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01-17-2016, 07:54 AM   #1
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DA 15mm Infinity focus question

Hello All, I noticed an unusual characteristic with my DA 15 mm SMC Limited. When I focus on a distant scene or object the focus ring never goes completely to infinity, stopping slightly before infinity. There is no mechanical issue with the focusing ring, it turns smooth end to end with no looseness or wobble. Distant Pictures at infinity look fine. Is this a normal characteristic of this lens or is there a potential problem? None of my other lenses behave like this and it is like this both on K-5 and K-3 so I know it is the lens, not the camera.

Thanks, John

01-17-2016, 08:59 AM   #2
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QuoteOriginally posted by jddwoods Quote
When I focus on a distant scene or object the focus ring never goes completely to infinity, stopping slightly before infinity.
John, my copy of this lens has the exact same quirk.
The true focus distance on mine is given about where the f/4 depth-of-field mark is,
not by the index mark on the scale.

Unless you want to focus manually using the distance scale,
it doesn't really matter.

If you do want to focus manually,
first use live view auto focusing with the center focus point on a contrasty distant object,
see where the infinity mark lands on the scale,
and use that point as your correct index mark.
01-17-2016, 09:34 AM   #3
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QuoteOriginally posted by jddwoods Quote
When I focus on a distant scene or object the focus ring never goes completely to infinity, stopping slightly before infinity.
Is that with viewfinder AF ?
01-17-2016, 11:27 AM   #4
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QuoteOriginally posted by kh1234567890 Quote
Is that with viewfinder AF ?
Both viewfinder, which I use most, and live view, which I only use occasionally behave the same when I focus on a distant object. The autofocus works fine and pictures are sharp. It is just that the lens infinity point is not on the infinity mark. Given that my pictures with this lens are fine, it is more a curiosity question rather than a worry about the lens. As Lytrytyr states I am not the only person with this observation so it may just be a characteristic of the lens. I attached an example with infinity focus. I may not show on the attached file, but if I zoom in on the picture you can make out the stars on the US flag. which would indicate the focus is accurate

Attached Images
View Picture EXIF
PENTAX K-3  Photo 
01-17-2016, 11:28 AM - 1 Like   #5
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QuoteOriginally posted by jddwoods Quote
Is this a normal characteristic of this lens or is there a potential problem?
John,
this does not indicate any potential problem. As for the first part of the question, it is a tricky one (assuming you carefully examined your photographs and you confirmed the lens focused properly at infinity. If you didn't, then you'd better have this done since you may just need to AF-adjust this lens, see K3/5 user manual for AF adjustment guide):
- many DA15 lenses behave this way;
- but some - lucky ones - do not;
- this is due to Pentax low quality control (but Pentax isn't the only one, see below);
- you can have this lens serviced at a shop or on your own, you only need screwdriver set (JIS preferred over Philips) and a rubber tool to remove the front ring. The lens disassembly procedure has been covered here on pentaxforums several times;
- many lads and lassies consider this behavior to be okay and normal, but the truth is that it isn't (this isn't normal even for short-tele lenses, let alone ultra-wide angle prime one), it's all because of cost savings which is the main trend of nowadays;
- if you don't do manual focusing and if you don't use any software in post-production which utilizes EXIF focus distance data to correct the perspective distortions, then you can leave this as is.

Cheers,
Zig
01-17-2016, 11:36 AM   #6
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Perfectly normal for lens like this (every manual focus lens I've ever owned did this).
01-17-2016, 11:43 AM   #7
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QuoteOriginally posted by jeffryscott Quote
Perfectly normal for lens like this (every manual focus lens I've ever owned did this).
My copy of the DA 15 is the only one of my many Pentax lenses, MF and AF, which noticeably exhibits this behavior
(of course, excluding those without a distance scale, which in my case only means the DA 40 XS, DA L kit lenses, and Q lenses).

It is not "normal", since it means that you cannot directly rely on the distance scale for quick-and-dirty, zone, or hyperfocal manual focusing.

---------- Post added 01-17-16 at 12:49 PM ----------

QuoteOriginally posted by Siegfried Quote
- if you don't do manual focusing and if you don't use any software in post-production which utilizes EXIF focus distance data
Do you mean that the way the distance scale ring happens to be attached
would correlate directly with how the lens reports focus distance?
A priori, they would seem to be two separate features,
even as they do move in tandem when the lens focuses.

Last edited by lytrytyr; 01-17-2016 at 11:50 AM.
01-17-2016, 11:57 AM   #8
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QuoteOriginally posted by lytrytyr Quote
My copy of the DA 15 is the only one of my many lenses, MF and AF, which noticeably exhibits this behavior
(of course, excluding those without a distance scale, which in my case only means the DA 40 XS, DA L kit lenses, and Q lenses).

It is not "normal", since it means that you cannot directly rely on the distance scale for quick-and-dirty, zone, or hyperfocal manual focusing.
Trust me, it is normal. Temperature variations, expansion/contraction, variations in materials and construction, etc ... No lens, no matter the manufacturer, can be built to such tight tolerances that a focus scale would be perfect in every conceivable use. Thus there is designed play, which is why lenses don't stop at infinity.

