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Minolta MC Rokkor-QE 100mm f3.5 maximum aperture question
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PostPosted: Fri Jan 14, 2022 10:46 pm    Post subject: Minolta MC Rokkor-QE 100mm f3.5 maximum aperture question Reply with quote
I just got a Rokkor-QE 100mm. It's a fixer upper and while I had it opened up I noticed that the aperture blades do not completely open when set to f3.5 (see pic). All the other lenses I've seen have aperture blades that open completely at the largest aperture. The edges of the blades are hidden by the iris pressure plates. Is this normal for this lens or does the aperture need an adjustment? Can any other owners take a look and see if it matches the aperture on their own lenses?

Thanks.



Edit: bbcode image broke so here's an alternate link.


PostPosted: Fri Jan 14, 2022 11:23 pm    Post subject: Re: Minolta MC Rokkor-QE 100mm f3.5 maximum aperture questio Reply with quote
vandergus wrote:
I just got a Rokkor-QE 100mm. It's a fixer upper and while I had it opened up I noticed that the aperture blades do not completely open when set to f3.5 (see pic). All the other lenses I've seen have aperture blades that open completely at the largest aperture. The edges of the blades are hidden by the iris pressure plates. Is this normal for this lens or does the aperture need an adjustment? Can any other owners take a look and see if it matches the aperture on their own lenses?

Thanks.



Edit: bbcode image broke so here's an alternate link.


No images first post -- should b okay now. Smile

Dunno about your lens; I have several lenses where aperture doesn't open completely but blades are hidden when lens is wide open.


PostPosted: Sat Jan 15, 2022 3:37 am    Post subject: Re: Minolta MC Rokkor-QE 100mm f3.5 maximum aperture questio Reply with quote
vandergus wrote:
I just got a Rokkor-QE 100mm. It's a fixer upper and while I had it opened up I noticed that the aperture blades do not completely open when set to f3.5 (see pic). All the other lenses I've seen have aperture blades that open completely at the largest aperture. The edges of the blades are hidden by the iris pressure plates. Is this normal for this lens or does the aperture need an adjustment? Can any other owners take a look and see if it matches the aperture on their own lenses?

Thanks.



Edit: bbcode image broke so here's an alternate link.


I believe that is not uncommon.


PostPosted: Sat Jan 15, 2022 4:00 am    Post subject: Re: Minolta MC Rokkor-QE 100mm f3.5 maximum aperture questio Reply with quote
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Last edited by Blazer0ne on Tue Feb 22, 2022 6:26 pm; edited 1 time in total


PostPosted: Sat Jan 15, 2022 11:39 am    Post subject: Re: Minolta MC Rokkor-QE 100mm f3.5 maximum aperture questio Reply with quote
Blazer0ne wrote:
Your lucky day.

https://www.ebay.com/itm/313771390703


Almost Wink That is the manual for the 100mm f/3.5 MACRO ROKKOR QE. The lens here is the regular 100mm f/3.5 ROKKOR QE, which is an entirely different optic, both optically and mechanically.

As for this lens (100mm f/3.5 MC/AUTO TELE ROKKOR-QE), I have multiple AR I, AR II and MC samples of this lens. On all of them the aperture opens fully; a not fully opening aperture as you show in the image is not normal for this lens.


There are usually three possible causes for aperture misalignment on Rokkor lenses:

a) The arc/wedge aperture adjustment plate mounted on the aperture ring itself is misaligned. This is attached to the aperture ring with 2 screws, and the position and angle can be adjusted to get both maximum and minimum apertures correctly set. This adjustment is relatively easy to access once the aluminium decorative dust-shield around the lens mount is removed.

b) The rotational offset adjustment plate on the aperture ring is misaligned. Very similar to a) apart from this only allowing for a fine-adjustment of the rotational positioning of the aperture ring relative to the internal aperture mechanism.

c) The aperture assembly itself is misaligned inside the inner barrel. If this is not set correctly before assembly of the lens, the adjustment possible under a) or b) will not be sufficient to get the apertures correct. The aperture assembly can be rotated inside the inner lens barrel itself by loosening three small set-screws on the inner lens barrel (you can see one pointed end of them in your image, they fix the aluminium clamp-down ring to secure the aperture assembly). Unfortunately, access to these screws will require disassembly of the lens. From your image I would guess two of those screws appear to be missing, hard to see. It may be this lens has only one, I can't remember off the top of my head without opening one up again further...


