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05-06-2021, 09:21 PM - 2 Likes   #1
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Testing Pentax 28mm infinity focus.

Some people have been discussing the difficulty of focusing Pentax 28mms recently.
I was aware that some of mine may not be reaching infinity so I devised a test that would show that.
Little did I realise how bad some were.
In fact I rejected the results of one of my m42 Takumars altogether because it wasn't focusing beyond about 3 metres.
I suspect that everyone using these lenses should do this check.
And I am not going to listen to anyone saying the Tak Bayonets are not up to standard anymore!
It is easy enough to adjust infinity. The biggest issue is likely to be whether the vanity ring wants to be unscrewed.
Remove the vanity ring. Loosen the backmost set of 3 screws and twist the focus ring. The direction to twist is a bit unintuitive. But from memory set your lens up against the infinity stop , loosen the screws and turn the focus ring to closer. Then tighten the screws.It is up to you whether you have it set to infinity or just a fraction past. But having it go past gives you the confidence it is achieving infinity.
It was nicely overcast today and I underexposed a little to keep the white under control.
So all these comparisons are done with the lens wide open.
It looks like all the Ms could do with some adjustment. Crops are close to 100%.
Note the nearest sign at bottom right of crops. It is the 6 metre one and is probably the best indicator of a lens not reaching infinity.

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05-06-2021, 09:57 PM   #2
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Here I have focused on the 12 metre sign (the one with the lens id). Some of the lens were already up against the infinity stop. Some adjustment needed. I think this focus adjustment would make all the lenses pretty even in sharpness. I have to say the old m42 super tak is a sweety though. Oh and correction - it is a S-M-C Tak.
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05-06-2021, 10:00 PM   #3
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I also recommend checking infinity focus for manual Pentax manual lenses if distant objects don't appear to be resolving as expected. I've found it necessary to re-calibrate several, most recently a M 50/1.4 that clearly 'wasn't right' at distant focus. Much improved once re-calibrated although infinity focus isn't a strong point for the M50/1.4 (the M50/1.7 is a better landscape lens).

Not sure if this should be attributed to a factory calibration issue or the fact that manual focus lenses acquired over the last decade or so may have passed through the hands of people adapting them to other camera systems and who may have played with the infinity focus setting to suit their adapted system. Probably a bit of both.
05-06-2021, 10:04 PM   #4
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QuoteOriginally posted by southlander Quote
I also recommend checking infinity focus for manual Pentax manual lenses if distant objects don't appear to be resolving as expected. I've found it necessary to re-calibrate several, most recently a M 50/1.4 that clearly 'wasn't right' at distant focus. Much improved once re-calibrated although infinity focus isn't a strong point for the M50/1.4 (the M50/1.7 is a better landscape lens).

Not sure if this should be attributed to a factory calibration issue or the fact that manual focus lenses acquired over the last decade or so may have passed through the hands of people adapting them to other camera systems and who may have played with the infinity focus setting to suit their adapted system. Probably a bit of both.
I am guessing they have slipped a bit over the years. Also I am wondering if in the case of these 28s that they may have been factory set at the old f2.8 hyperfocal distance. And our demands now are higher than that.

05-07-2021, 06:11 AM   #5
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Major points for actually doing a scientific test!!!!
05-07-2021, 01:03 PM   #6
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QuoteOriginally posted by ProfessorBuzz Quote
Major points for actually doing a scientific test!!!!
It is the only way I could work out how to test for infinity focus.
05-12-2021, 12:44 PM   #7
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QuoteOriginally posted by southlander Quote
I also recommend checking infinity focus for manual Pentax manual lenses if distant objects don't appear to be resolving as expected.
I agree! The cause is most likely a matter of manufacturing, though not sloppiness at the factory, and was explained to me once as a matter of convention related to user experience with the modest viewfinder magnification common to all SLRs. My memory is a little foggy on the specifics, but I believe that Pentax lenses have historically been calibrated with the infinity stop at a distance of 2000x the focal length. My Japan-made 28s and 50s (S-M-C Takumar 28/3.5 and early ST 55/1.8 included) appear to follow that rule.* Past about 50 meters with a 28mm, everything that distance and further appears equally sharp in the viewfinder.

