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09-03-2012, 04:11 PM   #1
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Repairing Pentax-A 35mm 2.8 (totally 'slack' focus ring)

Anybody ever done a similar repair on an older manual focus lens?

I have a 'Pentax-A 35mm f2.8' lens I'd like to have a go at but not sure what to expect or what I'll need once I get in there...

The focus ring 'spins' (not a full spin) from infinity (where it stops) to 1ft/0.3m (where it stops again) but appears to be disconected from the barrel of the lens; whilst being actually attached... The aperture blades open an close as they should but the lens 'will not' focus... the ring moves (slack) but the barrel does not...

'Guide me' LOL!

09-03-2012, 05:23 PM   #2
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Dave,
Here is a thread that has some pictures, I think they are working on a 50mm though so it might be different. The Classic Camera Repair Forum: fungus removal on Pentax lens

It has been awhile since I took an A lens apart.
I think you need to remove the rubber grip, then loosen the screws that hold the name plate on.
Now you should be able to see the three screws that hold the focusing ring to the body. My guess is these have loosened. They are only holding the focusing ring by the edge of the screw or washer so that the ring is adjustable.
You will need to move the ring to the correct place then tighten the screws and test your focus. When it looks good you can close up. You might think about a drop of lock tite on the screws to keep them there, or not.

That's what I remember and I worked on a 50mm so go slow as something might be different.
09-04-2012, 01:22 AM   #3
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That should be a nice relaxing job

You need to take photo of each step of dis-assembling to guide yourself back.
Be careful to mark/photos approx where the lens aligns when turning.

Seems like a simple focus/infinity adjustment.
09-04-2012, 02:48 PM - 2 Likes   #4
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I took apart an A501.4 which is (I believe) the same construction. As was said your fine focus screws came loose (the damn things love to do that). You need a JIS (looks like phillips) screwdriver set and a super tiny flat blade with the sides filed a bit to make it skinnier. A series are not the same as other series I have found. Considering how much trouble I had and how easy it is to screw it up I figured some detail might be helpful.

1. Remove rubber grip, it slides off and has some sticky spots to help keep it in place, its tight enough where I didn't need to replace those when I put it back on.

(you could remove metal filter ring or trim ring next)

2. You need to slide the plastic trim ring off the end of the lens (part with the name on it). There are 3 holes around the outside with little buttons in them that were hidden by the rubber grip, the buttons are tensioned plastic tabs which are part of the trim ring and hold it on. You need to push one of them in just far enough where a knife inserted between the trim ring and the rest of the lens will open a tiny gap, then go on to the next one. What will likely happen is that you will push too hard because its a stupid design and old brittle plastic and you will bust the tab off the trim ring, it can be super glued later though I had to actually glue the tab in place on the plastic lens part then glue the ring to the tab as I put it back on. Any future removal requires busting it off again, but you can scrape the dry glue off with a knife and glue it again. (ask me how I know).

3. Either before or after the plastic trim ring is off you need to unscrew the filter ring because it partially blocks the screws, it is held in by a tiny flat blade set screw in the side which is visible when the lens is fully extended (I forget which focus direction that is) I like to take it off after the trim ring so the lens cap can stay on while I'm fighting with the trim ring. The set screw is impossibly tiny (you have no idea) be incredibly careful not to lose it, if you can unscrew the filter ring without completely removing it that's the better way though I think it drags if its not completely out, I forget.

4. I assume since you didn't mention a rattle that all 3 of your adjustment hold down screws are loose but still in place, but if they came out make sure they don't escape. To adjust the lens you want them all screwed in but just loose enough where if you put a screwdriver in one of them you can turn the whole inside ring like a pencil on a rotary phone, look that up if you are not old enough to have used a pencil to dial a rotary phone , the points of a drafting compass in 2 of the screws could also work (I'm sure there's a special tool for this but I didn't need it) With the lens outer parts held so its stuck at infinity you have to turn the adjustment ring and screws so that the lens is actually focused on infinity, I used a high voltage power line super far off in the distance with a split prism on my film camera to do this. (didn't trust the DSLR due to shimmed Katzeye screen) I actually set it so it could go the tiniest almost imperceptible bit past the infinity focus point, though I'm told this is only necessary with the really long telephoto lenses which expand more in the sun, or with AF lenses so the AF system can hunt past infinity then come back to it.

5. Once the focus is set tighten all three screws, this really sucks because they are super ass tiny and you could bust them off with just 2 fingers if you wanted to, be VERY delicate, better to have them come loose again than to break one off. Before further assembly, if possible test the lens focus on other cameras for second opinion as it really sucks if you decide its wrong and want to change it later. (once again ask me how I know) Also try to force the focus ring past the stops a little (don't break it) to make sure the screws are tight. When screwing the filter ring on fully retract the lens to the stop first as I recall it being possible to screw it on too far otherwise but I might be wrong, then extend the focus again so you can put the set screw in. If the set screw came completely out it can be quite an experience holding it and getting it started, magnetizing the screwdriver helps. Putting the trim ring back on (and gluing it if necessary) is much easier than getting it off was, just work evenly on all 3 sides and don't flex anything too far, the writing on it is upright when the lens is mounted on a camera focused at infinity, btw. Then slide the grip ring back on till its flush in its little groove past the trim ring. Something else to note if your screwdriver created burrs on the hole for the trim ring set screw it might scrape a little bit on the plastic outer part when you adjust the lens all the way in (I think that's the direction of infinity) I had to de-burr it slightly to make it stop since I didn't have the flat screwdriver filed narrow enough the first time around.

I edited this about 50 times so if you read it immediately after posting there are minor clarifications.


Last edited by PPPPPP42; 09-04-2012 at 03:10 PM.
09-04-2012, 03:07 PM   #5
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Nice directions, and better than my memory could provide.
09-04-2012, 03:15 PM   #6
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It would be about 25 pages of warnings and 2 pages of actual instruction if he had to go farther than that, I did my A50 1.4 without having a clue what I was doing and took it down to nothing before cleaning lubing and reassembling. Getting the inner and outer helical parts properly clocked took HOURS since they are near impossible to get started and you basically have to fully assemble and disassemble the lens with each try before you know if its wrong. Shoulda marked the damn things with a pick so I knew which gears meshed with which.
Adjusting the fine focus is relatively easy by comparison but its just so damn easy to break off or lose the small screws and those plastic tabs are designed horribly, they shoulda been much sturdier, one of the big disadvantages of taking apart the A series is the outer plastic decorative parts, though they are handy for keeping the temp stable in the summer or winter compared to a metal exterior lens.

The pics in your link are super helpful to show the dangerous bits though, you can see the 3 plastic tabs sticking down off the trim ring in the pic of it all spread out. http://images52.fotki.com/v642/photos/4/1233394/8163544/IMGP0754b-vi.jpg
And this one: http://images61.fotki.com/v431/photos/4/1233394/8163544/Picture1-vi.jpg

And here you can see one of the focus ring screws though these look a little more sturdy than mine, plus you'd have to take that filter ring off or it cocks the screwdriver sideways. http://images27.fotki.com/v981/photos/4/1233394/8163544/Picture2-vi.jpg

Last edited by PPPPPP42; 09-04-2012 at 03:23 PM.
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