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09-17-2015, 11:20 PM   #1
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Problem with autofocus confirmation at Infinity on DA 40mm

Hi Folks,

I purchased a new lens and it arrived today, a HD DA 40mm F2.8. Unfortunately I seem to be having a small problem with it.

On my K5 ii I can't seem to achieve focus lock confirmation on objects in the distance. I don't appear to have any front focus or back focus issues (I tested by putting the camera on a tripod & focusing on a barcode using both contrast & phase detect AF and ensuring the results are the same), & if I put the lens on my k500 it works perfectly. This is the first lens I own where you cant focus past infinity (i.e. turn the focus ring past the infinity symbol), I wonder if this could have anything to do with it?

Has anyone got any ideas?

09-18-2015, 12:16 AM   #2
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If it works fine on one camera but not the other, a small AF adjustment will most likely help. I'd repeat the FF/BF test under more controlled conditions, but first why not try a -5 adjustment for starters to check if that solves the issue?

Adam
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09-18-2015, 01:49 AM   #3
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Thanks for the reply Adam. Increasing the AF adjustment resolved my infinity focusing issue, so I went back and did the FF/BF test again. This time I changed what I did (rather than using a single object and relying on the difference between phase & contrast to determine whether there was an issue I lined up 5 equally spaced apart diagonally) and it was quite obviously back focusing in my second test. In the end I adjusted to +9 which seemed to give me the results I wanted. I also carried out the same test with my sigma which although I never had any problems with in the field appears to be balanced at -2.

Is this something everyone checks when they get a new lens? I must confess up until now I have always just bolted it onto my camera and gone for a walk to see what the results are like.
09-18-2015, 08:29 AM   #4
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QuoteOriginally posted by barnzy Quote
Is this something everyone checks when they get a new lens? I must confess up until now I have always just bolted it onto my camera and gone for a walk to see what the results are like.
Usually only the most anal-retentive or bored of the Pentax Forum users immediately do a focus adjust

I am glad that you got the problem fixed, but surprised that your camera failed to lock on infinity with your new lens. I am also surprised that a direct comparison between CDAF and PDAF failed to detect backfocus*. The first question I must ask is if PDAF at distance (say 50 meters) is now accurate (pictures sharp). The second question is whether it is possible to attain manual focus at infinity using magnified live view. When I read your original post, the first thought that came to my mind was a poorly calibrated infinity stop on your lens or an optical issue (misaligned element) that manifests more at distance. With any luck all is well for manual focus too.

Enjoy your new lens!


Steve

* When testing focus calibration, I usually recommend using manual focus in magnified live view as the standard over CDAF. I also recommend a target having both horizontal and vertical lines such as the DotTune target (LINK) at 50x the focal length. That is far enough away to challenge the AF system, but close enough to allow reasonable travel of the focus system.

09-19-2015, 01:27 AM   #5
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Hi Steve,

Thank you for the additional information, I have re-completed my original FF/BF test using the information you provided above and this time have been able to identify obvious back focus that is resolved by increasing the AF fine adjustment. I think my original test was flawed, but my subsequent tests were successful.

I have also carried out a the other test you recommended. I used the BMW logo on the top of the dealership approximately 70 meters away as a point of reference and focused 2 on my lenses to infinity using x8 magnified live view. On my sigma I am able to go beyond infinity, however on my DA40mm i have to go all the way to the stop. both photos appear sharp, however if you zoom in much closer (x23) you can see ghosting round the letters in the shot taken with the DA40mm.

As such using manual focus to infinity with extreme magnification I am able to get a better result with the Sigma. I have included 2x 23x magnification crops from both lenses.

Sigma: 24-60mm (focal length for shot 40mm) at F2.8 (23x magnification of the logo on the sign)


Pentax DA 40mm at F2.8 (23x magnification of the logo on the sign)


for reference for the level of magnification in the crops this is the original sigma image:




What I don't know is whether this is a result of a problem with the lens (i.e. incorrectly calibrated infinity stop), or just a limitation of the optical ability of the lens + camera +me. I also wonder whether the pentax lens should focus past infinity in normal operation? As mine most definitely does not.

Cheers

Jon
09-20-2015, 06:41 PM   #6
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QuoteOriginally posted by barnzy Quote
I also wonder whether the pentax lens should focus past infinity in normal operation?
Most wide to moderate focal length primes don't and that extends to the AF lenses I own. That being said, I don't own the DA 70 and can't say.


Steve
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