[cg-4k50]

Would anyone know the procedure to adjust the infinity stop on this lens ? Will not quite reach infinity with my K50 Thanks...

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Last edited by cg-4k50; 09-06-2019 at 12:00 PM.

[FozzFoster]

I'm not 100% sure, but this lens looks different the ones I've seen before - I think.
Are you sure it's m42 and not m39? Do the threads fit very well on a m42 camera?
Maybe the register distance is different for m39 lenses?
I'm honestly just spit ballin' and idea here...

[cg-4k50]

I'm not 100% sure, but this lens looks different the ones I've seen before - I think.
Are you sure it's m42 and not m39? Do the threads fit very well on a m42 camera?
Maybe the register distance is different for m39 lenses?
I'm honestly just spit ballin' and idea here...

I bought the M42 adapter for my K50 and its a good fit. I also bought the 135mm f3.5 that was with this kit to the old Sears film camera that also did not reach infinity with my K50 but found directions on how, this lens is different however.

[paulh]

Did you get the OEM type Pentax adapter or the flange-style adapter? The flange type does not allow infinity focus.

[stevebrot]

I bought the M42 adapter for my K50 and its a good fit. I also bought the 135mm f3.5 that was with this kit to the old Sears film camera that also did not reach infinity with my K50

Two lenses and neither will attain infinity focus? I would put the blame on the adapter. Do you have a photo of it? A proper adapter will allow the lens to take its flange registration from the camera's mount face. If the lens rear flange does not contact the camera side of the mount, the adapter is at least partly and probably totally to blame. FWIW, your lens is probably 1970s vintage. Late 1960s or early 1970s.

Edit: I am reviewing previous posts regarding these two lenses. At one point, the OP was thinking they needed a T-adapter for the 135mm. It may also be that the failure to attain infinity focus applies to astrophotography. If so, very few manual focus primes at either 28mm or 135mm will have been calibrated to provide fine focus beyond relative close distances (less than half a kilometer).


Steve

[cg-4k50]

Two lenses and neither will attain infinity focus? I would put the blame on the adapter. Do you have a photo of it? A proper adapter will allow the lens to take its flange registration from the camera's mount face. If the lens rear flange does not contact the camera side of the mount, the adapter is at least partly and probably totally to blame. FWIW, your lens is probably 1970s vintage.

Edit: I am reviewing previous posts regarding these two lenses. At one point, the OP was thinking they needed a T-adapter. It may also be that the failure to attain infinity focus applies to astrophotography. If so, very few manual focus primes at either 28mm or 135mm will have been calibrated to provide fine focus beyond relative close distances (less than half a kilometer).


Steve

I'm sure I have the correct adapter, no flange,lens screws tight to camera mount face. Here is a pic set to infinity, does it look in focus ? The tree just above the roof peak is about 100+ feet away but looks out of focus. I know not the best photo to check focus.

[stevebrot]

I'm sure I have the correct adapter, no flange,lens screws tight to camera mount face. Here is a pic set to infinity, does it look in focus ?

At Web resolution...yes, it appears to be in focus for all elements in the frame. That being said, it would not surprise me if the lens were calibrated such that the plane of focus at the infinity stop might be as close as 55 meters distant and perhaps even closer.* You may be able to adjust to a further distance appropriate for astro, but I can provide no guidance for that task. Be aware that 28mm lenses of that vintage often have a fair amount of field curvature such that the the corners and edges may not attain the same quality focus as the center.


Steve

* This is not unreasonable given the DOF at commonly used apertures, the capabilities of the viewfinder focusing systems without supplemental magnification, even with a split image, and the mechanical limitations of the focus mechanism. I just checked an early 1970s Kipon-made Vivitar 28/2.5 and it is calibrated to about 30 meters at the infinity stop.

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Last edited by stevebrot; 09-06-2019 at 01:49 PM.

[cg-4k50]

Did you get the OEM type Pentax adapter or the flange-style adapter? The flange type does not allow infinity focus.

Yes the adapter inserts flush to the camera mount face...

[cg-4k50]

Was able to adjust and now can go past infinity all is good. First pic at infinity before adjustment, distance about 200'

[ZombieArmy]

A lot of older manual lenses aren't calibrated for proper infinity because you couldn't really tell on film when stopped down anyways. I've got quite a few I need to adjust myself.

[Ontarian50]

A lot of older manual lenses aren't calibrated for proper infinity because you couldn't really tell on film when stopped down anyways. I've got quite a few I need to adjust myself.

Oh, you can tell.

However, I do agree that many older lenses aren't properly calibrated. A lot of third party lenses, like the Sears one in this thread, likely didn't get exact calibration and quality control on their way down the assembly line. And it's true that if the average punter who bought budget optics took all vacation snaps at F8 or F11, they might never notice it didn't quite reach infinity.

But given that most old lenses of this type use a friction ring to clamp the focus ring onto the helical, a small slip here and there over time results in a lens that either goes well past infinity, or comes up short.

A top grade Takumar or Nikkor should have been spot on when it left the factory. But time, cold and hot weather, sticky greases (although Takumars pretty much never get sticky), can mess up that precise calibration.

[cg-4k50]

At Web resolution...yes, it appears to be in focus for all elements in the frame. That being said, it would not surprise me if the lens were calibrated such that the plane of focus at the infinity stop might be as close as 55 meters distant and perhaps even closer.* You may be able to adjust to a further distance appropriate for astro, but I can provide no guidance for that task. Be aware that 28mm lenses of that vintage often have a fair amount of field curvature such that the the corners and edges may not attain the same quality focus as the center.


Steve

* This is not unreasonable given the DOF at commonly used apertures, the capabilities of the viewfinder focusing systems without supplemental magnification, even with a split image, and the mechanical limitations of the focus mechanism. I just checked an early 1970s Kipon-made Vivitar 28/2.5 and it is calibrated to about 30 meters at the infinity stop.

Well was able to focus on the MW last night and like you said not a great lens for AP. Don't need to look far from the center of image to see the field curvature. Stopped down to F8, F4 was vary bad. Two min exposure ISO 800 bortle 8 sky.

[stevebrot]

However, I do agree that many older lenses aren't properly calibrated.

Define properly calibrated.* Limitations regarding infinity stop are common to every prime lens on my shelf save two, my DA 50/1.8 and Sigma EX DG 50/2.8 Macro. None of the others (my recent FA 77/1.8 Limited and FA 35/2.0 included) can attain focus past about 2000X the focal length distance.** Based on an article I read several years ago (have not been able to find it since) detailing the industry conventions regarding infinity calibration, I believe this is intentional and that my lenses have not "drifted close" with age.


Steve

* I should have put a smiley there because there are differences between practical (based on CoC), desirable (the stars), and absolute (optical bench at arbitrary precision) with desirable being the most difficult to satisfy.

** Pentax K-3, magnified live view, 21° C at maximum aperture confirmed with actual exposure.

[ZombieArmy]

I believe this is intentional and that my lenses have not "drifted close" with age.

I'd say this is generally true but it's not hard to tell when a lens is out of spec. For example my A50 1.4 simply won't focus past about 20 feet and needs repair.

Personally I've just come to like lenses that focus past infinity, makes my life easier.