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12-31-2017, 08:51 AM   #1
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DA20-40 weird AF issue

Hi,

I have noticed strange issue with my da20-40 (paired with K-3II) - when the aperture is set to bigger numbers the lens frequently faces front focus. I have performed the following excercise:
- aperture f4.0, lock AF on tree in distance ~20m
- taken picture is sharp

- set aperture to f16, lock AF on the same tree, tree in OVF does not look quite sharp
- set to MF WITHOUT even touching the lens, change aperture to f4 and take the picture
- the picture clearly reveals slight FF

I repeated it several times, with the same result. FF on my lens depends on aperture set on my body (K-3II)
It is the only one lens with such a strange behavior. The lens in after adjusting in official Pentax/Ricoh service.
:
Are you guys able to explain this? Asking because I can't :-P

Regards,
Paulo

12-31-2017, 09:19 AM - 1 Like   #2
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AF is conducted at widest aperture irrespective of aperture set. Very odd behaviour
12-31-2017, 10:05 AM   #3
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Hmm. Not something I've noticed with my own copy, but it sounds like "focus shifting"...
12-31-2017, 10:31 AM   #4
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QuoteOriginally posted by pschlute Quote
AF is conducted at widest aperture irrespective of aperture set.
That's why I created this new thread. I can't understand what happens at all. Either the lens requires further adjustment in service or it is problem with the body (tried 2 other brand new zooms: other da20-40 and also da*16-50 and all of them had BF on short end).

I'll call Pentax service in the next week and talk about it...

Regards,
Paulo

12-31-2017, 10:46 AM   #5
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QuoteOriginally posted by Paulo Quote
That's why I created this new thread. I can't understand what happens at all. Either the lens requires further adjustment in service or it is problem with the body (tried 2 other brand new zooms: other da20-40 and also da*16-50 and all of them had BF on short end).

I'll call Pentax service in the next week and talk about it...
I don't see that it can be a problem with the body, if you're able to focus accurately and take a sharp photo at f/4 (it's also not unusual for zoom lenses to show different fine focus adjustment at different focal lengths).

It sounds like your HD20-40 is exhibiting classic focus shifting... where the distance of accurate focus changes as the diaphragm is stopped down. If you're not familiar with this problem, the following article is a good place to start: What is Focus Shift?

I'm not aware of any reports that the HD20-40 suffers from this problem. That said, I haven't heard of it happening with the DA50 f/1.8 either, but it does with my copy.
12-31-2017, 11:46 AM - 1 Like   #6
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Thank you, BigMackCam! Just read the article about focus shift and wonder that the behavior of AF in my lens could be intentional. As mentioned it was recently adjusted in Pentax service, maybe they programmed it to compensate focus shift effect... Pictures taken at aperture previously pre-set during autofocusing are sharp, problem is only when photo is made on lower aperture values (especially wide open - shallow DOF) than used for autofocusing, as described in my initial post.

Anyway, it is the 1st time I hear about focus shift in my ~30 years play with photography...

Regards,
Paulo

Last edited by BigMackCam; 01-01-2018 at 05:55 AM.
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