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09-12-2012, 03:03 PM   #1
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Back Focus in DA*300 F4 with K-5

Received the DA*300 f/4 yesterday afternoon and had some less than stellar initial results with resolution. A test shows it is focusing behind the selected single center point by a significant amount requiring all 10 of the available micro adjustment units whereas my other lenses range from +/- 6.

I tried searching the forum for posts regarding this topic, and none turned up; If it's been discussed before, I apologize for introducing it again, but I was just wondering if anyone else has seen this degree of autofocus shift for this particular lens.

Resolution tests were shocking, and I won't even report the lppm value I observed, but it definitely exceeded my expectations, which were quite high given the glowing reviews here and elsewhere. So I'm definitely keeping it, but I am just a little concerned about being at the limit on AF microadjustment.

09-12-2012, 03:06 PM   #2
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I would send it back if it is that far off, wonder if it was mishandled at some point. My DA*300 requires no AF adjust on either of my K-5 bodies, and it is a real gem in terms of IQ and performance.
09-12-2012, 03:08 PM   #3
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QuoteOriginally posted by chicagojohn Quote
Received the DA*300 f/4 yesterday afternoon and had some less than stellar initial results with resolution. A test shows it is focusing behind the selected single center point by a significant amount requiring all 10 of the available micro adjustment units whereas my other lenses range from +/- 6.

I tried searching the forum for posts regarding this topic, and none turned up; If it's been discussed before, I apologize for introducing it again, but I was just wondering if anyone else has seen this degree of autofocus shift for this particular lens.

Resolution tests were shocking, and I won't even report the lppm value I observed, but it definitely exceeded my expectations, which were quite high given the glowing reviews here and elsewhere. So I'm definitely keeping it, but I am just a little concerned about being at the limit on AF microadjustment.
My FA77 MIJ is also at +10 but it does seem spot on now. So while it seems worrying to be at the limit if the af adjust makes it accurate should be fine. (+10 sounds like a lot but in realty it is a miniscule adjustment)
09-12-2012, 04:09 PM   #4
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I'd be very careful about doing your AF testing properly before making any adjustments in relation to the DA*300, or any lens for that matter.

These forums are littered with too many examples of folks who have drawn the wrong conclusions about their AF from sloppy testing. If you really feel there is an issue with the lens, either triple-check and verify your testing procedure, or send the lens off to Pentax with your K-5 body to have them adjusted together.

FWIW, I haven't made any adjustments for my DA*300, and it's sharp as a tack.

09-12-2012, 04:22 PM   #5
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Yeah I just assumed if someone is throwing out 'line pairs per millimeter' they might know what they are talking about, so that is why I think it may be a damaged copy.
09-12-2012, 04:41 PM   #6
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Thanks for your thoughts and observations, guys. I have a fairly rigorous procedure that, again, produces results varying from between +/- 6 increments for all of my other lenses, only one of which was "spot on". I think I'll call B&H and talk it over with them. The way it is working after the adjustment I'm satisfied with. Following are two representative images. The first one is the base of a maple tree shot from a distance of about 15 meters at f/4 on a tripod with gimbal head on my deck an hour or so ago.

I usually evaluate my charts at 100% crop and do the calculations of line pairs per millimeter using the appropriate magnification ratio, but in this case, I was impressed with the 200% crop taken from the base of the first image; an image that has a width in real life of only 15 cm. Although not tack sharp at this magnification, the detail in the lichens is truly amazing to me, and this image was shot using AF, not LiveView and the sharpen filter was not used.
So this lens is now the best I have in resolution, followed by my Tamron 90mm 1:1 macro as a close second.







Anyway, thanks again, and if you're interested, I'll let you know the outcome of my conversation with B&H. Calibration by Pentax is always an option, I suppose, but I'd hate to be without my K-5 for the time that would take, and it seems to be working fine now (whereas yesterday before the adjustment, the AF image was definitely soft at the same magnification).
09-12-2012, 04:44 PM   #7
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QuoteOriginally posted by crewl1 Quote
so that is why I think it may be a damaged copy.
Sure, it's happened to me too with other lenses that have been damaged in transit. But the DA*300 is built like a tank.

Anyway, I don't want to sound like a grump about it but I've read so many reports of folks claiming they need +10 or more AF adjustment to their lens, only to find out they did their 'tests' without a tripod, just pointing at their cat or something.
09-12-2012, 04:57 PM   #8
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Right on, I get you.

09-12-2012, 06:20 PM   #9
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QuoteOriginally posted by rawr Quote
Sure, it's happened to me too with other lenses that have been damaged in transit. But the DA*300 is built like a tank.

Anyway, I don't want to sound like a grump about it but I've read so many reports of folks claiming they need +10 or more AF adjustment to their lens, only to find out they did their 'tests' without a tripod, just pointing at their cat or something.
I'm sure some people do that kind of thing, which as you say, can be very misleading, although I'm sure the cat images are much more interesting than the ones I take. In my case I use a rock solid tripod, mirror up, remote release after about 10 seconds, shooting at a graphic that puts the camera pretty exactly at a 45 degree angle with respect to scales on either side which allow for fairly precise placement of the plane of sharp focus by comparing the balance of blur in front and behind it (the plane of sharp focus is inferred, the way I do it, not directly observed, since it is only a theoretical construct.) I usually do this with a B&W, high contrast target, well lit, center point focus priority, and the maximum aperture to minimize the range of acceptable focus.

But I will try this again tomorrow when I get home from work to see how well the result replicates: It wouldn't take much of a deviation from ideal alignment to produce an erroneous result.