Added: If one has a film camera with a known good lens, infinity collimation with two cameras is very cool. http://rick_oleson.tripod.com/index-123.html


Steve

* I checked before I posted and while not tested on the soccer/football field it is close enough for discussion.


Last edited by stevebrot; 05-12-2021 at 12:59 PM.
06-18-2021, 03:40 PM   #8
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Do you have a Spotmatic around to check if your lenses won't actually focus past infinity on that?
How do you know it's the lens that's bad and not, say, the adapter or even the camera mount?
For example, this guy tried to say something, but his bombastic way of speaking combined with not coming with any irrefutable proof resulted in him getting shot down in flames quite quickly:
An Open Letter to Pentax: K-1 Infinity Focus Problems - PentaxForums.com
To me the topic is interesting because I was shocked my SMC Takumar 28/3.5 and SMC Pentax-A 28/2.8 are so far from infinity on the K-1 II. I calibrated the Takumar for infinity focus using the live view on the K-1, and what do you know, it goes past infinity on my Spotmatics...
[ yeah, yeah, before you ask, I'm not using a flanged M42 adapter ]
06-19-2021, 07:06 PM   #9
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QuoteOriginally posted by kcobain1992 Quote
Do you have a Spotmatic around to check if your lenses won't actually focus past infinity on that?
How do you know it's the lens that's bad and not, say, the adapter or even the camera mount?
For example, this guy tried to say something, but his bombastic way of speaking combined with not coming with any irrefutable proof resulted in him getting shot down in flames quite quickly:
An Open Letter to Pentax: K-1 Infinity Focus Problems - PentaxForums.com
To me the topic is interesting because I was shocked my SMC Takumar 28/3.5 and SMC Pentax-A 28/2.8 are so far from infinity on the K-1 II. I calibrated the Takumar for infinity focus using the live view on the K-1, and what do you know, it goes past infinity on my Spotmatics...
[ yeah, yeah, before you ask, I'm not using a flanged M42 adapter ]
Of course there is only one lens in that lineup that will fit on the spotmatic. But anyway I put the tak on the spottie and it didn't appear to go beyond infinity. Which is what it should do (seeing it is bang in the top image) if the K-1 was different. Of course with the spottie's granular centre circle one can only see so much .
I tried the M mk11 28 2.8 on a MZ-M that was lying around and I could see nothing in the prisms that suggested that it wasn't getting to infinity. But I also feel I wouldn't see the difference anyway.
I wouldn't surprise me that there a slight variation but I would expect the K-1 to be built to a tighter tolerance because it is demanding more of the lens.
06-19-2021, 07:33 PM   #10
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QuoteOriginally posted by kcobain1992 Quote
How do you know it's the lens that's bad and not, say, the adapter or even the camera mount?
When I read that thread what got me sceptical was this comment.
" You only need to spend a little time looking at the Pentax monitor in LIVE VIEW to know something is wrong."
This meant the geometry for both the viewfinder and the sensor light paths conformed with each other.
I feel that is the test when you are deciding whether it is the lens or the camera components. It is unlikely to be the camera mount because that is precision hard-mounted on the metal body. If that was wrong then everyone's camera would be out.
06-20-2021, 02:10 AM - 1 Like   #11
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QuoteOriginally posted by GUB Quote
Some people have been discussing the difficulty of focusing Pentax 28mms recently.
As one of those people, I can add I have spent even more time trying out various MF lenses on my K3 and KP cameras. The latest being a K 35mm F3.5. It too has the same type of problem I have experienced with the A 28mm. I have come to the conclusion that that the small, compact lenses are harder to focus through the viewfinder because of three reasons.
