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Minolta MD 75-200 4,5
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PostPosted: Tue Nov 21, 2023 4:17 pm    Post subject: Minolta MD 75-200 4,5 Reply with quote
I bought this zoom and optical quality seemed fine. I notice on purchase that I had to readjust focus when zooming on a object situated at 10 meters and that should have been an alert of something wrong.

Upon further verification I realised that the lens reached perfectly infinity at the hard stop point at 200mm but could not reach infinity beyond approx. 100 mm and that 75mm was quite fuzzy.

Is there a simple way to adjust this lens to have it parafocal again?


PostPosted: Tue Nov 21, 2023 7:33 pm    Post subject: Re: Minolta MD 75-200 4,5 Reply with quote
lumens pixel wrote:
I bought this zoom and optical quality seemed fine. I notice on purchase that I had to readjust focus when zooming on a object situated at 10 meters and that should have been an alert of something wrong.

Upon further verification I realised that the lens reached perfectly infinity at the hard stop point at 200mm but could not reach infinity beyond approx. 100 mm and that 75mm was quite fuzzy.

Is there a simple way to adjust this lens to have it parafocal again?


My immediate thought would be that the adapter you are using isn't quite the correct thickness, but you are an experienced member here so I assume the adapter register/thickness you are using isn't the problem.

Most of these Minolta 70~ish to 200~ish MD zoom lenses have no internal adjustment points for par-focality, other than that the master lens register is adjusted/set by a factory-selected stack of metal shims under the mount, and there may be an adjustment shim underneath the master lens cell itself.

The behaviour you suggest is indicative of one of the following:

- Incorrect adapter thickness. On these older zoom lenses par-focality was never 100% perfect, so on a factory-fresh lens I would expect infinity focus to be a little off at both 70mm and 200mm, and perfect somewhere in the middle.
- The master lens (here the rear lens cell) hasn't been tightened enough after service; see if the rear optical cell is screwed in tight. If it could be tightened up by more than a 15 degree turn or so, you may need to re-collimate the lens.
- The mount screws aren't tight enough. These become visible once the decorative ring around the actual bayonet mount is removed (the decorative ring is held in place by three tiny grub-screws visible from the side)
- Incorrect shim stack thickness underneath the the mount (or underneath the master lens rear cell, not sure which ones have been used on this lens). This may happen if someone misplaced one of the shims during service.

The usual adjustment procedure in the factory is to first select the correct shim stack thickness; I assume they used to use a calibrated collimator for this to check for par-focality throughout the zoom range. Then after that the lens would be collimated for infinity.

Note that the lens being properly collimated for 200mm as it is, means it may need a collimation adjustment after you have fixed the par-focality problem. Collimation adjustment on these lenses is done by loosening the three little grub screw visible around the perimeter on the side of the front barrel, and realigning the front lens cell (and helicoid) relative to the focus grip.

Note on the shims: They can get very thin! They usually consist of aluminium ones for thicker shims, and brass for the thinner ones, down to 0.01mm. The thinnest ones are very fragile and easily damaged, so do take care.


PostPosted: Tue Nov 21, 2023 10:27 pm    Post subject: Re: Minolta MD 75-200 4,5 Reply with quote
RokkorDoctor wrote:
lumens pixel wrote:
I bought this zoom and optical quality seemed fine. I notice on purchase that I had to readjust focus when zooming on a object situated at 10 meters and that should have been an alert of something wrong.

Upon further verification I realised that the lens reached perfectly infinity at the hard stop point at 200mm but could not reach infinity beyond approx. 100 mm and that 75mm was quite fuzzy.

Is there a simple way to adjust this lens to have it parafocal again?


My immediate thought would be that the adapter you are using isn't quite the correct thickness, but you are an experienced member here so I assume the adapter register/thickness you are using isn't the problem.

Most of these Minolta 70~ish to 200~ish MD zoom lenses have no internal adjustment points for par-focality, other than that the master lens register is adjusted/set by a factory-selected stack of metal shims under the mount, and there may be an adjustment shim underneath the master lens cell itself.

The behaviour you suggest is indicative of one of the following:

- Incorrect adapter thickness. On these older zoom lenses par-focality was never 100% perfect, so on a factory-fresh lens I would expect infinity focus to be a little off at both 70mm and 200mm, and perfect somewhere in the middle.
- The master lens (here the rear lens cell) hasn't been tightened enough after service; see if the rear optical cell is screwed in tight. If it could be tightened up by more than a 15 degree turn or so, you may need to re-collimate the lens.
- The mount screws aren't tight enough. These become visible once the decorative ring around the actual bayonet mount is removed (the decorative ring is held in place by three tiny grub-screws visible from the side)
- Incorrect shim stack thickness underneath the the mount (or underneath the master lens rear cell, not sure which ones have been used on this lens). This may happen if someone misplaced one of the shims during service.