Some Rokkor lenses have adjustments a) and c), others have adjustments b) and c). I have quickly taken the back off one of my 100mm f/3.5 QEs to have a look; your lens will have adjustments b) and c).


The usual procedure for aperture adjustment on Minolta Rokkor lenses is:

1) When mounting the aperture assembly inside the inner lens barrel, it is rotated and fixed such that the blades can close just a little smaller than the required minimum aperture, but not so small that the maximum aperture cannot be attained.

2) Once the lens has been fully assembled apart from the front & rear lens groups & decorative ring around the lens mount:
on lenses with adjustment a) the arced wedge aperture adjustment plate is adjusted to fine-tune the maximum and minimum aperture opening of the blades. This can be a very sensitive adjustment, and takes a little practice.
on lenses with adjustment b) the rotational offset adjustment plate on the aperture ring is set such that the minimum aperture is set correct. This should normally mean the maximum aperture can also be attained, as the arc that determines the diameters-curve for all apertures is mounted on the inner lens-barrel and is non-adjustable on these lenses.

On your lens, if adjustment b) doesn't fix it, unfortunately the aperture assembly is misaligned in the inner barrel and you will need to make and adjustment to c).


Required aperture diameters:

The service manuals usually specify the required aperture diameters at the different f/# stops, but lacking that, they can be calculated from the maximum opening, which is usually (but not always!) the same as the opening of the aperture blades holder. The other aperture opening diameters are as follows:

aperture diameter @ f/a = maxdiameter x fmax/a

so, e.g. here for this f/3.5 lens: aperture diameter @ f/8 = maximum aperture diameter x 3.5 / 8

These calculations have always been nearly spot-on when I compare them with the diameters mentioned in the numerous Rokkor service manuals I have.


Sadly, many YouTube tutorials on servicing Rokkor lenses pay no heed to these adjustments at all, and some of these adjustments are very sensitive & precise. I am not at all surprised that many DIY serviced Rokkor lenses (and likely other ones too) end up with badly mis-calibrated aperture openings... Sad

Last edited by RokkorDoctor on Sat Jan 15, 2022 12:08 pm; edited 2 times in total


PostPosted: Sat Jan 15, 2022 11:42 am    Post subject: Re: Minolta MC Rokkor-QE 100mm f3.5 maximum aperture questio Reply with quote
Blazer0ne wrote:
Your lucky day.

https://www.ebay.com/itm/313771390703


No, that's not the right service manual!!

Th OP was referring to the (non-macro) MC Rokkor-QE 3.5/100mm, and not to the MC Rokkor-QE 3.5/100mm Macro!

Back to the original question: The aperture should be completely open when the aperture ring is set to f3.5. I just have checked that with my sample.

S


PostPosted: Sat Jan 15, 2022 12:02 pm    Post subject: Reply with quote
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Last edited by Blazer0ne on Tue Feb 22, 2022 6:26 pm; edited 1 time in total


PostPosted: Sat Jan 15, 2022 1:36 pm    Post subject: Re: Minolta MC Rokkor-QE 100mm f3.5 maximum aperture questio Reply with quote
RokkorDoctor wrote:


As for this lens (100mm f/3.5 MC/AUTO TELE ROKKOR-QE), I have multiple AR I, AR II and MC samples of this lens. On all of them the aperture opens fully; a not fully opening aperture as you show in the image is not normal for this lens.



Thank you! Exactly what I was looking for.


PostPosted: Sat Jan 15, 2022 8:28 pm    Post subject: Reply with quote
An update just to share the solution...