First is the short focus throw of the lenses. My K 28mm f3.5 has a large body and also a relatively large focus throw. My HD DA 35mm Ltd also has a (very) long focus throw. Neither lens is hard to focus through the viewfinder. Indeed, none of my macro lenses are very hard to focus.
Second is the the nature of the modern viewfinders and their screens. They are designed for use with AF lenses which are also relatively slow. This provides a nice bright image for viewing, but is rubbish for manually focusing. The focus confirmation light helps, but it is easy to overlook, and it only flashes briefly. APSc cameras are even worse because of the small viewfinder image. If you compare looking through an old film camera like an MX or ME Super with their large bright finders and screens designed for manual focussing you will easily see what I mean. The difference is like night and day.
Third is the limitation of our own eyesights.. As we get older our eyes deteriorate. So are we certain that our dioptre adjustments are set correctly? I have had to adjust mine recently. It is easy to forget that one's eyes are getting worse as it is a gradual decline that one may not notice for a long time. The other problem with our eyes is that they can find it hard to focus on something if the contrast between it and its background is not that great. Staring through the viewfinder for longer and concentrating harder does not help for me as my eyes will still struggle to see which part of the scene is in or out of focus. Try for long enough and my vision will go swimmy and nothing will look in focus!. Bright lenses though old bright film viewfinders do not present this type of problem.
Having said all that though, I think there may well be infinity focus issues with some of the lenses themselves. I have just sent my A 35mm f2 to be re-calibrated. I would have done it myself, but I can't get the vanity ring off. The lens is so far out of calibration, it almost has macro capabilities at close range!
These focusing issues I have found to apply to not just 28mm or 35mm lenses, but also to telephoto ones. I have given up trying to use MF lenses above 55mm on my KP/K3 cameras, unless they are macro ones. It is just easier to use either my X-T1 or X-H1. The latter recently acquired for just this task. I am not prepared to give up using more standard lenses on my Pentax cameras though, because a) I generally prefer to use an optical viewfinder and b) I just can't get some of the colours I like out of Fuji's without significant post processing. That is another story though!
Your post is very helpful I feel, regarding these matters, and with a little bit of work, hopefully we can start to enjoy these old lenses more, rather than remain being frustrated by them.
06-20-2021, 04:26 AM   #12
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Without a proper master camera or lens, you cannot decide what is right or wrong/ camera or lens. Same for AF fine tuning. The story is quite simple. You need to find the proper distance to get a sharp image on the sensor. Camera and lens need to be in alignment. Typically more care is taken to build the camera to specs, but not always. The SLR viewfinder is a separate system that need to be aligned to the sensor as well. There are shims available for focus screens. Live view has not much to do with alignment, it just shows reality. Sometimes flange distances change slightly, this is worst for wide angle lenses as tiny adjustment changes in the back have large impact in focus precision - at first this sounds counterintuitive, as we expect larger DOF for wider angle lenses.
You should check any lens and camera for correct focus at infinity and also check af precision as well as viewfinder alignment for manual focus.
06-22-2021, 05:50 PM   #13
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Thanks for this post.

