Mamiya Sekor 500/5.6 not focussing to infinity

[simato73]

I've got this lens that I bought last year to shoot the solar eclipse.
Unfortunately I cannot get it to focus to infinity! It gets close but not quite there; very stopped down I can make a shot passable if not inspected closely but clearly this is frustrating.
I use the lens on a Fuji X-T1 using adapters (Mamiya to Nikon and Nikon to Fuji). I don't think that the adapters are to blame because I have two of each kind and all permutations give the same result.

The lens has a big element in front, which does not rotate with focus and that gets closer to the sensor plane at infinity (but, critically, not close enough?).
The front element can be unscrewed off fairly easily.
I have done it once a year ago to take out the sliding hood to fix a dent (Erik advised back then).
The front bit pictured below can actually be further unscrewed to let the hood out but I haven't done it this time because it felt quite tight even using a strap wrench (which admittedly I am not too good at using).

Overall the construction seems pretty simple and maybe someone handier than me could easily find if and where it is possible to fine tune infinity focus.
Any clues? I am planning to take this one to Scotland to beg for some help.

[Bjørn Rørslett]

The pertinent quest now is: at what position does the lens provide infinity focus? From your description, the overall length of the adapter stack is slightly too long.

Mamiya 7/7II has register distance 59 mm and Mamiya 645 has 63.3 mm. Fuji X is 17.7 mm and Nikon F is 46.5 mm.

Can you measure the flange-flange distance of your adapters? That'll provide a rough estimate as to the total length of the combined adapters.

[Mongo]

Mongo's instant thoughts were that the adaptors are not precisely made and are keeping the rear element of the lens a little too far from the camera's sensor. If this is the case, and if the adaptors do not have the complication of contact points, Mongo would think to carefully machine the mount(s) on the adaptor down by minute fractions at a time until it does focus to infinity. Mongo thinks this should not effect the ability of the lens to focus throughout the rest of its focus distances (except maybe a fraction at the closest focus distance).

The other thing Mongo should have asked you first was, did it focus to infinity BEFORE you did the work on the front of the lens. If it did, then, it is likely the front element may not have been reinstalled correctly and the problem may need to be corrected at the front of the lens instead of shortening the mount(s) as mentioned above..........not really sure in your case.

[simato73]

The pertinent quest now is: at what position does the lens provide infinity focus? From your description, the overall length of the adapter stack is slightly too long.

Mamiya 7/7II has register distance 59 mm and Mamiya 645 has 63.3 mm. Fuji X is 17.7 mm and Nikon F is 46.5 mm.

Can you measure the flange-flange distance of your adapters? That'll provide a rough estimate as to the total length of the combined adapters.

I should add that I have another lens in Mamiya mount (it is a Hartblei 45mm superrotator) and with that one I can focus to infinity.

[simato73]

The other thing Mongo should have asked you first was, did it focus to infinity BEFORE you did the work on the front of the lens. If it did, then, it is likely the front element may not have been reinstalled correctly and the problem may need to be corrected at the front of the lens instead of shortening the mount(s) as mentioned above..........not really sure in your case.

I wish I could answer this question, I have asked it myself.
Simply, I don't remember if I checked or what was the result.

[Mongo]

I should add that I have another lens in Mamiya mount (it is a Hartblei 45mm superrotator) and with that one I can focus to infinity.

you can focus to infinity with the same adaptors.?? and did your 500mm focus to infinity before you fixed the front end ...

[Bjørn Rørslett]

If the combined stack of adapters measure reasonably within limits, then you have no option other than look at the lens itself. Probably this is a design with all the glass in front? The front cell might not be seated in the correct position. If there are smaller elements in the rear section, one or more might be flipped the wrong way. (yes, we once repaired a lens thought to be junk by locating the wrongly orientated element. Turned out to be element no. 11 deep down into the optical train).

[simato73]

you can focus to infinity with the same adaptors.?? and did your 500mm focus to infinity before you fixed the front end ...

Using the same adaptors the 45mm focuses to infinity (and beyond... a' la Buzz from Toy Stories ). The 500mm doesn't, the image of an item at infinity is not sharp even if I turn the focus collar all the way to the hard stop.

As for the second question, stupid me I do not remember if I checked before unscrewing off the front element.