Looking more closely at the aperture ring and the mechanisms on the rear of the lens, it looked like tab on the aperture ring that couples with the aperture mechanism was slightly bent. The slot that the tab sits in also looks like it might have opened up over time. These two things combined meant that the aperture ring could no longer open the iris to its maximum size before hitting the hard stop.

Here's the bent tab I'm talking about (although it's hard to show the shape of it in the pic)



The tab is attached with two screws through slotted holes (what RokkorDoctor was referring to in section a, I think). Luckily, there was enough room in the slots to simply loosen the screws and slide it all the way to one end. This was able to make up for the deformation of the tab and get the aperture to its fully open position.



The mechanism that moves the aperture blades is pretty stiff and I'm guessing that is what led to the bent tab in the first place. It may need to be disassembled and cleaned for a longer term fix.

Cheers and thanks again for the help!


PostPosted: Sat Jan 15, 2022 9:25 pm    Post subject: Reply with quote
vandergus wrote:
An update just to share the solution...

Looking more closely at the aperture ring and the mechanisms on the rear of the lens, it looked like tab on the aperture ring that couples with the aperture mechanism was slightly bent.


Yes, I had omitted the fourth possible reason: damaged parts Wink

vandergus wrote:
The tab is attached with two screws through slotted holes (what RokkorDoctor was referring to in section a, I think).


That is actually an example of adjustment type b); both type a) and type b) adjustment plates are usually mounted with two screws.

vandergus wrote:

The mechanism that moves the aperture blades is pretty stiff and I'm guessing that is what led to the bent tab in the first place. It may need to be disassembled and cleaned for a longer term fix.


Yes, that would need to be fixed. Likely old gummed-up migrated oil on the brass aperture parts on the back of the inner lens barrel.

I have serviced many of these Rokkors, so in case it is of any use (since the original lubricants Minolta specified for their lenses are no longer available):

After cleaning, a good lubricant to use for the moving brass parts & levers on the back of the inner barrel near the aperture blades would be a dry-lube like powdered graphite, powdered molybdenum disulfide or possibly powdered tungsten disulfide or powdered teflon. Alternatively I have found a soft 6B or 8B pencil rubbed lightly over the moving contact surfaces also to work fine (less messy to apply too). If preferred, a minute amount of grease like S40 or S250 (Japanhobbytools/microtools) can also be used on the inside cylindrical sliding surface and edge of the arc/wedge of the wedge-shaped brass part. Only use a dry-lube on any other levers in that area.

The contact surface of the bent part you showed preferably need a heavier grease like e.g. S1500, something similar to what you would use for the helicoid keys. S40 on the aperture ring contact surface, Helimax-XP for both threads on the double helicoid.

Those are my tried-and-tested lubricant choices, but others may have different preferences, especially. re. focus damping.

Glad to hear you got it sorted for now Like 1 small Smile


PostPosted: Sat Jan 15, 2022 10:17 pm    Post subject: Reply with quote
RokkorDoctor wrote:
I have serviced many of these Rokkors, so in case it is of any use (since the original lubricants Minolta specified for their lenses are no longer available):

After cleaning, a good lubricant to use for the moving brass parts & levers on the back of the inner barrel near the aperture blades would be a dry-lube like powdered graphite, powdered molybdenum disulfide or possibly powdered tungsten disulfide or powdered teflon. Alternatively I have found a soft 6B or 8B pencil rubbed lightly over the moving contact surfaces also to work fine (less messy to apply too). If preferred, a minute amount of grease like S40 or S250 (Japanhobbytools/microtools) can also be used on the inside cylindrical sliding surface and edge of the arc/wedge of the wedge-shaped brass part. Only use a dry-lube on any other levers in that area.

The contact surface of the bent part you showed preferably need a heavier grease like e.g. S1500, something similar to what you would use for the helicoid keys. S40 on the aperture ring contact surface, Helimax-XP for both threads on the double helicoid.

Those are my tried-and-tested lubricant choices, but others may have different preferences, especially. re. focus damping.

Glad to hear you got it sorted for now Like 1 small Smile


Excellent. Thanks for the extra knowledge.
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