Home

Please support mflenses.com if you need any graphic related work order it from us, click on above banner to order!

SearchSearch MemberlistMemberlist RegisterRegister ProfileProfile Log in to check your private messagesLog in to check your private messages Log inLog in

Minolta MDIII 100 2,5
View previous topic :: View next topic  


PostPosted: Sun Mar 13, 2022 4:44 pm    Post subject: Minolta MDIII 100 2,5 Reply with quote

I have serviced this lens and removed some very small fungus and haze from the front of the rear group.

I managed of course to loose the mini ball that clicks the diaphragm. Is there anything of comparable size that can be used as replacement?

I do not mind so much a declicked lens but I also have another problem. I realised that holding the aperture ring wide open does not allow for full open diaphragm. I am closer to f2,8 than wide open.I did not felt I messed with something here since the disassembly process is quite straightforward but I obviously committed some mistake.

Any clues as to how I could have a full opened diaphragm again?

Cheers. (Maybe I should stop servicing lenses)


PostPosted: Sun Mar 13, 2022 5:27 pm    Post subject: Re: Minolta MDIII 100 2,5 Reply with quote

lumens pixel wrote:
I have serviced this lens and removed some very small fungus and haze from the front of the rear group.
I managed of course to loose the mini ball that clicks the diaphragm. Is there anything of comparable size that can be used as replacement?

Most probably an MD-III 1.7/50mm or a 3.5/35-70mm will have the same size ball. Otherwise a supplier of ball bearings certainly will have a suitable diameter. I suspect they are about 1mm, but I never did actually measure them.

Quite cheap:
https://simplybearings.co.uk/shop/p35904/1mm-Diameter-Grade-100-Hardened-52100-Chrome-Steel-Ball-Bearings/product_info.html[/quote]



lumens pixel wrote:

I do not mind so much a declicked lens but I also have another problem. I realised that holding the aperture ring wide open does not allow for full open diaphragm. I am closer to f2,8 than wide open.I did not felt I messed with something here since the disassembly process is quite straightforward but I obviously committed some mistake.

Any clues as to how I could have a full opened diaphragm again?


I've disassembled the MD-III 2.5/100mm myself. I suspect a slight misalignment between the coupling elements (there are several, I guess) which transmit the force from the aperture lever to the aperture itself. RokkorDoctor certainly will know the solution Wink

lumens pixel wrote:

(Maybe I should stop servicing lenses)

No. Such mistakes do happen, and you learn from them ... However I know (also learnt from experience) that I do need a calm mind and enough time ("unlimited time") before starting serious work. If I'm in hurry, nothing works ...

S


PostPosted: Sun Mar 13, 2022 7:12 pm    Post subject: Re: Minolta MDIII 100 2,5 Reply with quote

stevemark wrote:
Most probably an MD-III 1.7/50mm or a 3.5/35-70mm will have the same size ball. Otherwise a supplier of ball bearings certainly will have a suitable diameter. I suspect they are about 1mm, but I never did actually measure them.


Almost all MD-III use the same size click-ball, 1mm. If you service many Minolta Lenses, you learn to keep a few aperture click balls & springs spare Wink

stevemark wrote:
I've disassembled the MD-III 2.5/100mm myself. I suspect a slight misalignment between the coupling elements (there are several, I guess) which transmit the force from the aperture lever to the aperture itself. RokkorDoctor certainly will know the solution Wink


I haven't serviced the MD-III 2.5/100 myself yet (all the other MC/MD-II 2.5/100's, yes). However, if it is like most MD-III lenses, then this is almost always an issue.

Background: when Minolta introduced their MD line lenses, in order to reduce overshoot of the aperture for shutter-speed priority and program modes, they changed the aperture mechanism to one with less moving mass involved. They did that in stages as lenses were updated, but ultimately the arced guide plate that determines the aperture size moved from a location near the aperture on the inner lens barrel, to a place directly screwed onto the aperture ring.

EDIT: some MC lenses already had the arced guide plate mounted directly on the aperture ring, but this became increasingly common as the successive generations of MD lines were introduced.

In the process, the usual well-supported and stable arced guide plates of the earlier MC lenses, on the MD lenses now became arced guide plates mounted on the aperture ring which is subject to more play. Combined with additional leverage in the mechanism, this made the aperture calibration affected by the guide plate adjustment much more sensitive.

All this made the mechanism lighter and more responsive with less overshoot, but also reduced accuracy & repeatability of the aperture, especially at smaller aperture sizes. The later introduction of the slightly flexible plastic Rolling Eyes aperture rings on some MD-II/MD-III lenses only made matter worse.

What that means is that with MD-III lenses, even if you reassemble them exactly according to all the marks you made (short of just taking the cells out and reinserting), most likely the aperture will need recalibration. On some MD-III lenses (some zooms in particular), the mere act of loosening the mount screws a bit, and re-tightening them is enough to necessitate aperture adjustment, due to the slop in the oversize screw through-holes..

Aperture adjustment on these MD-III lenses is usually done by adjusting the arced aperture guide plate screwed onto/into the aperture ring. As already hinted at; this is fiddly...; tiny adjustments of the plate have significant impact on the aperture size, esp. smaller apertures. My usual procedure is to remove one or all lens cells so I can clearly see/measure the aperture, and then readjust to the correct aperture sizes (which can be estimated from full aperture diameter and f/# stop ratio's, except for a few special lenses)

In your case though, I would hold off until you have that click-ball replaced. The tension in the spring behind it causes enough deformation of the plastic aperture ring to affect aperture calibration. Sign if anything that Minolta's switch to plastic aperture rings was a bad design choice. If you don't believe me: take some MD-III lenses, set the aperture to f/22, and then squeeze the plastic aperture ring in a few different places. The aperture will vary noticeably... Rolling Eyes

IMPORTANT NOTE: just check that the aperture click-ball hasn't ended up in the grease in the slot that forms the end-stops of the movement of the aperture ring. That is another possible reason why the aperture wont' open fully as then you may not be able to fully rotate the aperture ring to the open f/2.5 position. That's happened to me before on other lenses Wink

lumens pixel wrote:

(Maybe I should stop servicing lenses)


No, Stephan is right, this is how you learn lens repair, and nowadays is pretty much the only way you can learn lens repair. He is also right that you need patience.


PostPosted: Sun Mar 13, 2022 10:38 pm    Post subject: Re: Minolta MDIII 100 2,5 Reply with quote

stevemark wrote:
No. Such mistakes do happen, and you learn from them ... However I know (also learnt from experience) that I do need a calm mind and enough time ("unlimited time") before starting serious work. If I'm in hurry, nothing works ...

S


Absolutely. Having a dedicated work space is almost a must, as things may turn out tricky and can't be finished until the next day. Doing this at the dinner table when your partner has just started cooking dinner is a really bad idea... Wink


PostPosted: Mon Mar 14, 2022 1:45 am    Post subject: Reply with quote

Knowing when to stop "repairing" is another learned skill -- repair sometimes proceeds quite a bit easier after a hiatus such as a good night's sleep.


PostPosted: Mon Mar 14, 2022 10:58 am    Post subject: Reply with quote

visualopsins wrote:
Knowing when to stop "repairing" is another learned skill -- repair sometimes proceeds quite a bit easier after a hiatus such as a good night's sleep.


Not too long a hiatus though, or you had better make some good notes instead of relying on your memory. Especially when it comes to lens & lens spacer orientations... Confused


PostPosted: Mon Mar 14, 2022 1:45 pm    Post subject: Reply with quote

These are very helpful answers, thank you all.