[simato73]

If the combined stack of adapters measure reasonably within limits, then you have no option other than look at the lens itself. Probably this is a design with all the glass in front? The front cell might not be seated in the correct position. If there are smaller elements in the rear section, one or more might be flipped the wrong way. (yes, we once repaired a lens thought to be junk by locating the wrongly orientated element. Turned out to be element no. 11 deep down into the optical train).

The big elements are all in the front. The majority of the barrel is hollow and near the mount there are a few elements, which focusing seem to be moving in unison with the front element, i.e. away from the sensor when focusing close. Overall the design is 5 elements in 4 groups according to a link I found; I could not find an optical scheme yet.

[Mongo]

Mongo suspects that the adaptor is probably quite correctly made. Mongo uses an after market adaptor to use his Pentax 645 lenses on his Nikon bodies and all  work perfectly. It is reasonable to think all adaptors when designed are tested and should work correctly. Also, in your case, it has proved it works correctly on your 45mm lens.

The problem is in the 500mm. If you do not have any images with this lens in focus at infinity, then, it could be the lens was faulty when you got it . Someone could have tried to dismantle and fix things in it and not reassembled it correctly. If this is the case, it could be anywhere in the lens.This is highly unlikely as the lens gives good images with your 45mm

Probability would support the theory that the problem is likely in the front of the lens as a result of the work you did on it to correct the dent. If this is the case, it should be able to be reversed. The amount of error is probably very tiny and you may not readily see it. There is probably now no harm in disassembling the front again and reassembling it - keeping an eye out to see that all surfaces that are screwed together close fully and evenly. This may actually correct the problem if you are lucky. It can probably do no more harm. Best case, it fixes it - worst case, you should be no better off. 

[Mongo]

one quick thought. When you get an opportunity, mount the lens with adaptor onto the camera. Set it to infinity as much as possibly. Then, while looking through the viewfinder, unscrew the front element group slightly (as if you were going to remove it). Does this make the infinity focus better or worse

[simato73]

Mongo suspects that the adaptor is probably quite correctly made. Mongo uses an after market adaptor to use his Pentax 645 lenses on his Nikon bodies and all  work perfectly. It is reasonable to think all adaptors when designed are tested and should work correctly. Also, in your case, it has proved it works correctly on your 45mm lens.

The problem is in the 500mm. If you do not have any images with this lens in focus at infinity, then, it could be the lens was faulty when you got it . Someone could have tried to dismantle and fix things in it and not reassembled it correctly. If this is the case, it could be anywhere in the lens.This is highly unlikely as the lens gives good images with your 45mm

Probability would support the theory that the problem is likely in the front of the lens as a result of the work you did on it to correct the dent. If this is the case, it should be able to be reversed. The amount of error is probably very tiny and you may not readily see it. There is probably now no harm in disassembling the front again and reassembling it - keeping an eye out to see that all surfaces that are screwed together close fully and evenly. This may actually correct the problem if you are lucky. It can probably do no more harm. Best case, it fixes it - worst case, you should be no better off.

What you say makes sense to me.
As you said, I was suspicious that is this might be my doing from the time I fixed the dent; but I was also thinking that in my ignorance I could be delusionally optimistic.
After taking the pictures of the disassembled lens tonight I screwed it back quite tight, so much that I am struggling to open it again.
Maybe this is enough; it is too dark to check now, I'll try tomorrow.

[Mongo]

yes, it is possible to over tighten or under tighten screw connections that will vary ( in very tiny amounts) some critical distances. Sometimes, there can be small debris introduced while disassembling which stops the connections from fully being screwed together etc. Let us know tomorrow.

[Hugh_3170]

Simone, do you have any friends or second hand camera shop contacts that might loan you another Mamiya Sekor lens to check your adapters? 

I am thinking that another lens nearer the focal length of your problem lens rather than 45mm might be in order.  Just a thought.  Good luck.

[bjornthun]

My thought is that the front group is to far away, so as to make the lens near-sighted. The other option is that the focus group doesn't move as freely as it should, i.e. the focus group can't travel far enough toward the film/sensor plane. Is there a way to adjust the travel of the focus group? The lens should focus slightly past the infinity mark.