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03-26-2018, 10:51 AM   #16
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QuoteOriginally posted by BruceBanner Quote
What's the deal with infinity adjustments... like... how can you tell if you suspect the lens you own may benefit from it?

I own a SMC Pentax-A 24mm 2.8, I enjoy a lot of the shots it produces but sometimes I feel that when I push it to focus infinity there is still a softness there that shouldn't be, like it needs to go a tad farther. It could be me of course (user error), but I did note that a 'For Sale' post recently for a partially broken A 24mm 2.8 (missing piece to make it work in 'A' mode) also had an infinity adjustment made during it's attempted repair. It got me thinking... maybe mines needs it to?

a) how to test?

b) how to adjust?

Cheers!

Bruce
Your lens is a vintage one and may need servicing.
I recently had a F135 f2.8 serviced for other reasons, and when it came back, it could not focus further than 30 meters away. Like other posters have stated above, the guy who serviced it told me in these film era lenses, there are some inner screws to calibrate the focus, and he had just forgotten to check these. Now it is OK.

03-26-2018, 01:07 PM - 1 Like   #17
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CRKennedy

QuoteOriginally posted by BruceBanner Quote
Maybe it's worth posting off to Pentax Australia for an inspection (I forget the people that handle that stuff for Australia, they're really good IIRC).
Pentax Repair Facilities (Wordwide listing) - PentaxForums.com
03-26-2018, 10:26 PM - 2 Likes   #18
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I took this pic just now;



Please have a look at the full res version and pay attention to the telegraph pole on the right vs the large tree behind it on the left.

I took this shot at f8 and set to infinity. I'm just tripod mounted on my garden path looking towards the direction of my neighbours house on the left (red bricked). I measured the distance from where I roughly stood and took the shot to the telegraph pole on the right (the one that is sharp and in focus), that's 20 metres. I cannot accurately gauge the distance from where I stood to the large tree on the left but looking at the distance from where I stood to the telegraph pole (20m) I would say the telegraph pole is approximately halfway between me and the tree, a little less perhaps, at a round guess I would say the tree is 50-60m from the position I took the shot, it is definitely not 100m or more.

Now my eyes are telling me that the telegraph pole is really the last object that is sharp (furthest away). I would have expected the large tree on the left which is further away to be sharper and the pole less. This is only f8 mind you, perhaps its hyperfocal stuff and I need to change aperture etc, but I think this is why I continually feel as though this lens needs an infinity adjustment because anything from 0-20m is a great shot, further than 20m and I just cannot get sharpness or focus.

Thoughts?

I took this picture below with the FA50mm 1.4, with an aperture of f8, set to infinity focus;



The nearest cliff face on the left is sharp, the infinity cliff faces on the horizon are sharp. Of course I tried the same shot with the SMC Pentax-A 24mm 2.8 and both cliff faces are blurry and out of focus when using same aperture, location and set to infinity.
03-26-2018, 11:05 PM   #19
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Part of me wants to say its Ok and your just over pixelating the image. But then again , this image is a bit better than mine when I realized mine was coming up short.
Its a tough call , but I think its a hair short of infinity.
Now you've got me wanting to go check out my replacement copy of the SMC-A 24mm Ive barely even looked at......lol.


Last edited by Ronald Oakes; 03-26-2018 at 11:18 PM.
03-27-2018, 12:25 AM   #20
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QuoteOriginally posted by Ronald Oakes Quote
Part of me wants to say its Ok and your just over pixelating the image. But then again , this image is a bit better than mine when I realized mine was coming up short.
Its a tough call , but I think its a hair short of infinity.
Now you've got me wanting to go check out my replacement copy of the SMC-A 24mm Ive barely even looked at......lol.
Maybe I need to go to a golf range or something with distance markers, and then tripod up, fix the aperture right and then focus infinity and see what you get.

I guess the issue for me here is expectations of what I think infinity should manage on the 24mm lens in question, it really feels I can only work 0-20 meters, which makes landscapes and the like not really plausible. I compare what I can get with my DA 15mm mounted on my KP and in terms of infinity sharpness (like those mountains) and capturing details of things miles in the distance... it's night and day difference. It's why I stopped taking the Pentax-A 24mm around with me as everything was soft when trying landscapes with great distances involved.

I've contacted CR Kennedy and am awaiting on a quote if such an adjustment is warranted. I think I paid around $200AUD for mine, so there's a limit before I think whether it's worth spending too much on it or not.
03-27-2018, 01:16 AM   #21
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Why you don't use AF confirmation on MF lenses?
With AF lenses you can use the AF to set the focus point ohn the horizon.
03-27-2018, 02:09 AM   #22
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QuoteOriginally posted by BruceBanner Quote
I took this pic just now;



Please have a look at the full res version and pay attention to the telegraph pole on the right vs the large tree behind it on the left.

