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10-30-2016, 02:52 AM   #1
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How to adjust focus ring of a Pentax A 50:1.4?

Hi

I just received this lens, bought on ebay. When focus ring is in infinity position, the lens is focussing to about 10m. It seems the focussing ring is not adjusted correctly.

I have seen these instructions to correct this for the M 50/1.4:
https://www.pentaxforums.com/forums/10-pentax-slr-lens-discussion/204073-help...-infinity.html

Has anybody adjusted the focus ring of the A 50/1.4, or knows a site about disassembling this lens?

Best regards
Klaus

10-30-2016, 03:38 AM   #2
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typically focus can be adjusted through removing the front ring on the lens with the name "SMC-A Pentax 1:1.4 50mm S/N ASAHI OP.CO.,Japan" this has to be carefully turned counter-clockwise to access the focusing adjustment screws. Other lenses use a ring on the front of the focusing grip to cover the screws that couple the focusing helicoid with the focusing ring.
10-30-2016, 04:21 AM - 1 Like   #3
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With the A50/1.4 lens, the ring with the name engraved on it is NOT threaded.

You first need to remove the rubber focus grip and then you'll see three holes. You then push a rod or screwdriver in the hole to release the catch. Be careful that you don't push too hard as the ring is made from plastic and you can easily break the catch.

It would appear that someone has taken the lens apart previously and re-assembled it without any regard for its proper operation. I'd be worried about the internal state of this lens. Why was the lens dis-assembled in the first place? It's not normal for a lens to shift the infinity focus so much under regular use.

Before you start disassembling the lens, check to make sure there are no other surprises like fungus or clouding by looking through the lens and making careful observations. If you discover something amiss that was not mentioned by the seller, then I suggest you contact him/her and have a discussion as to what to do. Once you start tampering with the lens, you've pretty much accepted the lens as is and returning the lens for refund would be questionable as the item is now not in same condition as it was shipped.

Last edited by tom1803; 10-30-2016 at 04:33 AM.
10-30-2016, 04:56 AM   #4
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QuoteOriginally posted by tom1803 Quote
With the A50/1.4 lens, the ring with the name engraved on it is NOT threaded.
I wasn't aware of that* - so the ring on the front of the lens, inside the filter thread will be the ring that will need to be removed to access the adjustment screws. So it would be possible to adjust focus in a similar fashion to the SMC-A 50mm f/1.7

* strangely, the SMC-A and K 50mm f/1.2 lenses that ring is removable to adjust focusing.

10-30-2016, 05:22 AM - 1 Like   #5
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QuoteOriginally posted by Digitalis Quote
I wasn't aware of that* - so the ring on the front of the lens, inside the filter thread will be the ring that will need to be removed to access the adjustment screws. So it would be possible to adjust focus in a similar fashion to the SMC-A 50mm f/1.7

* strangely, the SMC-A and K 50mm f/1.2 lenses that ring is removable to adjust focusing.
The ring inside the filter thread holds the front element. The filter mounting ring is threaded and is held in place by a single small grub screw accessible from the outside. You willl then see the three screws that adjust the focus but you can't quite reach them with a screwdriver. Removing the name ring then gives access to the screws as per K50/1.4, M50/1.4 and M50/1.7.

The K and A series 50/1.2 have slightly different construction to above mentioned lenses.

Last edited by tom1803; 10-30-2016 at 05:28 AM.
10-30-2016, 06:07 AM   #6
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Thank you, Rod

The lenses are clean, the diaphragm moves well, and overall the lens looks like new.

It it strange that it has this misadjustment.

I will try to move the rubber focus grip. Just lifting and pushing, I suppose.

Best regards
Klaus

QuoteOriginally posted by tom1803 Quote
With the A50/1.4 lens, the ring with the name engraved on it is NOT threaded.

You first need to remove the rubber focus grip and then you'll see three holes. You then push a rod or screwdriver in the hole to release the catch. Be careful that you don't push too hard as the ring is made from plastic and you can easily break the catch.

It would appear that someone has taken the lens apart previously and re-assembled it without any regard for its proper operation. I'd be worried about the internal state of this lens. Why was the lens dis-assembled in the first place? It's not normal for a lens to shift the infinity focus so much under regular use.

