[The shooter]

Hello

I have a fine Pentax-A 15mm 1:3.5 - it looks like a new lens. I tried it out on a K1 at different apertures. The corners are muddy, smeared and soft. The centre is reasonably sharp. I then set the K1 to 'catch' focus and I observe that when the lens is set to infinity on the focus ring (there is a hard stop) that the object in focus through the viewfinder is literally only about 3-4 ft in front of the lens. I suspect that the focus helicoid alignment for infinity is way out.

Also tried LV with magnification and focus on distant object at the infinity hard stop.
Close by subject behave better than distant subjects.

When inspecting the lens up close with a loupe through the back and aperture set to f/3.5 no element separation is visible. Some through the front using a strong LED wand light and a loupe.

Anyone have a scan of a service manual or bulletin for this lens? If not anyone had experience of adjusting the helicoid on this model of lens before? Another option could be if there are any lens technicians out there doing such adjustments on old MF lenses?

regards,
Alan

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Last edited by The shooter; 09-20-2022 at 06:32 AM.

[pschlute]

I then set the K1 to 'catch' focus and I observe that when the lens is set to infinity on the focus ring (there is a hard stop) that the object in focus through the viewfinder is literally only about 3-4 ft in front of the lens

But can you tell that objects in the far distance (infinity) are not in focus? The lens has such a wide DOF, and such a wide angle (small subject size) that I would find it hard to trust CIF.

Using LV zoomed 100% may help confirm your findings.

[sblake]

I have a very good copy of this lens and tend to use hyperfocal distance for focusing on the K1ii. I would agree with pschlute that CIF might not work well with this lens, although I have never tried it. I've also taken the lens apart to clean for dust but have never found a manual for it. I don't recall that there was any adjustment for the helicoid, but I am certainly no expert.

[The shooter]

Hi Peter, thanks for your post. Objects in the far distance are fairly sharp - not tack sharp, but considerably sharper than on the edges. Attached is a jpg over the garden at 1/200 and f11 with IS on, hand held. Shot at the infinity hard stop on the focus ring. No adjustments. If you zoom in on the central palm tree, you will see that its reasonably sharp. This is about 50m (160 ft) from the camera. The sides are very soft / blurred mush. At f11 I expected the whole frame to be sharp. I also tried the different turret filters (UV, skylight and no filter) - does not seem to make a difference. Needless to say at f/3.5 it is a lot worse.

Insights appreciated.....
Alan

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Last edited by The shooter; 09-20-2022 at 06:27 AM. Reason: add info

[pschlute]

The sides are very soft / blurred mush. At f11 I expected the whole frame to be sharp

I own the "K" version of this lens (optically identical) and it is certainly a lovely bit of kit to work with. However if you are expecting the whole frame to be equally sharp, you are going to be disappointed, The edges will be noticeably softer than the centre. Having said that, there is something going wrong with your image.The edges should not be "that" soft !

Can you link to an image on a site like Flickr, so we can see a full size image (unfortunately posting directly is no good as the forum resizes).

I suggest checking the infinity focus using Live View, magnified 100%. Same for closer objects. This will help to rule out a simple mis-focus. It is quite possible the lens has been dropped or taken apart and not reassembled correctly.

I took this image with the K 15mm 3.5 at f8 The edges are perfectly acceptable. Like sblake above, I just use hyperfocal distance with f8 and rarely try to actually focus the lens itself.

[pid]

is there lens separation or smud on the backside lensgroupe?

[The shooter]

I own the "K" version of this lens (optically identical) and it is certainly a lovely bit of kit to work with. However if you are expecting the whole frame to be equally sharp, you are going to be disappointed, The edges will be noticeably softer than the centre. Having said that, there is something going wrong with your image.The edges should not be "that" soft !

Can you link to an image on a site like Flickr, so we can see a full size image (unfortunately posting directly is no good as the forum resizes).

I suggest checking the infinity focus using Live View, magnified 100%. Same for closer objects. This will help to rule out a simple mis-focus. It is quite possible the lens has been dropped or taken apart and not reassembled correctly.

I took this image with the K 15mm 3.5 at f8 The edges are perfectly acceptable. Like sblake above, I just use hyperfocal distance with f8 and rarely try to actually focus the lens itself.

Hey Peter - iconic shot of the concorde nose - even the individual rivets are visible - at the edge too.
Sadly my lens is not doing anything close to that! Will pop a shot onto Flickr tomorrow and send the link.
Thanks for the input. I am starting to wonder if this lens was opened up and one of the elements put back incorrectly (eg a singlet put backwards could do this). No signs of dings and no dents or cracks visible on close inspection through the lens with a loupe against the light fully open. Also no separation, dendrites, haze, fungus or dust. Actually too clean for an old lens which suggests it may have possibly been opened up to clean out dust between elements. I got it 10 years ago from a dealer in Hamburg, Germany and its been in a controlled cabinet with my collection since then. (...when I got this lens there was not even a K1 on the horizon!).
No to find a lens element diagram so that my technician can open it up and compare!

PS I tried live view as well at close focused subject as well as distant - still the same characteristic, but worse on distant subjects than close subjects. Using LV looks like focus on the infinity hard lock position is about 2m (6ft) from the object focused in LV. Very strange - Perhaps supports the view that the lens has been opened and incorrectly reassembled.

Kind regards from cape Town, Alan

---------- Post added 09-20-22 at 12:35 PM ----------

is there lens separation or smud on the backside lensgroupe?

Hi pid. No visible separation under close examination. No 'smud' on the back lensgroup either - elements appera very clean and clear.

[pschlute]

No to find a lens element diagram so that my technician can open it up and compare!

Here you go my friend. http://kmp.pentaxians.eu/wp-content/uploads/bojidar/lenses/primes/_optics/15f3.5-ii.gif

[The shooter]

Here you go my friend. http://kmp.pentaxians.eu/wp-content/uploads/bojidar/lenses/primes/_optics/15f3.5-ii.gif

Thank you for the lens diagram - very helpful.