The usual adjustment procedure in the factory is to first select the correct shim stack thickness; I assume they used to use a calibrated collimator for this to check for par-focality throughout the zoom range. Then after that the lens would be collimated for infinity.

Note that the lens being properly collimated for 200mm as it is, means it may need a collimation adjustment after you have fixed the par-focality problem. Collimation adjustment on these lenses is done by loosening the three little grub screw visible around the perimeter on the side of the front barrel, and realigning the front lens cell (and helicoid) relative to the focus grip.

Note on the shims: They can get very thin! They usually consist of aluminium ones for thicker shims, and brass for the thinner ones, down to 0.01mm. The thinnest ones are very fragile and easily damaged, so do take care.


That is a lot of useful info. I am pretty sure about the adapter I have shimmed with a micrometer and my other lenses are spot on.

This let me with the shims... And that makes me nervous.


PostPosted: Tue Nov 28, 2023 4:31 am    Post subject: Re: Minolta MD 75-200 4,5 Reply with quote
lumens pixel wrote:
RokkorDoctor wrote:


Note on the shims: They can get very thin! They usually consist of aluminium ones for thicker shims, and brass for the thinner ones, down to 0.01mm. The thinnest ones are very fragile and easily damaged, so do take care.


That is a lot of useful info. I am pretty sure about the adapter I have shimmed with a micrometer and my other lenses are spot on.

This let me with the shims... And that makes me nervous.


I have had similar issues with my "new" MinAF 2.8/70-200mm APO G SSM. Helluvallot of focus shift when zooming while using it on A7II plus (slightly too short) Sony A => Sony E adapter, and no problems at all when sing it on the A900 series SLR (no adapter, of course).

That said, I have seen shims made of three different metals (alu, copper, brass) in one single Minolta lens. Add a bit of a slightly acidic fluid there, and you have a perfect battery (which leads to corrosion and drastically changing thickness of the whole stack of shims, of course). Maybe that's what was happening with your lens?

S


PostPosted: Tue Nov 28, 2023 9:02 am    Post subject: Re: Minolta MD 75-200 4,5 Reply with quote
stevemark wrote:
lumens pixel wrote:
RokkorDoctor wrote:


Note on the shims: They can get very thin! They usually consist of aluminium ones for thicker shims, and brass for the thinner ones, down to 0.01mm. The thinnest ones are very fragile and easily damaged, so do take care.


That is a lot of useful info. I am pretty sure about the adapter I have shimmed with a micrometer and my other lenses are spot on.

This let me with the shims... And that makes me nervous.


I have had similar issues with my "new" MinAF 2.8/70-200mm APO G SSM. Helluvallot of focus shift when zooming while using it on A7II plus (slightly too short) Sony A => Sony E adapter, and no problems at all when sing it on the A900 series SLR (no adapter, of course).

That said, I have seen shims made of three different metals (alu, copper, brass) in one single Minolta lens. Add a bit of a slightly acidic fluid there, and you have a perfect battery (which leads to corrosion and drastically changing thickness of the whole stack of shims, of course). Maybe that's what was happening with your lens?

S


Stephan makes a valid point; there are copper shims around as well.

Galvanic corrosion is certainly possible if the lens was subjected to sea water splashes or a spilled soft drink...


PostPosted: Fri Dec 01, 2023 8:46 am    Post subject: Reply with quote
Thanks for these replies. The lens looks as new but you never now. I have from time to time serviced fixed focal Minolta lenses. In some cases I have lost the little steel ball that sequences the diaphragm and in one case I was unable to put back in place the diaphragm cam since my skills are not that great. I might very well mess up with a zoom.


PostPosted: Fri Dec 01, 2023 12:09 pm    Post subject: Reply with quote
lumens pixel wrote:
Thanks for these replies. The lens looks as new but you never now. I have from time to time serviced fixed focal Minolta lenses. In some cases I have lost the little steel ball that sequences the diaphragm and in one case I was unable to put back in place the diaphragm cam since my skills are not that great. I might very well mess up with a zoom.


Every lens repair enthusiasts has a story to tell about spring-loaded tiny steel click balls Wink

Zooms vary a lot in their ease of disassembly / reassembly. Stuck zoom-cams are the usual trouble, as are press-fitted or glued parts.

Shimming the master lens should be relatively straightforward, but you need good eyes & dexterity and it may need some tedious trial & error.

