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12-17-2013, 06:17 PM   #1
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Pentax 20-40 Focus Issue?

I notice that, every so often, my new Pentax 20-40 tends to have focus errors at 20 f2.8/4.0. This happened both with my K-01 and K-5 with fine focus adjustment set OK. Hmmmm

I saw a pic on another thread with misfocus at 20/2.8. Just wondering if this is a fluke or possible weakness of the lens.

12-17-2013, 06:21 PM   #2
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Of maybe the image just isn't sharp at F2.8? If the K-01 confirms focus then the focus should definitely be locked, since it uses CDAF.

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12-17-2013, 06:30 PM   #3
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Adam, after extensive testing, I can say that the weak point of the 20-40 is at 20/2.8 and 20/4.0. It's not really bad a 20/4.0, but a lot of less expensive lenses have no problem matching it's performance. Unfortunately for me, I would be using it at 20 most of the time, from 2.8 to 5.6. Not ideal. My Pentax 17-70 gives it a run for the money, except at the extreme edges. If you get a centered copy of the 20-40, I will say it probably has pretty decent edges....but the centers on my lens are nothing particularly special. This lens is really nice, no doubt about it, from 24 to 40.

FYI, the misfocus errors I experienced are really out of focus issues....not just softness of the lens.

I having a tough time figuring this lens out. I could just carry my FA 20/2.8 and DA 35 Ltd macro, and I would beat this lens with better macro and more speed at 35. and way better pics wide open at 20mm. Plus these primes are a lot smaller. I really want to like this 20-40 lens, but it's range is pretty limited. One surprise is how good my Sigma 18-250 macro (latest version) was between 20 and 40, at f4.0 and up. Centers were just as good, sometimes better, than the 20-40. I was shocked. (-: And the Sigma lens is not much bigger and less than half the price. For a travel lens, I hate to say this, but the Sigma is really not bad and the range is really flexible.

Pentax strayed from their limited formula with the 20-40 in terms of lens hood. The lens hood looks like a step up ring, and has NO filter threads on it. Really dumb design. I always remove the Limited lens caps and use Pentax pinch caps for faster, easier cap removal. Plus, if lost not a major cost to replace. On this lens, you either lose the hood or have to buy a step up ring to 58/62.

Last edited by rdj92807; 12-17-2013 at 06:43 PM.
12-17-2013, 06:44 PM   #4
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QuoteOriginally posted by rdj92807 Quote
My Pentax 17-70 gives it a run for the money
How can the 17-70 beat it at f2.8? The 17-70mm is f4 wide open.
Also, did you update your camera to the latest firmware? It includes updates for the CD AF of new HD lenses.
QuoteOriginally posted by rdj92807 Quote
I could just carry my FA 20/2.8 and DA 35 Ltd macro,
Well of course, primes should be better than zooms. That is the whole point of primes, IQ over convenience. The zoom has the advantage that you don't need to swap lenses to go from 20mm to 40mm or anything in between. It would be really amazing if a zoom lens would beat macro lenses (especially since the DA 35mm ltd. is a high-end lens, optically, even for 35mm primes)

And I really cannot believe a 18-250mm f3.5-6.3 could match the DA 20-40mm limited. I'm sure a test in proper light, with tripods and a clear subject would show the 20-40mm to render better images. But hey, if you enjoy the 18-250mm more, use it and sell the DA 20-40mm limited.. but maybe you just expected too much and need to use the lens for a while to learn how to squeeze the most out of it. Every time I get a new lens it takes a couple months of work to start getting good photos with it.

12-17-2013, 07:21 PM   #5
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Na, your reply is not really helpful since you do not own a Pentax 20-40. i was referring to the Pentax 17-70 at f4 and above, obviously.
All my firmware is updated, and firmware only impacts focus, not image quality.

Again, you don't seem to own a new Sigma 18-250 macro version. I do, and I actually tested it so I have reviewed the pics....you can believe it or not. Side by side, stopped down to f5.6 for example, it's not easy telling the difference...colors are exactly the same also.
If you test lenses a lot, like I do, you realize that many lenses are surprisingly good at certain focal ranges and f stops. The Sigma has been very well tested and reviewed, and it is a strong performer in the 18-80 range for example. Also pretty good at 250. Not so great at some intermediate stops. My Pentax 16-50 is a great lens, but not at 2.8. The 20-40 has better edges at 2.8, which is not saying that much but it's better than the 16-50. However, stopped down to f4 and beyond, the 16-50 does very well. Each lens has it's pros and cons.
12-17-2013, 08:04 PM   #6
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QuoteOriginally posted by rdj92807 Quote
I notice that, every so often, my new Pentax 20-40 tends to have focus errors at 20 f2.8/4.0. This happened both with my K-01 and K-5 with fine focus adjustment set OK. Hmmmm

I saw a pic on another thread with misfocus at 20/2.8. Just wondering if this is a fluke or possible weakness of the lens.
Is the lens parfocal?

So focus at 40mm, then zoom out for the shot.
12-17-2013, 08:15 PM   #7
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QuoteOriginally posted by rdj92807 Quote
Na, your reply is not really helpful since you do not own a Pentax 20-40
Burn
12-17-2013, 09:26 PM   #8
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I'm not zoom specialist, but I saw rather sharp pictures from 20-40 only in one short review.
Pentax HD 20-40mm f2.8-4 DA Limited Lens: Image Gallery and First Look

But is it level of DA21, 35 and 40?

The rest photos from users are not satisfied for me in terms of sharpness.

12-17-2013, 09:27 PM   #9
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QuoteOriginally posted by Adam Quote
Of maybe the image just isn't sharp at F2.8? If the K-01 confirms focus then the focus should definitely be locked, since it uses CDAF.
There are CDAF mistakes too.
12-17-2013, 10:55 PM   #10
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QuoteOriginally posted by Adam Quote
Of maybe the image just isn't sharp at F2.8? If the K-01 confirms focus then the focus should definitely be locked, since it uses CDAF.
Adam, my K-01 had focus errors as well, with pretty contrasty subjects at that. That's why I returned it.
12-17-2013, 11:41 PM   #11
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QuoteOriginally posted by Giklab Quote
Adam, my K-01 had focus errors as well, with pretty contrasty subjects at that. That's why I returned it.
Well, the implementation certainly isn't perfect- sometimes it reports things in focus that are completely blurry. But from a theoretical standpoint / most of the time, CDAF is pretty reliable since it's not prone to front or back focus like DSLR autofocus.

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12-18-2013, 01:11 AM - 1 Like   #12
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Adam, please get an in-depth review out on the Pentax 20-40 before year end. It seems like a pretty good lens, but clearly some of us have some decentering issues and perhaps some focusing issues. The question is: are these quality control problems or inherent in the design?
Thanks.
01-06-2014, 03:51 PM   #13
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QuoteOriginally posted by rdj92807 Quote
My Pentax 17-70 gives it a run for the money, except at the extreme edges.
I am interested in some of these comparisons -- in particular I'm wondering about how the 20-40 compares to the DA16-45/4 or DA17-70/4, the DA21/3.2, and possibly the FA20-35/4 (I own all these). Given the somewhat lackluster showing in the PF in-depth-review, I'm wondering just how much better or worse this new zoom is, particularly since this zoom plays in the range where the 16-45/4 and 17-70/4 should be fairly strong.
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