Do a google search, these questions arise with Nikon, Canon, etc ...., the answer is the same, it is normal.

01-17-2016, 12:04 PM   #9
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QuoteOriginally posted by jeffryscott Quote
Trust me, it is normal. Temperature variations, expansion/contraction, variations in materials and construction, etc ... No lens, no matter the manufacturer, can be built to such tight tolerances that a focus scale would be perfect in every conceivable use.
We are talking about _noticeable_ deviation, not obviously unavoidable normal minor variations.
QuoteOriginally posted by jeffryscott Quote
Thus there is designed play, which is why lenses don't stop at infinity.
Lenses with "hard infinity" are designed to do just that.
I have several, and they're very useful for nighttime landscapes!
01-17-2016, 01:12 PM   #10
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QuoteOriginally posted by jddwoods Quote
which would indicate the focus is accurate
Interestingly, the EXIF Makernotes value of the focus range index on that shot is 2, rather that 4, which it should be at infinity.
01-17-2016, 01:17 PM   #11
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My DA15 does this too, never been a problem I worried about. If handheld I use the viewfinder, if tripod live view. If used at night, I know to very slightly back it off from the infinity mark. I have a manual Bower 14mm lens with distance marks not even close. While there are ways to move the distance scales to correct the reading, I went a different route. I placed a piece of tape over the scale, then used live view on a tripod to focus at different distances, from 1.5 feet to infinity, and made my own marks on the tape to use. The field of focus is so wide on this lens that all I have to do is line up the distance with the new marks. If on a tripod I can still use live view to confirm. This makes it very easy to use at night when I can't see much.
01-17-2016, 06:36 PM   #12
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QuoteOriginally posted by lytrytyr Quote
Do you mean that the way the distance scale ring happens to be attached
would correlate directly with how the lens reports focus distance?
lytrytyr,
yes, I do. Those focus distance tracks which focus distance fork runs on are designed to mean what the distance scale is telling you, though Pentax doesn't do this thing that precise as Canon does, so far I've seen only 4 codes: macro as MFD, close focus, distant focus and hyperfocal a la infinity while Canon EXIF data can be used almost as a ruler. My DA15 copy was so badly factory-adjusted that it never showed me infinity in the EXIF data (or I had to manually twist it beyond infinity[*]) until I cracked it up and had it adjusted by myself: https://www.pentaxforums.com/forums/10-pentax-slr-lens-discussion/145797-da15...ml#post3428544


(photo snatched from DCShooter's DA15 disassembly guide)

Zig
[*]
update:
I just looked at the published photo and it appears that my memory may no longer serve me right: probably it is the MFD (= Min Focus Distance) focus distance EXIF code that never appeared in the EXIF data until I had the lens adjusted.

update2: I just checked my old shots and it looks like I was more correct at the former rather then later. It is the infinity focus which was coded as 'close focus' with factory infinity lens focus setting.

Last edited by Siegfried; 01-17-2016 at 06:53 PM.
01-17-2016, 10:01 PM   #13
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Mine does the same. It's never bothered me.
01-18-2016, 02:51 AM   #14
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QuoteOriginally posted by Siegfried Quote
so far I've seen only 4 codes: macro as MFD, close focus, distant focus and hyperfocal a la infinity
Mine gives six codes (0-4 and 7). The difference between 3 and 4 is really small and 4 covers only a tiny rotational angle.

4 is actually spot on infinity - confirmed by tests using liveview and test shots. With a properly shimmed Canon S-type focusing screen you can actually tell through the viewfinder when you hit infinity focus - you get a brief 'shimmer' in the view. 7 is beyond infinity or when the camera had not found focus.

On my unadjusted DA15 the infinity focus point physically corresponds to about the middle of the right loop of the infinity sign.

PDAF on my K-5ii often gets the focus wrong. If you start AF from somewhere near the proper focus position the camera often does not move the lens at all. The only (semi)reliable way to AF is to manually turn the lens to some near distance first. Even then it often stops rotation short of the true infinity focus point, suggesting that the camera PDAF can't distinguish the best focus point for whatever reason. My old K-7 behaves similarly, but is slightly better.

No doubt someone will tell me that it makes no difference because of some dubious depth of field calculation and suggests that I stop the lens down. It makes a difference to my eyes and I don't see the point of having a f4 lens which you can only use at f8 and above. My DA16-45 has no such problems.
01-18-2016, 03:49 AM   #15
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QuoteOriginally posted by kh1234567890 Quote
.

PDAF on my K-5ii often gets the focus wrong. If you start AF from somewhere near the proper focus position the camera often does not move the lens at all. The only (semi)reliable way to AF is to manually turn the lens to some near distance first. Even then it often stops rotation short of the true infinity focus point, suggesting that the camera PDAF can't distinguish the best focus point for whatever reason. My old K-7 behaves similarly, but is slightly better.

No doubt someone will tell me that it makes no difference because of some dubious depth of field calculation and suggests that I stop the lens down. It makes a difference to my eyes and I don't see the point of having a f4 lens which you can only use at f8 and above. My DA16-45 has no such problems.
Mine doesn't do that on either K-30 or K-S1, Kh, and no weak corners/edges with centre focus that you've reported with yours ... must be a dud copy.
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