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Minolta APO Tele Rokkor 400: Looking for parts or a miracle
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PostPosted: Mon Jul 17, 2017 6:51 pm    Post subject: Minolta APO Tele Rokkor 400: Looking for parts or a miracle Reply with quote

I recently acquired a pretty rare Minolta gem: The MC APO Tele Rokkor 400 mm f/5.6. When the lens arrived, it was obvious that it had a fungus infection and the rearmost element was hit pretty hard. I managed to clean the mycelium, but the fungus already etched into the glass. The lens is still usable, but I have the impression that the image is slightly clouded and shows a tiny amount of ghosting at f/5.6 and f/8. Since I have no other APO Tele Rokkor to compare it to, it might be normal. But judging from the looks of the rear element, it probably isn't.



Long story short: The lens needs a new rear element. I know that this is pretty hopeless, and for any other "regular" lens I wouldn't even dare to ask. But since the lens is quite rare and a small legend amongst Minolta users, I have to try. Do you - by any chance - know a dealer who might have some Minolta MC/MD spare parts left? Or have you maybe seen a beaten APO Tele Rokkor with other defects which might serve as a donor? Other ideas are welcome, too!


PostPosted: Tue Jul 18, 2017 10:23 am    Post subject: Reply with quote

I don't know about the telephoto lenses too much in practice, but their 24mm and 35mm are extremely prone to ghosting - even when the source is quite far out of the frame. Minolta's multicoating did not prioritise such performance, so it's quite possible that it's normal. You can certainly have the rear element re-polished for low cost, which may reduce the etching, if it is only etched into the hard-coating and not the glass itself. Otherwise, you might consider a professional service that can polish and then re-coat the rear element but that would be vastly more expensive than the former option. I doubt that you'll find another lens for parts as not many of these lenses would have been sold, and even less will be in a parts condition.


PostPosted: Tue Jul 18, 2017 6:10 pm    Post subject: Reply with quote

Teemō wrote:
I don't know about the telephoto lenses too much in practice, but their 24mm and 35mm are extremely prone to ghosting - even when the source is quite far out of the frame. Minolta's multicoating did not prioritise such performance, so it's quite possible that it's normal. You can certainly have the rear element re-polished for low cost, which may reduce the etching, if it is only etched into the hard-coating and not the glass itself. Otherwise, you might consider a professional service that can polish and then re-coat the rear element but that would be vastly more expensive than the former option. I doubt that you'll find another lens for parts as not many of these lenses would have been sold, and even less will be in a parts condition.


Ghosting may have been the wrong term, as it usually refers to some kind of flare. What I meant to say was, that the image gives you the impression of a having a ghost image in it. Details have sort of an outline and the bokeh is quite nervous directly behind and in front of the plane of focus. I'll see if I can manage to upload a decent sample pic. However, I'm pretty convinced the rear lens element needs to be replaced / refurbished.

As you are speaking of it - can you recommend any service in Europe which polishes (and re-coats) lenses?


PostPosted: Tue Jul 18, 2017 7:10 pm    Post subject: Reply with quote

I managed to get some usable test shots at a range of about 200 m. Below are 100% crops from an area close to the center of the frame. The effect is most noticeable in the f/5.6 crop - it's still rather subtle on this subject, but if you look closely, you will see a slight ghost image / double lining at the edges. And yes, this was correctly focused, taken on a tripod, with self timer and all precautions.

f/5.6, 1/640 s, ISO 100:


f/8, 1/320 s, ISO 100:


f/11, 1/160 s, ISO 100:


PostPosted: Wed Jul 19, 2017 3:16 pm    Post subject: Reply with quote

I'm not sure how to describe the effect either but I believe it is normal - I think it is veiling flare, almost seems like colour fringing which it shouldn't be because it's an APO lens. Possibly an element is out of alignment... I don't think fungus could ever be so detrimental to the image quality, as it will mainly concern overall contrast. I think it is just residual spherical aberration and low microcontrast - you can see how many details come out on the gold at each aperture while the fringing seems to be minimised by F8 already. What camera are you using the lens with and how does the effect manifest itself on different targets and distances?


PostPosted: Wed Jul 19, 2017 4:04 pm    Post subject: Reply with quote

I'm using an A7II (24 MP, full frame), IBIS was deactivated for the test shots. On foliage for example, the effect is quite dramatic - it looks extremely nervous due to the ghost contours. I agree that something could be misaligned - maybe some element spacing is wrong. But I wouldn't know how to identify or repair such a defect.

I'm certainly not used to this amount of image degradation - let alone on one of the "better" Minolta lenses. An MD 200 mm f/4, for example, will show a bit of glow wide open (likely from SA) and consequently gain contrast when stopped down. But it's pretty good already at f/4 and very good at f/5.6.


