Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
01-06-2016, 12:01 AM   #1
Senior Member
apfhex's Avatar

Join Date: Feb 2013
Location: Northern California
Photos: Gallery
Posts: 117
20-40mm Ltd focusing: CDAF vs PDAF

I'd very much like to hear from other people who own this lens, what camera they're using, and how their focusing with this lens is using phase-detect autofocus vs contrast-detect autofocus and if they have any focusing adjustments set.

I'm on my second copy from B&H and have been doing many focus chart tests at 20/30/40mm, wide open each time. The lens is always very sharp using CDAF (at least in the center, have not been checking corners). But PDAF is a mixed bag, it seems to back focus at the wide end and front focus at the other, though the back focus is a little more drastic. I've currently ended up with a +4 adjustment which gets the majority of the shots sharp enough that I think it can be worked with, while stopping down will hopefully help eliminate any discrepancies, but it is still an annoyance and disappointment.

I'm especially interested if there seems to be more focus issues with K-30/K-50 vs K-3/K-3II, as I eventually plan to get the later. I can get access to a K-50 myself which I'm curious to check. The ONLY other lens I've owned that needed any focus adjustment is the Tamron 17-50mm, which only needed a -1 to be sharp across the whole range.

The first lens had the same general problem so I'm wondering if it's just going be an issue with any copy I get.


Last edited by apfhex; 01-06-2016 at 12:14 AM.
01-06-2016, 07:37 AM   #2
Loyal Site Supporter
waterfall's Avatar

Join Date: Apr 2013
Location: Northern Minnesota
Photos: Gallery | Albums
Posts: 1,812
My copy on the K-3 required no focus correction and is quick focusing on PDAF. Corners are soft wide open at 20mm but sharp at F5.6 and above. At 30-40mm there is no softness at edges wide open (F > 2.8). Color and contrast are excellent on auto WB.
01-06-2016, 08:07 AM   #3
Senior Moderator
Loyal Site Supporter
Loyal Site Supporter
BigMackCam's Avatar

Join Date: Mar 2010
Location: North-East of England
Posts: 12,478
No problems with PDAF on my K3 & 20-40.

What apertures are you testing at, and how are you assessing sharpness - centre only, or frame-wide, and are you using a single focus point?

As Dick mentions above, the lens is soft in the borders / corners wide open at 20mm. In fact, you really need to stop it down to f/5.6 for decent performance at all focal lengths, and ideally f/8 between 20-30mm.

I recall having read something about field curvature at 20mm with the 20-40... not sure if this was ever substantiated, or just a question someone raised...
01-06-2016, 12:11 PM   #4
Senior Member
apfhex's Avatar

Join Date: Feb 2013
Location: Northern California
Photos: Gallery
Posts: 117
Original Poster
QuoteOriginally posted by BigMackCam Quote
What apertures are you testing at, and how are you assessing sharpness - centre only, or frame-wide, and are you using a single focus point?
Single center focus point, yeah. I said in my post but, wide open at each focal length (so f2.8/3.5/4) and just center sharpness, I'm aware it won't have sharp corners. And I do plan some more tests at maybe f4, f5.6 and f8... I did do a couple like that and the PDAF was still missing, but I definitely need more samples and maybe in better light.

01-06-2016, 12:28 PM   #5
Senior Moderator
Loyal Site Supporter
Loyal Site Supporter
BigMackCam's Avatar

Join Date: Mar 2010
Location: North-East of England
Posts: 12,478
QuoteOriginally posted by apfhex Quote
Single center focus point, yeah. I said in my post but, wide open at each focal length (so f2.8/3.5/4) and just center sharpness, I'm aware it won't have sharp corners. And I do plan some more tests at maybe f4, f5.6 and f8... I did do a couple like that and the PDAF was still missing, but I definitely need more samples and maybe in better light.
Hmmm. More samples in better light would be a very good idea, as you say. Testing on a second body, if you have access to one, would also be useful, but if you get the same results then it does indeed sound like you have a second bad copy. It does happen... I had two 16-85s, both with problems that had been reported by a handful of other users. At least you had the good sense to buy from a reputable retailer who will replace / refund if necessary.

One final thought - what are you focusing on for your test? I've run round in circles performing focus testing with lenses before realising that it was the subject confusing the camera's PDAF, even with just one specific lens. I now use either an angled chart specifically designed for the purpose, or a wooden ruler with camera pointed toward it at an angle. I'm sure you've already thought of this, but I though it worth asking.

