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11-14-2008, 04:24 PM   #1
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DA` 50-135 focus problem

I just bought my DA` 50-135mm lens that I've been drooling over for a while (from Adorama) to use with my K10D. Today was the first day I got to take it outdoors in daylight and take a few snaps. I was expecting to see wonderfully sharp photos with great colors. It's an overcast, foggy day and there weren't any vivid colors to be found, but the colors I did get were reasonable for such a miserable day.
The thing that bothers me is the pictures don't look sharp. As less than full screen they're OK. At full screen they are not. At 100% blow up they are bad. Some of the pictures appear sharper when I look behind the focus point (BF I guess).
I have not noticed this with my other lenses at all. I've got (and been very satisfied with) a Sigma 17-70, DA 21mm Ltd., SMC F100-300, and a Vivitar Series 1 70-210 V3. I've also used a Vivitar Series 1 28-105 (before I bought the 17-70) and an old Samyang 100-500 that has a minor Fungus problem. None of these lenses have this focus problem.
I've just been reading about a way to get into the K10D's Diagnostic program and adjust the focus (BF FF issue). I don't really want to do this since all of my other lenes are fine. I guess I should also mention that the 50-135 looks sharp when I'm looking through the camera, but the resulting pictures aren't. I also noticed that the Pentax Photo Browser doesn't identify the lens by name, but it does identify the focal length and the aperture. I also noticed that a few shots I took with flash during my indoor tests were sharp, but the aperture was up at 8 so the depth of field might be hiding the focus issue.
Please give me some advice. Do I exchange the lens at Adorama and try the next one from the shipment? Do I enter Diagnostic focus adjustment, adjust for this lens and hope for the best with the rest of them? Should I try to deal direct with Pentax USA?
HELP PLEASE, my wife will shoot me for spending this much on a lens that isn't good.
Thanks,
Brian

11-14-2008, 04:29 PM   #2
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QuoteOriginally posted by calicojack Quote
I have not noticed this with my other lenses at all.
Were you shooting t f/2.8 with the 50-135? None of the other lenses are capable of that at these same focal lengths, so you're dealing with rather less DOF than you're accustomed to. That plus any possibility that the camera simply chose to focus on something other than what you expected would explain a lot.

As usual, posting images (with EXIF intact!) would help. And check out the test chart at focustestchart.com
11-14-2008, 04:44 PM   #3
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There was a thread a while back dicussing something similar. Were all the shots no in foucus, or just some? If some, were they wider angle shots? If yes, try zooming in, selecting your focus point, and then going wide for the shot and see if it's corrected.
11-14-2008, 06:33 PM   #4
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You didn't mention if you read other threads on the lens. So I'll ask, anything like this? Great colors but focus is off across the frame:
https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/41432-testing-...-50-135mm.html
I really got myself psyched to get his lens a few weeks ago instead of a 100mm-ish AF prime. Now, not so much. :ugh:

11-15-2008, 06:58 AM   #5
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Samples

9401, 9402, 9417, 9418 (pictures 2,3,4,&5) were shot as .pef, converted to jpg in Pentax photo lab, then reduced in photoshop elements 5. 9351 (picture 1) was shot as jpg and reduced only. No other processing.
In the first (351) the focus point was the middle of the picture, on the wire. The wire is sloped further away at the bottom. The focus is sharper at the bottom. (bokeh is pleasing though)
Next is 9401 & the focus point is the can. The sharpness is several inches behind the can.
9402 is the broken tree limb. The focus point is the wood grain just lower left of the red mark. To the right, further back on the limb is sharper.
9417 is the shot on the right. The focus is on the near center of the tree trunk. It is sharper to the right which is half way back through the trunk.
9418 is my GMC Jimmy. The focus is on the napkin stuck in the grill. It is sharper by the rear bumper.
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11-15-2008, 07:05 AM   #6
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QuoteOriginally posted by Marc Sabatella Quote
Were you shooting t f/2.8 with the 50-135? None of the other lenses are capable of that at these same focal lengths, so you're dealing with rather less DOF than you're accustomed to. That plus any possibility that the camera simply chose to focus on something other than what you expected would explain a lot.

As usual, posting images (with EXIF intact!) would help. And check out the test chart at focustestchart.com
The Sigma 17-70 goes to 2.8, as well as both of my Vivitar Series 1's.
But the answer is no, I was in MTF and thge camera adjusted the aperture to 4. I also have some shots where I left the camera in Matrix focusing (or whatever it's called) but all of these were in spot focusing which is where I always keep it. Thanks for the link to the test chart. I remember reading about it a while ago and planned to find it this morning.
I hope the exif is in the pictures I posted.
Thanks,
Brian
11-15-2008, 07:10 AM   #7
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QuoteOriginally posted by Jodokast96 Quote
There was a thread a while back dicussing something similar. Were all the shots no in foucus, or just some? If some, were they wider angle shots? If yes, try zooming in, selecting your focus point, and then going wide for the shot and see if it's corrected.
Almost all of my test shots show similar results. The only ones that are sharp at my focus point were done with flash and apertures about 6.3 - 8.
I tried the zoom in first trick with no difference. All of my shots appeared sharp in the viewfinder. I couldn't improve them by trying the focus override.
Brian
11-15-2008, 07:14 AM   #8
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QuoteOriginally posted by m8o Quote
You didn't mention if you read other threads on the lens. So I'll ask, anything like this? Great colors but focus is off across the frame:
https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/41432-testing-...-50-135mm.html
I really got myself psyched to get his lens a few weeks ago instead of a 100mm-ish AF prime. Now, not so much. :ugh:
Yes, I did read the other thread, just after buying my lens. There has been so much great news and comments on this lens, that I was really looking forward to it. I've been saving up for this lens for a while and the rebate I had from Sandisk for use at Adorama was just about to expire, so I had to pull the trigger when I did.
Brian