I took this shot at f8 and set to infinity. I'm just tripod mounted on my garden path looking towards the direction of my neighbours house on the left (red bricked). I measured the distance from where I roughly stood and took the shot to the telegraph pole on the right (the one that is sharp and in focus), that's 20 metres. I cannot accurately gauge the distance from where I stood to the large tree on the left but looking at the distance from where I stood to the telegraph pole (20m) I would say the telegraph pole is approximately halfway between me and the tree, a little less perhaps, at a round guess I would say the tree is 50-60m from the position I took the shot, it is definitely not 100m or more.

Now my eyes are telling me that the telegraph pole is really the last object that is sharp (furthest away). I would have expected the large tree on the left which is further away to be sharper and the pole less. This is only f8 mind you, perhaps its hyperfocal stuff and I need to change aperture etc, but I think this is why I continually feel as though this lens needs an infinity adjustment because anything from 0-20m is a great shot, further than 20m and I just cannot get sharpness or focus.

Thoughts?

I took this picture below with the FA50mm 1.4, with an aperture of f8, set to infinity focus;



The nearest cliff face on the left is sharp, the infinity cliff faces on the horizon are sharp. Of course I tried the same shot with the SMC Pentax-A 24mm 2.8 and both cliff faces are blurry and out of focus when using same aperture, location and set to infinity.
One thing to note too is that these old wides (typically these 24mm ) have field curvature and often, the sides are nearer in focus than the center.
So if your lens can't reach infinity for the center, it can be much worse on the off center.
Add to that, the off center having weaker sharpness that the center and it can look bad.

The right side looks better than the left, so there might be some decentering too.

I do think it can be better though and the infinity of the lens is probably too short.

For the price of servicing, its often cheaper to buy another lens.
Lets see what the quote for the lens is.....

03-27-2018, 02:23 AM   #23
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QuoteOriginally posted by angerdan Quote
Why you don't use AF confirmation on MF lenses?
With AF lenses you can use the AF to set the focus point ohn the horizon.
Can you explain this a little more, I'm not sure what you're getting at? With MF lenses all you can do is get AF confirmation on centre point, I'd have to aim the centre of the lens at the horizon, I'm pretty sure it would 'beep' to confirm focus, it's just not that 'focused' tho huh.

---------- Post added 03-27-18 at 08:28 PM ----------

QuoteOriginally posted by pinholecam Quote
One thing to note too is that these old wides (typically these 24mm ) have field curvature and often, the sides are nearer in focus than the center.
So if your lens can't reach infinity for the center, it can be much worse on the off center.
Add to that, the off center having weaker sharpness that the center and it can look bad.

The right side looks better than the left, so there might be some decentering too.

I do think it can be better though and the infinity of the lens is probably too short.

For the price of servicing, its often cheaper to buy another lens.
Lets see what the quote for the lens is
.....
That might be true, but I bet the next A 24mm would 'feel' the same and could also probably benefit from a service.

It's a shame, because I'm looking at this A 24mm as my K-1 equivalent of the HD DA15mm on my KP (they both have a similar FoV). The A 24mm prolly gives more pleasing sun/star burst effects, and I think it feels a lot sharper around the edges than the DA15mm can get. Factor in that it's also about the same size as the DA15mm and 52mm thread means it takes my current line up of ND filters I have. It would also therefore give 36mp shots of a similar FoV vs the 24mp the KP tops out at. It's real nice to get a semi decent landscape lens in a small package, I would just like to make sure it works to the best of it's ability.

It might be I continue to use the DA15mm for very far distances, and more of the A 24mm for close to medium stuff (street photography and the like).

If it's around the $100AUD mark I think I would definitely do it, I've come this far...
03-27-2018, 02:32 AM   #24
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QuoteOriginally posted by angerdan Quote
Why you don't use AF confirmation on MF lenses?
If you have ever tried it....there is quite a bit of area it will "Beep" or confirm in. Not so good.
03-27-2018, 02:37 AM   #25
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QuoteOriginally posted by Ronald Oakes Quote
If you have ever tried it....there is quite a bit of area it will "Beep" or confirm in. Not so good.
That's what I'm getting at, it's a pretty big 'AF focal zone', I'm not sure it helps much in this case.
03-28-2018, 02:03 AM   #26
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From the same location (garden path, shooting towards the the telegraph pole and large tree on the left) I took some more shots this evening. I also kept the tripod where it was and tested the KP with its DA15mm f4 attached, to compare the FoV as well as sharpness at similar apertures to the K-1 with the A 24mm 2.8 etc.
I was quite surprised, I would have thought the DA15mm easily trump the A 24mm but really they came very close in focus and quality. So maybe the lens doesn't have an issue after all. But then perhaps the game changes substantially when shooting out towards very long distances miles away (mountains on the horizon etc), because there seemed to be a reason why I switched to predominately using the DA15mm.

