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05-23-2015, 02:35 PM   #1
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Pentax 50mm f1.2 repair

I'm currently having a right laugh trying to fix a 50mm lens. It's a Pentax f1.2 that came with a K2 I bought recently. The barrel was a bit wobbly, but also the aperture didn't seem to adjust past f8 or so - the ring turned but the blades didn't close any further. I had it in a lot of bits tonight, cleaned plenty of grit and sand out, but in the end put it back together. Now the ring won't turn past f8 and I've screwed up the focusing (must have replaced the lens in the helical guide in the wrong place it's focusing very short), but the wobble has mysteriously gone. I didn't get into the front group and aperture blades but I guess that must be the next port of call. The reviews for this lens seem good and it's a huge bit of glass, so hopefully try again and figure it out! I must get on Flickr and post the various repair pics. So, any words of wisdom? Otherwise I'll let you know what happened in the end.

05-23-2015, 04:07 PM   #2
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QuoteOriginally posted by Johnny Rod Quote
So, any words of wisdom?
The only words of wisdom I have would be to send it off for professional repair. I've worked on a good number of lenses, and that is one I would not touch. YMMV
05-23-2015, 04:20 PM   #3
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I;ve fixed several lenses of this model for PF members. Drop me a PM if you'd be interested in my help.
05-23-2015, 05:42 PM   #4
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You could contact Eric Home . He just fixed my A50/1.2 from slow aperture blades.

05-23-2015, 07:31 PM   #5
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Definitely send it to someone with experience. That lens is too nice to be practicing repairs on
05-24-2015, 02:01 PM   #6
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Jatrrax - why not?

dc shooter - I'm based in the UK so could be a bit pricey to send it over!

I've had a few goes now and am still a bit baffled, and not fixed it. It still won't close past f8, but to me it looks like the aperture is shut as far as it will go. I ended up with the diaphragm mechanism out (didn't dismantle it), all looked very clean and everything moved from one end stop to the other. The rest of the lens had a fair bit of crud in it and the filter ring has a small dent in it, but all the glass is perfect. So I had to try to refit the diaphragm mechanism back in the right place, I think this is still not quite right however. Getting the lens back into the helix to focus right is still a mystery - it now focuses at 3ft and reads 3.5ft so infinity is still a bit off, but in all honesty I'm not sure quite how I got it on.There appears to be a very fine thread as well as the coarse one for focusing and this I think is the problem. The barrel wobble is still there but the main focusing threads seem almost devoid of grease so that might be it.

I certainly can't find any good info on t'internet. I have an exploded parts diagram for a f1.7 which is very similar but it lacks the adjustment info. I even found a video of someone dismantling the f1.2 but I can do that no problem! They didn't seem to put it back together again.

I know a lot of the above probably sounds like mindless fumbling but in reality I've fixed just about everything I've come across from a truck or a house to complex lab instruments, so this is doable but it's a knotty one alright. I know the stock PF answer is send it to Eric but half the fun of these is the DIY. So, further thoughts?
05-25-2015, 03:08 PM   #7
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Send it to Pentax Camera Repair Specialists - Repair Prices - Harrow Technical
05-25-2015, 03:46 PM   #8
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Robin at ^^ Harrow Technical is your 'Eric.'

05-25-2015, 08:12 PM   #9
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QuoteOriginally posted by Johnny Rod Quote
Jatrrax - why not?
Because it is a rare and very valuable lens that I would not trust myself to service without messing it up. I have repaired a A 50mm f/1.7 with no issues. But a $600 - $700 lens? Nope, not me. Well worth having a good technician take a look.
05-26-2015, 12:37 AM   #10
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Thanks chaps/chapesses. In total hoensty I bought a K2 with this lens attached from a guy selling his great-uncle's stuff for 20/$30, mainly interested in the K2 but thought it was a nice looking lens. In photography terms I am very much the amateur. Somewhat belatedly I looked on ebay and saw these selling for upwards of 250/$400 which was more than a bit of a surprise and maybe I wouldn't have been so cavalier. Thanks for the link for Harrow, I might well avail myself!