Good luck and let us know how you get on?
01-06-2016, 01:15 PM   #6
Senior Member
apfhex's Avatar

Join Date: Feb 2013
Location: Northern California
Photos: Gallery
Posts: 117
Original Poster
This to check if it was front or back focus, and then this to check sharpness after making adjustments (also some random objects around the room). They have worked well for testing other lenses with consistent results. I've never had to do this much testing with other lenses to get sharp results. There does seem to be at least one other user here with the same problem on the K-50.
01-06-2016, 01:51 PM   #7
Senior Moderator
Loyal Site Supporter
Loyal Site Supporter
BigMackCam's Avatar

Join Date: Mar 2010
Location: North-East of England
Posts: 12,478
QuoteOriginally posted by apfhex Quote
This to check if it was front or back focus, and then this to check sharpness after making adjustments (also some random objects around the room). They have worked well for testing other lenses with consistent results. I've never had to do this much testing with other lenses to get sharp results. There does seem to be at least one other user here with the same problem on the K-50.
Well, even my limited testing setup works well - and yours is certainly better than mine. You're clearly going about it the right way. Interesting that someone else is having the same problem. As I said, I had double trouble with the 16-85 so it's entirely possible there is a potential issue with the 20-40 that occasionally arises as a result of design / manufacturing tolerances / QC. I'll wait to hear your results with interest. All the best with it.
01-06-2016, 10:03 PM   #8
Senior Member
apfhex's Avatar

Join Date: Feb 2013
Location: Northern California
Photos: Gallery
Posts: 117
Original Poster
I can at least chalk up some of my bad results to inconsistent PDAF. This time I watched and made a note of where the focus ring stopped in CDAF, then took at least 3-5 PDAF shots while only observing the focus ring, and a good amount of them were wildly different (a certain CDAF shot landed exactly half way between 2 and 1.3ft, but one of the PDAF shots landed on 2ft! It was completely blurry). Some of them were in good focus though.

Tested against the DA 50mm on the same object with the same settings, the 50 landed focus on the majority of the shots while the 20-40 seems to be the opposite of that. I would be willing to believe the PDAF is just struggling in the low light if not for those different results. Will still need to test as soon as I have any daylight shooting time. The first copy of the lens was blurry outdoors in bright light though so I dunno.


Last edited by apfhex; 01-11-2016 at 09:18 PM. Reason: removed album link
01-07-2016, 12:39 AM   #9
zan
Junior Member




Join Date: Sep 2015
Location: Ljubljana-Slovenia
Posts: 48
Try with this chart http://chriscontiphotography.com/stuff/lens_test_chart.jpg in good daylight. I think you will get much better results. The book with the small white letters is not contrasty enough for AF testing.
01-11-2016, 09:52 PM   #10
Senior Member
apfhex's Avatar

Join Date: Feb 2013
Location: Northern California
Photos: Gallery
Posts: 117
Original Poster
Thanks, that worked better and with a little more light. Turned focus adjustments OFF and managed some much more uniform focusing. +1 and -1 adjustments didn't make any improvements so I'll leave it at 0 and try some more real world shooting (lots of gloomy days here, hard time to test).

100% crops with no sharpening. F5.6 at 1/30sec. First of each is CDAF.
20mm
30mm
40mm

I did try some tests at f2.8 20mm and f4 40mm and results were the same as above, so maybe my prior methodology wasn't as good as I thought (it still stands that my other lenses seem to have an easier time with PDAF. And the first copy of this lens was definitely bum).

In the mean time, CDAF is producing great shots with the kind of look I was hoping for, on the one sunny day last week.
Attached Images
 
Reply

Bookmarks
  • Submit Thread to Facebook Facebook
  • Submit Thread to Twitter Twitter
  • Submit Thread to Digg Digg
Tags - Make this thread easier to find by adding keywords to it!
20-40mm ltd, adjustment, apertures, autofocus, cdaf, contrast, copy, corner, experience, focus, focus correction, k-mount, k3, lens, mentioned, pdaf, pentax lens, performance, slr lens, vs
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
Comparing PDAF & CDAF and other AF stuff dosdan General Photography 4 04-27-2015 06:17 PM
20-40mm Ltd and the 31mm Ltd ? kentishrev Pentax SLR Lens Discussion 16 02-02-2015 05:19 PM
DA* 16-50mm vs. DA 20-40mm Ltd. michael.colby Pentax SLR Lens Discussion 20 11-20-2014 03:49 PM
CDAF is less accurate than PDAF Zafar Iqbal Troubleshooting and Beginner Help 12 06-21-2013 12:12 AM
Why not use CDAF to adjust PDAF? Raffwal Photographic Industry and Professionals 2 01-13-2013 10:26 AM



All times are GMT -7. The time now is 09:01 PM. | See also: NikonForums.com, CanonForums.com part of our network of photo forums!
  • Red (Default)
  • Green
  • Gray
  • Dark
  • Dark Yellow
  • Dark Blue
  • Old Red
  • Old Green
  • Old Gray
  • Dial-Up Style
Hello! It's great to see you back on the forum! Have you considered joining the community?
register
Creating a FREE ACCOUNT takes under a minute, removes ads, and lets you post! [Dismiss]
Top