Before you start disassembling the lens, check to make sure there are no other surprises like fungus or clouding by looking through the lens and making careful observations. If you discover something amiss that was not mentioned by the seller, then I suggest you contact him/her and have a discussion as to what to do. Once you start tampering with the lens, you've pretty much accepted the lens as is and returning the lens for refund would be questionable as the item is now not in same condition as it was shipped.
10-30-2016, 06:32 AM   #7
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Many of the "A" lenses have the focal length engraved ring on the focusing barrel (outside of the filter threads), in which case disassembly is as outlined as above. Otherwise if it is inside the filter threads you unscrew it as with the majority of lenses.

The lens may have been used on a non-Pentax body with a glassless adapter and adjusted for infinity focus for that body.

Is the auto aperture lever intact? People will perform surgery on this appendage too.
10-30-2016, 06:49 AM   #8
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If I get it correctly, Klaus meant the marks on the focusing ring does not match the real focusing distance, but the lens still can achieve focusing to infinity and has a hard stop at or slightly pass infinity.
If that is true, adjusting the focusing at the front might further mess up the lens.

10-30-2016, 07:02 AM   #9
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QuoteOriginally posted by grahame Quote
If I get it correctly, Klaus meant the marks on the focusing ring does not match the real focusing distance, but the lens still can achieve focusing to infinity and has a hard stop at or slightly pass infinity.
If that is true, adjusting the focusing at the front might further mess up the lens.
For all Pentax prime lenses that I have serviced, focus issues are adjusted by matching the focus ring to the focus helicoid at infinity. If you can't match it, then you have serious problems. Also, if you can match the infinity index but the lens won't focus at closer distances, then again you have problems, most likely an element mounted in reverse, etc.
10-30-2016, 07:12 AM   #10
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I once bought a lens from ebay. If I don't look at the focusing scale at all, it is just a perfect copy, focusing from min-infinity, and stops at both ends clearly. But the scale mark on the ring does not match the real distance.
All I need to do was to loss the screws on the focus ring and turn the scale to its correct position (infinity to infinity).
I wonder if Klaus is in the same situation.
10-30-2016, 07:27 AM   #11
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Hi, again
I got the plastic ring with the names unmounted, and I am adjusting the ring to the right position. Great. Thanks a lot for the explanation. It was easy. Lifted the rubber with a small knife. Pushed in the whole with a screwdriver, while I separated the front plastic ring from the rest using a small knife. And the 3 screws are accesible with a small screwdriver.

Before I could not focus to infinity, as when selecting infinity on the scale the lens focussed to about 10m. There was no way to focus further away.

Best regards
Klaus
10-30-2016, 07:38 AM - 1 Like   #12
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10 meters just happens to be the last engraved distance on the barrel before infinity. It's only 2-3 arc of travel between them. Setting the lens to the infinity stop and then loosening the screws and adjusting the ring to the 10 m mark should get you in the ball park. Tighten the screws but do not replace the ring and check the focus at infinity and the minimum of 0.45 m. If all is good lock down the screws and replace the trim ring and rubber ring.

Other things to look for on a improperly assembled lens are lack of click stops on the aperture ring (lost ball-bearing), missing or non-functioning "A" pin. The "A" pin on the lens mount should retract when the aperture ring is taken off the "A" setting. Even if it does retract and extend properly there may be grease in the works that make electrical contact intermittent.
10-30-2016, 08:11 AM - 1 Like   #13
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In my K55/1.8 the focusing scale has, with time and use, become separated from the barrel, as the glue dried out and lost grip. Friction with the rotating elements causes it to move.
Check it's not something like this before disassembling everything...
10-30-2016, 08:40 AM   #14
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Thank's to everybody for the advice.

Adjusting has been a bit tricky, as my focussing screens shimming seems not to be perfect (it is not the original one). I didn't have a 1.4 lens before ;-). It seems to be a little bit too thick. Now I got perfect focus on the photos with the focussing ring to infinity. To see best focus on the screen I have to move the ring a little bit towards closer distances.

Best regards
Klaus

---------- Post added 10-30-2016 at 05:01 PM ----------

QuoteOriginally posted by Not a Number Quote
The lens may have been used on a non-Pentax body with a glassless adapter and adjusted for infinity focus for that body.

Is the auto aperture lever intact? People will perform surgery on this appendage too.
The lever looks ok to me.

Thank's for the advice.

Best regards
Klaus
10-30-2016, 04:30 PM   #15
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QuoteOriginally posted by LensBeginner Quote
In my K55/1.8 the focusing scale has, with time and use, become separated from the barrel, as the glue dried out and lost grip. Friction with the rotating elements causes it to move.
yes, you will have to pull that one apart and re-adhere the focus scale. I'd recommend you get in touch with @dcshooter on the best method for restoring this particular lens.
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