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10-10-2010, 05:52 PM   #1
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SMC Pentax-M 50mm 1.4 Won't Stop Down

This is a problem I've just noticed; not sure how long it's been going on since I don't use the lens at extreme apertures very much.

As I turn the aperture ring, the lens stops down fine all the way to f/16. For some reason, it won't stop down the rest of the way to f/22 when I turn the ring any more.

I have tested, thinking it may be a problem with the ring, but the exposures are identical with the lens at f/16 and at f/22.

Has anyone else had a problem similar to this, and how did you fix it?

10-10-2010, 06:33 PM   #2
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You know what, I think mine might do that too... I'll have to check it tonight.
I think I may have noticed it once and then forgotten about it, as I don't think I have ever used f/22. Strange problem though.
10-10-2010, 07:15 PM   #3
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Interesting that two people have the issue. It's not a big deal to me either; I hardly use f/22 either.
10-12-2010, 06:46 PM   #4
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While this problem did seem familiar to me for some reason, I checked my M50 this morning and found it be operating OK. The physical distance that the aperture blades move from F/16 to f/22 is pretty small though.

10-12-2010, 06:52 PM   #5
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I think I shall need to look at this f1.4 to see if I have a solution to your problem. Not to worry, you'll have it back. All in the name of science and investigation you know.






Yeah. That never works.
07-30-2014, 05:06 AM   #6
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Pentax M 50/1.4

I have the same problem. It seems the aperture reaches full stop down at f16. Turning it to f22 does nothing.
Thought it was grease or oil blocking the blades, So opened the lens from the front to access the blades and cleaned the blades with white spirits, Still the same problem
Does anyone know how to re-adjust the aperture settings so that it fully stops down at f22 instead of f16?
07-30-2014, 06:26 AM   #7
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I have a few lenses that do this. I first noticed it over 20 years ago on some of my lenses. As you stop it down, the aperture stops getting smaller. This is not an issue. Move the ring to f22. Then actuate the aperture reduction lever on the rear of the lens just like it would if you are doing DOF preview or taking a TTL meter reading. Then release the lever. If the lens aperture goes to it's proper size than you ave no problem. My trusty old Tamron 90mm F2.5 macro lens doesn't close past f5.6 when I turn the ring and the lens is off the camera. Still works fine.
07-30-2014, 11:57 AM   #8
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QuoteOriginally posted by clie Quote
I have the same problem. It seems the aperture reaches full stop down at f16. Turning it to f22 does nothing.
Thought it was grease or oil blocking the blades, So opened the lens from the front to access the blades and cleaned the blades with white spirits, Still the same problem
Does anyone know how to re-adjust the aperture settings so that it fully stops down at f22 instead of f16?
There is an easy adjustment for this on the M50/1.7, but that lens is built differently. I can't tell from the disassembly photos I've seen how the 1.4 aperture works exactly. I'd just take it apart and see what parts of the aperture mechanism are adjustable.

07-30-2014, 10:18 PM   #9
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QuoteOriginally posted by Just1MoreDave Quote
There is an easy adjustment for this on the M50/1.7, but that lens is built differently. I can't tell from the disassembly photos I've seen how the 1.4 aperture works exactly. I'd just take it apart and see what parts of the aperture mechanism are adjustable.
Will it help if I post some pics of the disassembled lens (rear view)?
I just can't believe Pentax will make a mistake like this, so there must be some adjustment issue somewhere. Looking at the aperture size, it is definitely f22 (I compared against an SMC Takumar at f16, and the aperture in the Takumar is noticably larger)

Another thing I noticed is that when I start stopping up from f22, the blades do not move until f8. After that the blades continue opening at each aperture ring click all the way to f1.4.
08-01-2014, 11:37 AM   #10
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QuoteOriginally posted by brofkand Quote
As I turn the aperture ring, the lens stops down fine all the way to f/16. For some reason, it won't stop down the rest of the way to f/22 when I turn the ring any more. I have tested, thinking it may be a problem with the ring, but the exposures are identical with the lens at f/16 and at f/22.
Mine did something similar to this, but after sending it to Eric for a CLA, it now works normally. My concern was not really with f/16 and f/22, but if these two were messed up, were the other f-stops accurate? The click between f/16 and f/22 now noticeably changes the aperture.
08-03-2014, 12:32 AM   #11
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QuoteOriginally posted by clie Quote
Will it help if I post some pics of the disassembled lens (rear view)?
I just can't believe Pentax will make a mistake like this, so there must be some adjustment issue somewhere. Looking at the aperture size, it is definitely f22 (I compared against an SMC Takumar at f16, and the aperture in the Takumar is noticably larger)

Another thing I noticed is that when I start stopping up from f22, the blades do not move until f8. After that the blades continue opening at each aperture ring click all the way to f1.4.
Hooray. I've fixed the problem.
The internal brass lever (fork) that controls the aperture blades pin was out of alignment by about 1mm. I guess the brass lever bent over time due to constant use. I bent it back and everything is back to normal.
The brass fork I'm referring to is the one that looks like a fork and slots nicely into the main pin that controls the aperture blades (not the automatic aperture coupling lever that comes out of the back of the lens. It's inside the lens).

Here're the steps to fix it:
1. Remove the 5 silver screws at the back of the lens body
2. Remove the silver bayonet plate at the back
3. You'll see the brass fork roughly opposite the automatic aperture coupling lever (the fork is facing the front of the lens and inside a slot).
4. Take a flat head screwdriver and push sidewards (anticlockwise as you're looking from the back of the lens) against the long body of the brass fork. Apply firm but gentle pressure only. You don't want to break anything.
All you need is to be able to push the brass fork anticlockwise by about 1mm to unbend it.

That all. No other disassembly required. Mine works like new now.

Thanks's to everyone who messaged me with tips.

Last edited by clie; 08-03-2014 at 09:48 PM.
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