11-15-2008, 09:56 AM   #9
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Debug mode Focus adjustment?

I was just reviewing the K10D debug mode focus adjustment possibilities. I was wondering if anyone knows if that adjustment effects all focusing or just the SDM focusing. Since my problem is only with the SDM and my body screw autofocus lenses are OK (I reviewed them again this morning), this debug mode adjustment might be worth looking into for me.
Any advice?
Thanks,
Brian
11-15-2008, 12:20 PM   #10
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Focus Test

With thanks to Yvon Bourque and his published focus chart, I'm now sure that it's a back focus problem. These were taken mounted on a tripod, shake reduction off, 2 second shutter delay, with and without flash. They were taken in pef, converted to jpg and reduced Elements 5. (there isn't a noticable difference between starting with pef or jpg using raw +). It looks to mr to be about 5/16 BF.
Brian
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11-15-2008, 01:53 PM   #11
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QuoteOriginally posted by calicojack Quote
The Sigma 17-70 goes to 2.8
Yes, but onl;y at the very widest end, where DOF is much bigger.

QuoteQuote:
as both of my Vivitar Series 1's.
I didn't realize this. But, are those MF or AF?
11-15-2008, 03:45 PM   #12
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QuoteOriginally posted by Marc Sabatella Quote
I didn't realize this. But, are those MF or AF?
Both of my Vivitar Series 1's are manual focus, and like the Sigma, are only 2.8 at the wide end of the zoom.
Brian
11-15-2008, 03:59 PM   #13
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QuoteOriginally posted by calicojack Quote
Both of my Vivitar Series 1's are manual focus, and like the Sigma, are only 2.8 at the wide end of the zoom.
Brian
So, it isn't really meaningful to compare them to the 50-135. All evidence from what I see here is that that whatever problem you might have with backfocus in the AF system, it probably affects all your AF lenses that have a sufficiently shallow DOF to expose the problem. The focus chart you display is too small to say for sure, but yeah, it looks like it might be focusing a little bit behind where it should. This is more apparent in the poorly lit example than in the well-lit one, though, so I wouldn't be leaping to any coclusion. Multiple tests in good light (not flash, not good *constant* light for the AF system) would help. There is often a little bit of variation from shot to shot in where the focus system decided to focus. If it turns out to *consistently* produce results where the *entire* zone of acceptable sharpness is behind the focus target, I'd send it in for service. It it's just, as appear to be here, where the *center* of the zone is *sometimes* behind the target but the target is still in focus, I personally wouldn't worry about it - it shouldn't be enough of an issue in practice to be worth being without the camera for weeks.

The other examples you posted are pretty much useless, BTW. *You* may have wanted to focus on the tin can, but that's no guarantee the camera chose the can. it might just have well chosen the leaf in front of the dark shadow behind the can, which is the area of highest contrast, and *well* within the center focus point's zone. Ditto with the others. That isn't proof of BF, it's easily explained by the size of the focus sensor and the inherent difficulty in forcing a camera to focus on what you think it should focus on.
11-15-2008, 07:59 PM   #14
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Corrected

Thanks to Rulez and his Blog, Photography Rulez: Debug mode with latest firmware (YEAH!) (in English).
I was able to enter the debug mode in V1.3 and adjust the focus point using Yvon's chart noted above. It appears that this adjustment has not hurt the sharpness of my other lenses, so maybe they were all BF'ing. Of course this adjustment and testing was all done in my wife's kitchen, at night. I guess I'll have to do more "field" testing tomorrow.
Regards,
Brian
11-15-2008, 08:08 PM   #15
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QuoteOriginally posted by Marc Sabatella Quote
*You* may have wanted to focus on the tin can, but that's no guarantee the camera chose the can. it might just have well chosen the leaf in front of the dark shadow behind the can, which is the area of highest contrast, and *well* within the center focus point's zone.

Marc,
Thanks for your comments and help in figuring out what's going on. All of my shooting is done in center point only focus mode. I'll routinely focus on my desired subject and then recompose to shoot. Of course I realise that bthe center focus point must be larger than the little red dot. Do you have any idea how much larger?
Also, I would have thought that the difference between the bright colored can and the muted shades of the leaves would give the greatest contrast.

Regards,
Brian
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