I will say this however, I took a couple of pixelshifted images from the same point, and that made a really pronounced difference, like obviously perceived sharpness was better but actual focus looked better, blurry things appeared much less blurry. Sometimes with other lenses and I pixelshift, the change I feel is not that dramatic, like it's not overly worth it. It might be that the lens is already sharp and it doesn't help that much, but on other lenses it really assists?
03-31-2018, 06:22 AM - 1 Like   #27
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I own the same lens, it's not the best Pentax (it's the same project as the old second generation K series one), but it should perform at infinity as well as at mid distance. Adjusting the focusing helicoid should be cheap enough to justify the investment. Any decent repairman is perfectly capable to do that. If you can find one close to your house you'd also save on delivery expenses. No need to pay the premium for the official Pentax repair center.

If you decide to part with your lens, and you like to consider another one with A contacts, the Sigma Super Wide II 2.8/24mm is probably the best choice.
It's not as nicely built as the Pentax-A, but optically is at another level.
I have a pdf of an MTF test done in Germany in the early eighties, where the Sigma beats all the other contenders. Second is the Leica R.
The Pentax-A beats most third party lenses but as far as far as I remember was the last or second last of the big brands.
The differences, as it often happened with rather extreme wide angles, were quite impressive.
This is to say that the 24mm is a good lens stopped down, but it is not on par with modern wides of the same brand... and also the best 3rd party offerings. This is why I have a Samyang

If anybody is interested in the test, I will find it and post a couple of jpg's.

Cheers

Paolo

Last edited by cyberjunkie; 03-31-2018 at 11:59 AM.
04-01-2018, 04:46 PM   #28
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QuoteOriginally posted by cyberjunkie Quote
I own the same lens, it's not the best Pentax (it's the same project as the old second generation K series one), but it should perform at infinity as well as at mid distance. Adjusting the focusing helicoid should be cheap enough to justify the investment. Any decent repairman is perfectly capable to do that. If you can find one close to your house you'd also save on delivery expenses. No need to pay the premium for the official Pentax repair center.

If you decide to part with your lens, and you like to consider another one with A contacts, the Sigma Super Wide II 2.8/24mm is probably the best choice.
It's not as nicely built as the Pentax-A, but optically is at another level.
I have a pdf of an MTF test done in Germany in the early eighties, where the Sigma beats all the other contenders. Second is the Leica R.
The Pentax-A beats most third party lenses but as far as far as I remember was the last or second last of the big brands.
The differences, as it often happened with rather extreme wide angles, were quite impressive.
This is to say that the 24mm is a good lens stopped down, but it is not on par with modern wides of the same brand... and also the best 3rd party offerings. This is why I have a Samyang

If anybody is interested in the test, I will find it and post a couple of jpg's.

Cheers

Paolo
Interested
04-01-2018, 06:05 PM - 2 Likes   #29
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The lens may well need infinity adjustment. That may be relatively straightforward (simple reposition of focus ring relative to helicoid...done without disassembly) or fine adjust of optical block position ($$). Things that make this difficult to assess:
  • Relatively short focus throw for most vintage ultrawides (yes 24mm is UW on 24x36) makes infinity stop adjustment difficult
  • Arbitrary definition for "infinity" as "4000" times focal length for factory calibration*
  • Field curvature (sort of comes with the territory for 24mm lenses of that vintage)
  • Lateral chromatic aberration may defeat proper off-axis focus. The photo above has significant LaCA in the region of the large tree to the left of center. I would do a CA correction and post up a snippet, but don't have access to my regular tools.
  • Possible centering and/or axial alignment issues. Combined with field curvature, either can make infinity focus (fine focus in general) almost impossible.


Steve

* That may seem a little strange, but given that two posts 0.1 meter apart at 200m with a 50mm lens may well fail to resolve any better at that distance than at 2000m, depending on capture medium and format. It just sounds a bit unreasonable to calibrate infinity at 96m with a 24mm lens.

Last edited by stevebrot; 04-01-2018 at 06:10 PM.
04-04-2018, 04:46 PM - 1 Like   #30
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QuoteOriginally posted by BruceBanner Quote
Interested
German magazine Color Foto
January 1986
German language

Considering the type of lens, I think the most interesting values are the dark blue and the dark red.
The first is the performance at the center, wide open.
The second is the overall performance (center and borders) wide open.
The light red and light blue refer to f/5.6 performance.
Landscape photographers should consider the borders. I don't care very much, given the kind of pics I shoot, of course YMMV.
The Sigma wins almost everywhere. Impressive.

Credit is given to whoever posted the pdf on the Internet. I couldn't find the original URL, so I saved to jpg and now I'm posting it.

It's a huge file, better if you download it and browse it on your computer.
Here is the link:

1-86 Color Foto Test of 24mm lenses


Here is a low res one:



cheers

Paolo

Last edited by cyberjunkie; 04-05-2018 at 11:16 AM.
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