As a temporary measure you could try and see if you can restore par-focality by re-shimming the adapter. If you can, then you know the thickness-adjustment to be made to the shims underneath the lens' mount.


PostPosted: Fri Dec 01, 2023 9:32 pm    Post subject: Reply with quote
RokkorDoctor wrote:
lumens pixel wrote:
Thanks for these replies. The lens looks as new but you never now. I have from time to time serviced fixed focal Minolta lenses. In some cases I have lost the little steel ball that sequences the diaphragm and in one case I was unable to put back in place the diaphragm cam since my skills are not that great. I might very well mess up with a zoom.


Every lens repair enthusiasts has a story to tell about spring-loaded tiny steel click balls Wink

Zooms vary a lot in their ease of disassembly / reassembly. Stuck zoom-cams are the usual trouble, as are press-fitted or glued parts.

Shimming the master lens should be relatively straightforward, but you need good eyes & dexterity and it may need some tedious trial & error.

As a temporary measure you could try and see if you can restore par-focality by re-shimming the adapter. If you can, then you know the thickness-adjustment to be made to the shims underneath the lens' mount.


Since I do not reach infinity on the short end I guess I should reduce the length of the adapter and then loose precise infinity on the long end. So I guess the shimming parts in the lens are in excess. But then an adjustment should be made on the long end as well.


PostPosted: Sat Dec 02, 2023 11:13 am    Post subject: Reply with quote
lumens pixel wrote:
RokkorDoctor wrote:
lumens pixel wrote:
Thanks for these replies. The lens looks as new but you never now. I have from time to time serviced fixed focal Minolta lenses. In some cases I have lost the little steel ball that sequences the diaphragm and in one case I was unable to put back in place the diaphragm cam since my skills are not that great. I might very well mess up with a zoom.


Every lens repair enthusiasts has a story to tell about spring-loaded tiny steel click balls Wink

Zooms vary a lot in their ease of disassembly / reassembly. Stuck zoom-cams are the usual trouble, as are press-fitted or glued parts.

Shimming the master lens should be relatively straightforward, but you need good eyes & dexterity and it may need some tedious trial & error.

As a temporary measure you could try and see if you can restore par-focality by re-shimming the adapter. If you can, then you know the thickness-adjustment to be made to the shims underneath the lens' mount.


Since I do not reach infinity on the short end I guess I should reduce the length of the adapter and then loose precise infinity on the long end. So I guess the shimming parts in the lens are in excess. But then an adjustment should be made on the long end as well.


After shimming you will likely need to re-collimate the lens by adjusting the front end, so it is hard to tell whether the shim(s) need to be thicker or thinner.

Shimming the master lens is to ensure par-focality as far as I understand; at that point exact collimation such that it reaches infinity is not yet important.

When shimming I would ensure that par-focality is maintained throughtout the zoom range when focused on a 10m distant subject or so.

Once par-focality has been achieved, I would then adjust the infinity collimation using the front focus group such that (hopefully) infinity is reached and maintained throughout the whole zoom range as well.

Note that all my zoom lenses have always been sufficiently par-focal for my uses, so I have never had to make this adjustment on a zoom lens yet myself. The above guidance is only based of how I understand the procedure to work in theory . In practice it may turn out to be a more iterative process than the above suggests, since we lack the calibrated adjustment rig(s) that would have been used at the factory.


PostPosted: Tue Sep 17, 2024 2:41 pm    Post subject: Reply with quote
Thanks again to all for your various contributions.

As indicated in a post regarding the MD 45 2,0 I have shimmed again my adaptor subtracting two layers of scotch tape. That did well on my other lenses so out of curiosity I tried again this zoom.

It is perfect now. I can reach infinity on the whole range of focal lengths. Quite scary to note how little can mess so much with a lens.


Paris la Défense by lumens pixel, sur Flickr

Last edited by lumens pixel on Tue Sep 17, 2024 9:17 pm; edited 1 time in total


PostPosted: Tue Sep 17, 2024 8:43 pm    Post subject: Reply with quote
lumens pixel wrote:
... I have shimmed again my adaptor subtracting two layers of scotch tape. That dis well on my other lenses so out of curiosity I tried again this zoom.

It is perfect now. I can reach infinity on the whole range of focal lengths. Quite scary to note how little can mess so much with a lens.


Yep, indeed. I was really surprised how much a slightly off-the-scale adapter can influence the parfocalty of a zoom. That said, in case of my Minolta/Sony AF 2.8/70-200mm G SSM the actual performance of the lens didn't change much (i. e. it was as good as with a correct adapter when re-focuing after zooming).

S
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