Had another go last night though and have fixed what I had unfixed previously. The confusing thing is the original problem is still there, which is that it will open to full aperture (blade edges just disappear from view), but when closing it, it seems to reach minimum aperture by the time the adjuster ring reaches f11, and while the ring turns beyond there, the size of the aperture stays the same. I saw a video on Youtube of someone selling one of these and the aperture looks the same as mine when fully stopped down, also the same as f22 on my 50mm F1.7 on another Pentax, it's like it gets there too soon, although inside it everything seemed fine and I am fairly sure it isn't a case of it closing too early. I even found another sales video where he demonstrates at f.28 and it looks the same as mine - the aperture closes quite a lot between f1.2 and f4 so it's easy to see.

You'll be pleased to hear I'm disinclined to have another go as I can't figure out what the problem is and God knows I've had a good look at it. Anyone seen this sort of thing before though?
05-26-2015, 01:06 AM   #11
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QuoteOriginally posted by Johnny Rod Quote
You'll be pleased to hear I'm disinclined to have another go
I quite relieved to hear that, as I have a mental picture of many dismantled dysfunctional lenses on your kitchen table, that's my idea of a nightmare.
05-26-2015, 04:22 AM   #12
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I'll have to double check my notes, but IIRC, the position of the entire aperture assembly has some rotational play in it and can be fine tuned by loosening some screws holding it in, then adjusting the position so that at f/1.2, the aperture reaches fully open, then tightening the screws down. It sounds like yours is slightly over-rotated, so it is actually starting from a partially closed position (i.e. the actuating fork has pressure on teh lever even when fully open) and bottoming out before you get to the end of the ring.
05-26-2015, 08:25 AM   #13
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You're right about the setting of it and there are six tiny grub screws around the outside that hold the diaphragm housing in place once set. Annoyingly these are hidden by the filter tube and the mounting screws for that are hidden by the focusing ring. Anyhoo I tried a lot of possible positions along the same lines of thinking and for some reason I just couldn't figure out where it should sit. I think (?) it's in the right place as anywhere else meant it wouldn't fully open to f1.2, or wouldn't shut nearly far enough. Something is obviously up though. The range of adjustment is fairly small as sooner or later the long lever that holds the aperture open will interfere with the stops that set it when taking the picture. Considering how few parts there are in there it's surprising how much it can be fiddled with; it's quite an elegant design.
05-26-2015, 09:22 AM   #14
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The 50ish f/1.2s from pretty much all the brands can be a pain to work with, since everything is so integrated to accomodate all that glass. The Minolta Rokkor-PG 58mm f/1.2, for example has to have the helical removed just to adjust the infinity focus screws. The Pentax isn't quite as bad, but compared to it, a 50mm f/1.4 is luxuriously roomy.

Sounds like you are on the right track, though. Are you sure the aperture actuating lever is seated in the correct position on its cam?

QuoteOriginally posted by Johnny Rod Quote
You're right about the setting of it and there are six tiny grub screws around the outside that hold the diaphragm housing in place once set. Annoyingly these are hidden by the filter tube and the mounting screws for that are hidden by the focusing ring. Anyhoo I tried a lot of possible positions along the same lines of thinking and for some reason I just couldn't figure out where it should sit. I think (?) it's in the right place as anywhere else meant it wouldn't fully open to f1.2, or wouldn't shut nearly far enough. Something is obviously up though. The range of adjustment is fairly small as sooner or later the long lever that holds the aperture open will interfere with the stops that set it when taking the picture. Considering how few parts there are in there it's surprising how much it can be fiddled with; it's quite an elegant design.
05-26-2015, 01:03 PM   #15
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The short-travel one that closes the iris when you shoot? It seems to be. It's mounted on a flat ring which has a pivot on the opposite side of the body - see here:
http://www.pentax-manuals.com/manuals/service/smca5017.pdf
Password is Pentax (capital P), not the same lens but very similar. Part is O-X26 in the bottom left. Little return spring X37 seems to engage with a stud on the back of the ring and also on a screw on the body - not sure which one but it sits under the ring when viewed from the film side and it's not a clamping screw, seems to be just a spring rest. This fully returns okay. When I had the diaphragm assembly twisted too far the wrong way though, the lever would have done the job but the mounting point (which incorporates a spring) to the ring ran out of travel first. Too far the other way and the ring hit the inside of the body before going to full travel. The lever seems only to link the camera body to the diaphragm, there isn't a cam I can see that acts on it. Inside the diaphragm assembly there is a similar ring (not in the diagram I think) that works with the aperture setting lever, that is engaged okay. Hoping this makes some sense, bit verbose so sorry! There may be details in there that strike a chord to the more experienced eye.
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