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VLR
Joined: 05 Mar 2015 Posts: 86
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Posted: Mon Jul 17, 2017 6:51 pm Post subject: Minolta APO Tele Rokkor 400: Looking for parts or a miracle |
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VLR wrote:
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! _________________ http://vintagelensreviews.com/
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Teemō
Joined: 07 Apr 2016 Posts: 586 Location: Australia
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Posted: Tue Jul 18, 2017 10:23 am Post subject: |
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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. |
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VLR
Joined: 05 Mar 2015 Posts: 86
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Posted: Tue Jul 18, 2017 6:10 pm Post subject: |
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VLR wrote:
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? _________________ http://vintagelensreviews.com/
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VLR
Joined: 05 Mar 2015 Posts: 86
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Posted: Tue Jul 18, 2017 7:10 pm Post subject: |
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VLR wrote:
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:
_________________ http://vintagelensreviews.com/
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Teemō
Joined: 07 Apr 2016 Posts: 586 Location: Australia
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Posted: Wed Jul 19, 2017 3:16 pm Post subject: |
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Teemō wrote:
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? |
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VLR
Joined: 05 Mar 2015 Posts: 86
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Posted: Wed Jul 19, 2017 4:04 pm Post subject: |
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VLR wrote:
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. _________________ http://vintagelensreviews.com/
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Teemō
Joined: 07 Apr 2016 Posts: 586 Location: Australia
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Posted: Fri Jul 21, 2017 10:13 am Post subject: |
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Teemō wrote:
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. |
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VLR
Joined: 05 Mar 2015 Posts: 86
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Posted: Sat Jul 22, 2017 12:11 pm Post subject: |
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VLR wrote:
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):
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! _________________ http://vintagelensreviews.com/
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iangreenhalgh1
Joined: 18 Mar 2011 Posts: 15685
Expire: 2014-01-07
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Posted: Tue Jul 25, 2017 1:02 pm Post subject: |
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iangreenhalgh1 wrote:
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! _________________ I don't care who designed it, who made it or what country it comes from - I just enjoy using it! |
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Lloydy
Joined: 02 Sep 2009 Posts: 7785 Location: Ironbridge. UK.
Expire: 2022-01-01
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Posted: Tue Jul 25, 2017 4:10 pm Post subject: |
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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. _________________ LENSES & CAMERAS FOR SALE.....
I have loads of stuff that I have to get rid of, if you see me commenting about something I have got and you want one, ask me.
My Flickr https://www.flickr.com/photos/mudplugga/
My ipernity -
http://www.ipernity.com/home/294337 |
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VLR
Joined: 05 Mar 2015 Posts: 86
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Posted: Tue Jul 25, 2017 5:03 pm Post subject: |
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VLR wrote:
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 _________________ http://vintagelensreviews.com/
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Gerald
Joined: 25 Mar 2014 Posts: 1196 Location: Brazil
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Posted: Wed Jul 26, 2017 12:18 am Post subject: |
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Gerald wrote:
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. _________________ If raindrops were perfect lenses, the rainbow did not exist. |
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stevemark
Joined: 29 Apr 2011 Posts: 3754 Location: Switzerland
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Posted: Mon Dec 18, 2017 6:17 pm Post subject: |
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stevemark wrote:
VLR wrote: |
OK, so I took out the fungus affected rear element again ...
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):
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! |
Quite interesting. However i still don't understand how a simple axial shift of a lens group can cure central astigmatism (which only occurs if one ore more lenses are de-centered). Anyway, your result is impressive, and we should try to repeat it with another MD 5.6/400 APO.
If simply the lens spacing between front and rear element is the problem, there might be a quite simple explanation for the systematic "softness / double lines" of this and other Minolta 400 APO lenses: "not standard" temperatures ... if we assume a distance of 250mm between front and rear group, and if we assume the barrel to be aluminium, the distance between front an rear group may change significantly when tempertures are changing (something in the area of 0.1mm for 15°C temperature difference). In addition the known Fluorite sensitivity for temperature changes may play a role as well.
BUT then ... Why does the Canon FD 2.8/300mm Fluorite not have such problems ...?
Stephan _________________ www.artaphot.ch |
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