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11-01-2010, 02:14 AM   #16
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QuoteOriginally posted by cbaytan Quote
A micro-metric wear or misplacement on the lens' screws or aligning point makes the lens stop at the wrong place. Thats my guess.
That's not the correct explanation. Pentax AF is based on a closed-loop feedback. The camera would keep adjusting till the lens stops at the right place.

I believe the key to the explanation is that the AF systems "sees" differently than the sensor. It uses f/5.6 and thus ignores certain parts of the lens. If a lens has pronounced spherical aberrations at lower f-ratios this will lead to a focus shift.

QuoteOriginally posted by Cannikin Quote
Dunno, that method linked seems rather tedious. I use a much simpler, "low tech" method.
Your "low tech" method is way too imprecise. AF points are not points but rather areas. Unless you use a precise target you'll never know where the camera focused exactly. Your method might get you in the right ballpark but that's not the idea of "micro adjustments".

You may want to check out my Autofocus Adjustment Hints.


QuoteOriginally posted by Andi Lo Quote
This seems to be a very common problem for owners of bodies with AF adjustment though.
This problem occurs on all cameras. There is no connection to the availability of AF adjustment.

11-01-2010, 02:39 AM   #17
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As a K-7 owner with multiple lenses, I can say I have never needed to make any front/back focus adjustments. I dont know it that's telling of the camera itself or the lenses, but never any problems after having checked them all with a focus chart.

Jason
11-01-2010, 04:54 AM   #18
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QuoteOriginally posted by Andi Lo Quote
This seems to be a very common problem for owners of bodies with AF adjustment though. First time we hear someone complaining about this is on K20d. Lenses that would focus perfectly (or reasonably) will badly FF / BF on it.
Focus problems have been present since the beginning of auto focus, however on film, it was hard to pixel peep and tell if the issue with a soft photo was subject movement, camera shake or focus problems. Even with the 6 megapixel digital cameras that Pentax released in the beginning, softness wasn't as readily apparent. When you get up to 12 or 14 megapixels, it is a lot harder to get sharp phots and even small focus problems show up.
11-01-2010, 06:32 AM   #19
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QuoteOriginally posted by Class A Quote
I believe the key to the explanation is that the AF systems "sees" differently than the sensor. It uses f/5.6 and thus ignores certain parts of the lens. If a lens has pronounced spherical aberrations at lower f-ratios this will lead to a focus shift.
That sounds impossible, if it was so same batch of same lenses all would be need to same correction, but all individual lens might need different adjustment. So that tells us, in the focusing gear physically there are/is incorrect parts, or aging/worn parts needs to be adjusted IMO. In my logic there should be some lenses no need to be AF adjusted then eventually with aging AF adjustment should be necessary, if there is a such a lens I don't had it personally because I am new to AF/digital like 2 years, anyone???..

Unfortunately new AF lenses are more prone to misaligned parts because focussing gear is so small&short to save time and electric power, Old manual lenses have a long thread to turn gear to focus, but new AF lenses very short threads to focus, that makes me focusing manually with a DA lens practically impossible, a slight portion of milimetric turn changes the focus tremendously..

11-01-2010, 11:34 AM   #20
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QuoteOriginally posted by Class A Quote
Your "low tech" method is way too imprecise. AF points are not points but rather areas. Unless you use a precise target you'll never know where the camera focused exactly. Your method might get you in the right ballpark but that's not the idea of "micro adjustments".
If I can get a DOF that's less than 1 cm wide to appear in the dead center of the frame at ~100% magnification on every object I test on, and infinity is right on as well, I'd say that's plenty precise enough for me . Before that it would consistently produce a focus area about 1-2cm behind where the center point is.

I chose a target with a very dense and uniform micro texture across the entire center area. I do not use a ruler or test chart because I know that it would hunt for the numbers and lines on the target rather than the actual texture at the center.

Last edited by Cannikin; 11-01-2010 at 11:54 AM.
11-01-2010, 12:49 PM   #21
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QuoteOriginally posted by Rondec Quote
Even with the 6 megapixel digital cameras that Pentax released in the beginning, softness wasn't as readily apparent. When you get up to 12 or 14 megapixels, it is a lot harder to get sharp phots and even small focus problems show up.
AF issues or other blur-introducing factors can easily be seen on a 6MP image. A 14MP camera has 53% more resolution, that doesn't make it a lot harder to get sharp photos. It does make it harder but that doesn't mean you cannot readily see softness in a 6MP image.

QuoteOriginally posted by cbaytan Quote
That sounds impossible, if it was so same batch of same lenses all would be need to same correction, but all individual lens might need different adjustment.
Every single lens copy differs from another in slight ways even if they are from the same batch. The lens elements aren't perfect, sometimes rotating them can improve lens performance and there are tolerances in how the elements are fit into the barrel. Decentering is a known phenomenon with lenses and it isn't either present or not, it occurs in many forms of severity.


QuoteOriginally posted by cbaytan Quote
So that tells us, in the focusing gear physically there are/is incorrect parts, or aging/worn parts needs to be adjusted IMO.
No, I'm very sure this is not the reason. Note that lenses may have FF/BF when they are brand new. And again, unless the AF gear is in very bad shape (no tight transmission), the camera would keep adjusting until it has obtained focus.

QuoteOriginally posted by Cannikin Quote
If I can get a DOF that's less than 1 cm wide to appear in the dead center of the frame at ~100% magnification on every object I test on, and infinity is right on as well, I'd say that's plenty precise enough for me .
Your focus is dead centre in the frame but where has the AF system focused on? You have no way of knowing whether it picked up the structure to focus on before, at, or beyond the centre of the frame. Remember we are talking about AF areas and the AF system is happy if it can find focus within that area.