PostPosted: Fri Jul 21, 2017 10:13 am    Post subject: Reply with quote

Unless the element is covered in fungus and haze, any effective loss should be marginal. The only way to know is to compare it against another lens on a comparable camera but I agree it probably isn't normal performance for this lens. You might try to find a collectors group to inquire. Or maybe try emailing LensRentals/Roger Cicala with the images - he might recognise the optical defect because they test and calibrate lenses on an optical bench all the time.


PostPosted: Sat Jul 22, 2017 12:11 pm    Post subject: Reply with quote

OK, so I took out the fungus affected rear element again in cleaned it a second time, this time adding a very very careful polishing step with low-abrasive toothpaste (Elmex Sensitive). Seeing that the fungus had attacked most of the coating anyway, I figured it couldn't hurt. And I think it didn't:

f/5.6, ISO 100, 1/640 s, A7II:


Inspired by your advice (Thank you!), I then looked up an old Lensrentals post about testing decentering - just to rule that out. I made a simple test chart, set it to full screen on my monitor and photographed it from maybe 5.5 m.

Chart:


Top left corner, focused correctly:


Top left corner, slightly misfocused:


Following Roger cicala's guide, I'd say that looks round enough to be fine to my untrained eye. So there is probably nothing heavily decentered or tilted. I didn't want to bother him with this personally, thinking he's probably getting a dozen of similar mails a day already.

Next thing to check was element spacing. Since every lens element in the APO Tele Rokkor is held in place by one fixed collar and a spanning ring, there seems to be nothing to adjust. At least nothing I could see. I wasn't able to dismantle the front group, though. From other Minolta lenses I know a way to tune the infinity focus, but on a unit-focusing lens like this one, that won't change any spacing between the elements and is therefore not of interest. The last thing I could come up with was the rear group: By slightly loosening the threading, the distance between the front and the (loosened) rear group can be adjusted to some degree. That also causes a very slight tilt in the rear group because it's giving it some play in all directions, but I wanted to try this nonetheless. And this is where things get cool.

f/5.6, 1/500 s, after second clean, rear group tightened:


f/5.6, 1/500 s, after second clean, rear group loose (approx. 80° CCW):


Shocked I think the improvement is quite dramatic. I also testes multiple other rotations from 40° to 180° and around 80-90° is the optimum. There is some risidual glow, probably from SA. But other than that, it looks pretty good. I'm thinking about fixing the group in place like this by simply using Loctite 243. That would do no harm and could still be removed later on, if anybody decides to work on the lens again. I'm open for other suggestions, though!


PostPosted: Tue Jul 25, 2017 1:02 pm    Post subject: Reply with quote

Someone dropped it or gave it a good bang. If you want it to be 100% restored, you would need to have it collimated, there are companies that do it, no idea of cost though.

Well done on figuring out the issue, nice work!


PostPosted: Tue Jul 25, 2017 4:10 pm    Post subject: Reply with quote

If you have slacked off the retaining ring to loosen the group, and it shows improvement, then it might be possible to shim the group into the correct position. If you know how many degrees you have slacked the ring off, and the pitch of the thread, that will give you the distance you need to move the group which is the shim thickness. It's difficult to measure the pitch on tiny threads, but a good precision engineer should have thread pitch gauges.


PostPosted: Tue Jul 25, 2017 5:03 pm    Post subject: Reply with quote

Lloydy wrote:
If you have slacked off the retaining ring to loosen the group, and it shows improvement, then it might be possible to shim the group into the correct position. If you know how many degrees you have slacked the ring off, and the pitch of the thread, that will give you the distance you need to move the group which is the shim thickness. It's difficult to measure the pitch on tiny threads, but a good precision engineer should have thread pitch gauges.


I'm trying to get fitting shims right now. Loctite didn't seem like the most elegant solution after all. Instead of measuring the thread I fiddled around with stacks of clear Tesa (Scotch tape). 3 Layers get very close to the correct distance - that's approximately 0.13 mm. Considering that the tape is a bit compressible and one or two hundredth mm less wouldn't hurt even after compressing, 0.1 mm seems to be the correct distance. That's what I'll try first.

Having the lens collimated / serviced by a professional would certainly be the cleanest solution. But I'm close to being happy with the results, so it might not be worth the extra cash (to me). We will see how I feel about my finished repair Wink


PostPosted: Wed Jul 26, 2017 12:18 am    Post subject: Reply with quote

I believe that Stevemark, another member of the fórum, has the same lens as yours. Maybe he could say something about the performance of this lens.