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11-29-2020, 10:58 AM   #1
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20-40 LTD Autofocus Problems

I have a 20-40 LTD lens that I bought used back in June. I haven't used it a lot, but I've struggled with the autofocus from time to time. I tried a calibration a couple months ago and thought I had it pretty well nailed. But over the past few days I took some pictures and all of them that were at or near f/2.8 were back focusing. Just did some more testing, and on my K-3ii a +10 AF fine adjust still doesn't nail focus at f/2.8. It's clearly back-focusing.


Shifted to my K-30, and +10 just barely gets the subject in focus but clearly the area of sharpness starts at the subject and goes behind it. Almost zero in focus in front of the subject.

Test with live view and it looks much better. I should mention all of these tests today were done at 20mm.


So... it looks like the lens is the problem. Can this be fixed by Precision? And if so, a quick look at Precision's site seems to indicate that any lens repair is $300. I bought the lens for something like $325, so it may not be worth it. Does anyone else have experience getting a lens fixed by Precision because you couldn't fine adjust focus?

11-29-2020, 11:08 AM   #2
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Sadly this problem occurs sometimes.
I know it can be adjusted.
I purchased my 20-40 ltd. new and was able to sent it back as it wasn't perfect adjusted either.
11-29-2020, 05:38 PM   #3
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Does it make a difference if you make the lens focus from the infinity end versus the closest focus end ?
11-29-2020, 06:28 PM   #4
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The DA 20-40 I bought last year also had AF issues, as well as poor sharpness at 40mm outside of the central area of the frame, even stopped down to f/8.

QuoteOriginally posted by pschlute Quote
Does it make a difference if you make the lens focus from the infinity end versus the closest focus end ?
My Tamron 17-50/2.8 autofocused better this way.

11-29-2020, 06:38 PM - 1 Like   #5
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Further to the question from @pschlute, I've had a challenge in calibrating my copy of this lens.

As I have done with my other lenses, I conducted a rigorous AF Fine Adjustment (AFFA) for my DA 20-40, assessing the settings at 20, 30, and 40mm; at focus distances of at least 60 x FL; and variably focusing from the minimum focus distance and infinity positions for each calibration 'run'.

I found that the optimal AFFA setting at 20mm is significantly different from that at 30mm and 40mm. The only setting that gives the sharpest autofocus at all focal lengths is AFFA = +2 when focused from infinity. So, I simply preset the focus to infinity before each shot, which is not a big deal, but it's an extra step to remember in the field.

I find that my calibrated copy has a very high AF accuracy and shot-to-shot consistency. I shoot with a K-3 II.

Sorry, I don't have any experience with sending my lenses to a repair centre to adjust AF issues. I opted to keep my copy as-is and work around the calibration issue.


- Craig

Last edited by c.a.m; 11-29-2020 at 07:34 PM.
11-29-2020, 07:13 PM   #6
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For some reason there is a difference focus accuracy when focusing from minimum distance compared to focusing from infinity.
11-30-2020, 07:37 AM   #7
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QuoteOriginally posted by pschlute Quote
Does it make a difference if you make the lens focus from the infinity end versus the closest focus end ?
I will need to test. I also need to do more testing at different focal lengths.

---------- Post added 11-30-20 at 09:39 AM ----------

QuoteOriginally posted by c.a.m Quote
Sorry, I don't have any experience with sending my lenses to a repair centre to adjust AF issues. I opted to keep my copy as-is and work around the calibration issue.


- Craig
That's what I've always done before. I've never had a lens that required more than +10, and to tell the truth most of my lenses I don't even calibrate because they look fine. But it certainly looks like this one (at least at 20mm) still back-focuses at +10 across two different bodies.
11-30-2020, 11:41 AM   #8
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QuoteOriginally posted by DWS1 Quote
For some reason there is a difference focus accuracy when focusing from minimum distance compared to focusing from infinity.
That is correct, and is seen more often in screw drive lenses. The camera does not have a single position where it considers an image to be in-focus but a (very small) range when using PDAF. When the camera tells the lens to stop there is lee-way. PDAF is just not as accurate as we all thought it was before the advent of big megapixel cameras

11-30-2020, 03:53 PM   #9
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Send it back, Thor, for exchange or refund. We pay a lot of money for these things, you deserve on that's up to spec. My copy is excellent.
11-30-2020, 04:11 PM   #10
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I am fortunate that I've not encountered AF issues with my DA 20-40mm Ltd copy I would think method of testing to require great accuracy at 20mm, and since needing to be at f/2.8 then DOF being a factor at 20mm especially. Even so, mine checked out well with the front/back AF checks I've done.

Last edited by mikesbike; 11-30-2020 at 04:17 PM.
12-02-2020, 05:04 AM   #11
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QuoteOriginally posted by clackers Quote
Send it back, Thor, for exchange or refund. We pay a lot of money for these things, you deserve on that's up to spec. My copy is excellent.
I bought it in June, used from Lens Authority (Lens Rentals used sales arm). I don't think returning it five months later is an option. Their conditions indicate a 3-day inspection/return window and a 90 day warranty. My fault for not immediately running more comprehensive tests when it came.
12-02-2020, 03:40 PM   #12
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QuoteOriginally posted by ThorSanchez Quote
I bought it in June, used from Lens Authority (Lens Rentals used sales arm). I don't think returning it five months later is an option. Their conditions indicate a 3-day inspection/return window and a 90 day warranty. My fault for not immediately running more comprehensive tests when it came.
Yeah, I don't throw away the packaging until I've done microadjustment calibration and checked for decentreing (rotating camera for a scene, seeing if there's any softness that also follows the movement), and I won't buy from a vendor that doesn't offer a return.
12-02-2020, 03:56 PM   #13
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QuoteOriginally posted by ThorSanchez Quote
I bought it in June, used from Lens Authority (Lens Rentals used sales arm). I don't think returning it five months later is an option. Their conditions indicate a 3-day inspection/return window and a 90 day warranty. My fault for not immediately running more comprehensive tests when it came.
Wow, I wonder if this is the one I bought and returned to them?! I paid about $290, a year ago, for it after using a promo code. When I returned it I included a detailed description of my findings (tested with K-3 II, K-70, and K-01) but they posted it for sale 2 days after it arrived back to them.
12-02-2020, 06:39 PM   #14
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Did it consistently back focus? That would be pretty crappy of them.

12-02-2020, 06:54 PM   #15
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QuoteOriginally posted by ThorSanchez Quote
Did it consistently back focus? That would be pretty crappy of them.
Yes, it back-focused when shooting at/near infinity, unless I shot Live View, or if I set the lens to infinity and let it focus from there, as @c.a.m described.

I almost kept it anyway, because I'm comfortable shooting Live View with shorter focal lengths, and the image quality at 20-30mm was lovely. Basically it would have replaced my 21 Ltd. If the performance at 40mm had been good, I would have kept it.

I was surprised when I saw it back for sale.
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