QuoteOriginally posted by Cannikin Quote
I do not use a ruler or test chart because I know that it would hunt for the numbers and lines on the target rather than the actual texture at the center.
Rulers are a bad idea (as a focusing target) because again one has no way of knowing where the camera focused on exactly.

A good test chart has a very small focusing target (i.e., a black and white transition, not a small bar) which one can centre in the frame. Then it is clear where the focus was obtained and then it makes sense to check whether a ruler structure next to the target is sharp in the centre of the frame.
11-01-2010, 05:21 PM   #22
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QuoteOriginally posted by Class A Quote
the camera would keep adjusting until it has obtained focus.
I would have to agree with this. BF or FF should only be caused by a miss-adjusted/malfunctioning body, not a lens, providing there was adequate travel.
So far my K7 is better than my eyesight.
11-01-2010, 05:55 PM   #23
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QuoteOriginally posted by Ex Finn. Quote
I would have to agree with this. BF or FF should only be caused by a miss-adjusted/malfunctioning body, not a lens, providing there was adequate travel.
But note that the lens plays a part in this. Otherwise, there wouldn't be a need for individual adjustment values for individual lenses.

A camera body can be off (e.g., due to AF module misalignment) but that can be fixed by a single correction factor for all lenses. If one needs different values for different lenses than the latter contribute to FF/BF issues.

QuoteOriginally posted by Ex Finn. Quote
So far my K7 is better than my eyesight.
That's not hard if you are using the stock focusing screen. It doesn't show differences in DOF below f/2.8, i.e., things then will look always sharper than they are. Also, if the viewfinder magnification were larger, it would help.

11-01-2010, 06:51 PM   #24
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This is interesting because I found that my FA31 focusses better on my K5 compared to my KX. Will go and try to calibrate the KX if that is possible.
11-01-2010, 09:25 PM   #25
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just my worthless .02, but has been nagging at me since I read that method a long time ago. it is common knowledge that different specturms of light focus at different planes (on the film/sensor), thus focus is really only correct for one color of light, I assume 5500k.. which is why lenses had infrared focus marks.. with that said.. I've never been confident that using light from a monitor is the hot setup.. and I found trying to focus on that pattern, for me, to not be repeatable... so far I've found focusing on something slightly textured like pine wood with a black mark on it then zoomed in Adobe Bridge (or whatever) at 400+ magnification and looking at pixel contrast to be the most accurate...

as for the angled focus test chart.. I think that is worthless.. something flat and shot three times each starting at infinity seems best.. and is what Canon purports as the way to do it.

again, just my .02

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11-01-2010, 10:13 PM   #26
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QuoteOriginally posted by tsammyc Quote
Will go and try to calibrate the KX if that is possible.
You'll have to use the debug menu. Getting into the latter involves writing a special file to the SD card. I'm sure you'll find the details if you google for them.

QuoteOriginally posted by RogerD_AZ Quote
I've never been confident that using light from a monitor is the hot setup..
I agree. The K-7 & K-5 AF modules are probably alright because they adjust according to the wavelength but other cameras might be slightly miscalibrated after using this method.

QuoteOriginally posted by RogerD_AZ Quote
as for the angled focus test chart.. I think that is worthless.. something flat and shot three times each starting at infinity seems best.
Angled targets shouldn't be a problem, in theory. In practice, the AF calibration by Pentax also uses a flat target and only the ruler for checking where the focus is, is angled. I don't know why angled targets could be a problem and I don't think they are worthless but adjustment bracketing with a flat target is probably the safest bet.
11-01-2010, 10:56 PM   #27
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there are two professional focus chart products out there at the moment. Includes target and ruler at the proper angles. One by colorvision and ummm some smaller company who started making them first.
11-01-2010, 11:13 PM   #28
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QuoteOriginally posted by WerTicus Quote
One by colorvision and ummm some smaller company who started making them first.
By "smaller company" do you mean the one that produces the "LensAlign" tool?

Have you got information about the ColorVision Focus Calibration tool? I can only find a store that sells it but with zero info about the product. (except the price). EDIT: Is it this "SpyderLensCal"?
11-02-2010, 05:37 AM   #29
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QuoteOriginally posted by filmamigo Quote
Andi Lo, if you check the forums for different brands and go back a few years, you see that front focus/back focus issues were common. But the recommended solution was to keep returning lenses and try again and again until you got one that focussed perfectly on your body. I'd much rather have AF fine tuning. This was a key reason I upgraded my K10D to a K20D. I had a lens that I loved, and it was in perfect agreement with my *ist DL. With my K10D, the same lens was soft... actually, it was sharper than ever but it was backfocusing. The K20D rescued that lens for me.
Thanks for the info. My k10d is focusing perfect with all my lenses right now thankfully (and they're sharper than ever like you said). I won't be deterred going to K5 because of this issue anyway I'll just send the lens to pentax if this occurs in the future (knock on wood).
11-02-2010, 06:36 AM   #30
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QuoteOriginally posted by Ex Finn. Quote
I would have to agree with this. BF or FF should only be caused by a miss-adjusted/malfunctioning body, not a lens, providing there was adequate travel.
So far my K7 is better than my eyesight.
AF works by measuring phase differences on light entering the lens on the left and right side in two small spots. These spots are at a distance equal to about f/5.6. Problems start to appear when lenses are misaligned. The AF system could then measure perfectly in-phase light, but an image could still be out of focus on the sensor or the focus screen.

Here are 17 pages with technical information: http://www.dougkerr.net/pumpkin/articles/Split_Prism.pdf
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