was started at incorrect thread during re-assembly. These lenses are T2 mount, correct? Suggest put on T2 mount, disassemble helical, trying each starting position until infinity focus is achieved.

[Lloydy]

[visualopsins]

[pdccameras]

The resolution, for now.

Thank you all very much for your help with this lens!

Here is what I did as a quick fix:

I mounted the lens via an M42-E-mount adapter on a Sony A7 Mii. Then, I performed some focus tests by checking the focus on the distance scale of the lens when it was focused at various distances: 10m, 20m, and 30m. I made sure the image was in critical focus in A7 finder, and then read the scale. You can see the results in the pics below. Next I mounted the lens to the A7 via a short throw M42 to E-0mount helical adapter. I then measured the length of the helical adapter when critical focus of the target in the viewfinder matched the subject distance on the distance scale. I realized that the back focus of the lens was off by almost 11 mm, so I am assuming that Visualopolis is correct - the MTO's helical must be misadjusted. Instead of disassembling the lens, I removed the Russian M42 lens ring (held on with three grub screws like a T-mount ring) and then grabbed a spare M42-E-mount adapter, turned down its length a bit, removed the 42mm threads which are held in with grub screws. and mounted it directly to the back of the MTO in place of the Russian M42 ring. (see pic below)

This is an Ok solution for use on the Sony, but it won't let me use it on M42 or Canon bodies. I should really fix the helical, but that's a job I have always had difficulty with. - probably lack of patience on may part.

Thanks for all the great suggestions, hopefully this might prove useful to someone else at some point.

Best,

Paul

[Gerald]

I used the Gauss formula for thin lenses to calculate how much your lens is off. The results were as follows for the various distances:

30m: 11.3mm
20m: 10.0mm
10m: 10.2mm

There is some discrepancy for 30m compared to the other two distances, but we can say with good certainty that your lens is off by 10 ~ 11mm. This is too large to correct by reducing the thickness of the adapter. Everything indicates that there is a mistake in the focusing helicoid assembly. As the thread of the focusing helicoid has multiple entries, whoever assembled the lens probably did not notice that he was taking the wrong entry.

[pdccameras]

Thanks for running the numbers, Gerald! At least it's usable on the mirrorless camera!

Paul

Are these the formulas you used?

[Gerald]

Exactly, Paul. I used the formula (1), but you can use the (2) if you prefer.

When the focusing mechanism is perfectly calibrated, x is the distance the lens must be moved forward to achieve focus for an object at distance s' (or x').

To know how much the focusing mechanism is off, simply calculate the difference between the corresponding values of x for the indicated and the actual distance s'.