Anyway, I'll follow up with what the folks at B&H say about it too. Thanks again for your comments and observations.
09-13-2012, 04:28 AM   #10
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Final conclusion: I went to bed last night wondering if I had missed something in my AF test. I've been in materials science my entire career (to retire in Jan, and one quickly learns that despite attention to detail, extraneous factors often produce misleading experimental results. So I took rawr's concerns to heart, having been surprised myself at what I thought I'd observed initially from this expensive and highly touted lens.

This morning I woke up with an idea for a much better mounting for the AF target, tried it before getting ready for work, and that confirmed much more clearly that at +10 the AF is dead-nuts on. The initially soft lichen crop (not included in the thread) in combination with the image shot after the AF micro-correction is confirmatory evidence as is the AF resolution tests I did yesterday in comparison with LiveView.

This is by far the greatest expenditure I've made for a lens, and I just wanted to make sure I would be able to get optimum results from it. So I think things are fine for the time being, but I do plan to send it in in to Pentax in December to get it calibrated to the K-5 just so as not t be at the extreme limit which, as Deimos noted, is a bit of a concern just in case anything would change over time.

So thanks again crewl1, Deimos, and rawr; it seems I benefited from something in each of your posts in arriving at a final level of certainty that I'm comfortable with in the the short term, and a plan to get things brought to nominal in the longer term. In closing, here is a hand held shot I took off our deck last night at sunset. Can't wait to get out this weekend and take some pictures, some of which I'm sure I'll be posting elsewhere on the Pentax forums. Thanks once again for your assistance!
======================================================================================
Update: For anyone reading this in the future, I called B&H and they gave me the number to call at Pentax (800) 877 0155.
I could send my K-5 and several lenses in (their software can only accommodate one non-Pentax lens in the mix) for adjustment of the camera body. This process would require 3-4 weeks to complete. Just sending in the DA* 300 alone could result in a shift that would put some of my other lenses on the edge, so the technical person I spoke with recommended sending in as many as possible.

Secondly, it's camera body warranty that matters, apparently. Mine was purchased in March of this year, so I'll be OK for a while. I was also informed that as long as the +10 does make the adjustment complete, which it does, it won't change so it should continue to be good. So the net result is that I squeaked by; everything is good as-is, and I'm not sending anything in. I just don't feel the shipping costs, including insurance, length of turn-around time, and potential risks would justify it.

Finally, I was told that camera bodies and lenses are manufactured within given tolerances and that depending upon the combined deviation of a particular camera-lens pair, the micro-adjustments can vary more than the lens or body considered alone. So while someone may be lucky in receiving a body and lens that are well matched, in other cases, such as mine, the adjustment needs to be larger. The range provided should be sufficient to handle the vast majority of cases, as it was in mine, but if more is required, they said, that's when it has to be sent in.



Last edited by chicagojohn; 09-13-2012 at 11:24 AM. Reason: Update
09-18-2012, 01:37 PM   #11
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I don't see much description of your calibration technique.

I suggest that you shoot at a perfectly flat surface parallel to the film plane to test this kind of thing. Even better, use a LensAlign. You need to be very sure of the point the AF system is locking on to. The K-5 in particular seems to have a real knack for focusing on things you don't expect it to. I have the same lens and my attempts to calibrate it were very inconsistent til I got a LensAlign. I then discovered that the best calibration for it was actually 0, no calibration at all.
09-18-2012, 02:51 PM   #12
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QuoteOriginally posted by DDPhotos Quote
I don't see much description of your calibration technique.

I suggest that you shoot at a perfectly flat surface parallel to the film plane to test this kind of thing. Even better, use a LensAlign. You need to be very sure of the point the AF system is locking on to. The K-5 in particular seems to have a real knack for focusing on things you don't expect it to. I have the same lens and my attempts to calibrate it were very inconsistent til I got a LensAlign. I then discovered that the best calibration for it was actually 0, no calibration at all.
Thanks for your comments, DDPhotos,

This is what I use. If properly constructed and used as directed, it should provide the same result as LensAlign, and it is free. I'm glad your copy is at 0.

focustestchart.com/focus10.pdf
09-19-2012, 03:51 AM   #13
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For future reference, it may be useful to point out that PK_Tether makes it possible to easily enter Debug Mode, which enables you to effectively shift the zero point of the AF adjustment scale, thus making it possible to set an effective value beyond the +/-10 limits. Keep in mind, however, that shifting in one direction results in restricted range at the other end of the scale.
09-19-2012, 05:03 AM   #14
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QuoteOriginally posted by bxf Quote
For future reference, it may be useful to point out that PK_Tether makes it possible to easily enter Debug Mode, which enables you to effectively shift the zero point of the AF adjustment scale, thus making it possible to set an effective value beyond the +/-10 limits. Keep in mind, however, that shifting in one direction results in restricted range at the other end of the scale.
This is great information, bxf. I will definitely check this out.

After re-testing all my Pentax lenses (and a few Tamrons), the data suggest a correlation with lens focal length such that the longer the lens, the greater the intrinsic back focus with a front focus correction needed for shorter focal lengths. So even though all is working well currently, in January, I'm planning to send my K-5 in with a selection of lenses for calibration of the camera body. Amongst my current AF corrections, I do have three units available on the front side that I could use to shift the zero point for back focus using PK_Tether (which I will try to Google).

I find that I'm using the 300 more for close-ups than anything else, and DoF calculations suggest the need to have AF zeroed well to facilitate subsequent manual adjustments; e.g., at the minimum focus distance for f/4 it's +/- 2.5 mm and at 8 ft +/- 8.8 mm, and for f/16 +/- 5 and +/- 18 mm, respectively, while at 30 feet and +/- 5 and +/- 10 feet, respectively, minor deviations in AF are irrelevant.
09-19-2012, 05:10 AM   #15
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QuoteOriginally posted by chicagojohn Quote
PK_Tether (which I will try to Google).
PKTether lives here.
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