I too have noticed this on some of my lenses.


Will it be possible for you to post a tutorial on how to adjust the lenses (with some photos)? TIA
06-27-2021, 05:32 AM - 1 Like   #14
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QuoteOriginally posted by pinholecam Quote
Thanks for this post.

I too have noticed this on some of my lenses.


Will it be possible for you to post a tutorial on how to adjust the lenses (with some photos)? TIA
Sorry I missed this for a few days. The adjustment is not technically difficult.
Older prime lenses achieve focus solely by moving the entire element block closer or further from the sensor. There is no internal adjustment of elements as found with zooms.
This is achieved by a helicoid thread that telecopes the elements in and out. We do not dismantle the helicoid coz that gets complicated.
The focus ring of the lens turns that helicoid. With these Pentax lenses the focus grip has a friction fit on the helicoid that is tightened up via 3 little screws accessed from the front. These 3 screws just need to be loosened (not removed) to achieve adjustment.
To get to these three screws firstly you need to remove the "Vanity Ring". That is the washer like ring that carries the lens description (in older pentax and Taks.) next to the front element. (In the A series it is blank). You need a friction tool to remove this ring. A rubber bath plug may fit or you can buy a set on line.
Rubber Lens Tool 9pcs Set Camera Lens Ring Removal Optical Repair Gripping Spanner|Len Parts| - AliExpress
If you can not remove this ring then there is no work around - the job can't be done. It is up to you if you want to deface the ring to remove it.
Once the vanity ring is off you can see three little screws that hold the filter thread assembly in place. I think in all the 28s the filter thread assembly has to be removed - but in the 50mm lenses I think the next 3 screws are accessible without removing the filter assembly.
So once you have removed the filter thread assembly you should see a similar set of three screws. These are the little suckers we are after!
Probably good if you scratch a mark against one of the so you can go back if necessary.
So the lens is not reaching infinity.
Turn the focus ring to infinity then loosen the screws a little. Turn the focus ring back a little (watching to see the ring is in fact slipping past the screws). Retighten and check.
For me it is a trial and error approach.
It is impossible to know that you are at the perfect point of infinity and I find it gives me a little confidence in the setting if it turns slightly past.

Now I am writing this from memory and I think I have got the different models covered.
But please if anyone wants to add something then do so.
06-27-2021, 05:54 PM   #15
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QuoteOriginally posted by GUB Quote
Sorry I missed this for a few days. The adjustment is not technically difficult.
Older prime lenses achieve focus solely by moving the entire element block closer or further from the sensor. There is no internal adjustment of elements as found with zooms.
This is achieved by a helicoid thread that telecopes the elements in and out. We do not dismantle the helicoid coz that gets complicated.
The focus ring of the lens turns that helicoid. With these Pentax lenses the focus grip has a friction fit on the helicoid that is tightened up via 3 little screws accessed from the front. These 3 screws just need to be loosened (not removed) to achieve adjustment.
To get to these three screws firstly you need to remove the "Vanity Ring". That is the washer like ring that carries the lens description (in older pentax and Taks.) next to the front element. (In the A series it is blank). You need a friction tool to remove this ring. A rubber bath plug may fit or you can buy a set on line.
Rubber Lens Tool 9pcs Set Camera Lens Ring Removal Optical Repair Gripping Spanner|Len Parts| - AliExpress
If you can not remove this ring then there is no work around - the job can't be done. It is up to you if you want to deface the ring to remove it.
Once the vanity ring is off you can see three little screws that hold the filter thread assembly in place. I think in all the 28s the filter thread assembly has to be removed - but in the 50mm lenses I think the next 3 screws are accessible without removing the filter assembly.
So once you have removed the filter thread assembly you should see a similar set of three screws. These are the little suckers we are after!
Probably good if you scratch a mark against one of the so you can go back if necessary.
So the lens is not reaching infinity.
Turn the focus ring to infinity then loosen the screws a little. Turn the focus ring back a little (watching to see the ring is in fact slipping past the screws). Retighten and check.
For me it is a trial and error approach.
It is impossible to know that you are at the perfect point of infinity and I find it gives me a little confidence in the setting if it turns slightly past.

Now I am writing this from memory and I think I have got the different models covered.
But please if anyone wants to add something then do so.

Thanks a lot for this information.
I certainly will try this out on one of my 50/2 lenses.

There are some lenses in my collection which turn up to infinity, but I just think that it never reaches a peak of focus (so they are likely slightly off ).
The worst case so far was my M85/2 which was ok on apsc, but was widely off infinity on K1 (I had it sent for adjustment )
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