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07-02-2010, 03:20 PM   #16
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The first shot (AF) does not look very good regardless of where it was focused. So, I think there is a problem. Just not quite convinced where the problem is :-)

It was not quite the test I was expecting, though (along a wall or fence or something)...

07-02-2010, 03:22 PM   #17
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Nothing seems to be focused in the af image, you did take multiple shots and they all look alike?.
Yes ,a fence would make it easier to see where the focus is.
07-02-2010, 04:32 PM   #18
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In reference to the flower shots.

They both look lousy. I noticed that they are attached images. My experience on Pentax Forums is that an attached image may be munged by the forum software. The result is usually a significant decrease in sharpness. It is generally better to link to an image posted elsewhere. Do you have a copy posted elsewhere...say a gallery posting?

As noted above, a fence or some other surface at an angle is usually used for determining front/back focus issues. Centering issues are usually addressed with newsprint on a flat surface at 20x the focal length distance. Extreme care should be taken for the second case to make sure that the sensor is parallel to the target. Care should also be taken to make sure that the center is actually in focus.


Steve

Last edited by stevebrot; 07-02-2010 at 04:38 PM.
07-02-2010, 07:29 PM   #19
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Very helpful everyone! I'll take some fence shots on Sunday (I'm gone tomorrow). Should the sensor be parallel to the fence or what?

Here are a couple shots, cropped of course. These are center point focus, f/2.8 first is AF, second is manual focus. It seems from a bit more testing that f/2.8 is where it focuses the worst. I'll have to check into that more though...

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07-02-2010, 07:37 PM   #20
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QuoteOriginally posted by Ubuntu_user Quote
Very helpful everyone! I'll take some fence shots on Sunday (I'm gone tomorrow). Should the sensor be parallel to the fence or what?

Here are a couple shots, cropped of course. These are center point focus, f/2.8 first is AF, second is manual focus. It seems from a bit more testing that f/2.8 is where it focuses the worst. I'll have to check into that more though...
The idea of the fence is to shoot it at an angle so that at least one part of the fence is in focus. Front/Back focus can be determined by comparing where the focus actually is relative to the intended point of focus. If everything is blurry, the issue is not with the focus system. Be sure to do the test on a tripod with SR turned OFF to remove both sources of variability.


Steve
07-04-2010, 10:07 AM   #21
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Photoshop.com - IMGP4677.JPG

Photoshop.com - IMGP4678.JPG


The first is AF, second is MF. F/2.8 center point focus. BTW, the post right in the middle of the picture was exactly in the center of the view finder, directly in between the little ( ) thing. My D FA 100 Macro hits it right on with AF, why can't this lens do that?
07-04-2010, 10:59 AM   #22
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I can't say anything else, out of your examples can't find any kind of fault or explanation here. Yes, there's back-focusing, but you have a K-7 so the focus adjustment could help. Also, keep in mind that the 100mm has a lot less angle of view and depth of field so it's not necessarily a good direct comparison for this kind of fence. The last thing I can tell is take the lens out for the weekend if possible and shoot _only or mostly_ in AF to see what you can do with it.

That said, if you are not happy with it you might want to exchange your lens OR you might try contacting the seller to see if they can try to adjust it for you, but keep in mind that you might be still unhappy with the lens. In which case you might want to look at a different lens altogether.
07-04-2010, 11:15 AM   #23
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PENTAX DSLRs: PART-1. Autofocus Adjustment for the Pentax K20D, Custom Setting No 35

07-04-2010, 12:56 PM   #24
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That's no backfocusing. Backfocusing is the camera missing focus by a few millimeters on the target it chose. The discrepancy here is one of many meters. That's simply a case of the camera *choosing* to focus somewhere other than where you intended. It either saw right through the fence to the background and choose to focus there (a legitimate choice), or it is not using the center point even thoug you selected it (if so, a camera problem).
07-04-2010, 01:24 PM   #25
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QuoteOriginally posted by Marc Sabatella Quote
That's no backfocusing. Backfocusing is the camera missing focus by a few millimeters on the target it chose. The discrepancy here is one of many meters. That's simply a case of the camera *choosing* to focus somewhere other than where you intended. It either saw right through the fence to the background and choose to focus there (a legitimate choice), or it is not using the center point even thoug you selected it (if so, a camera problem).
Ah! I agree. It is as though the camera just "chooses" to disregard the center focus point and then focuses somewhere else. If it were a problem with the camera, then wouldn't I have the same issue with other lenses?
07-04-2010, 03:40 PM   #26
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QuoteOriginally posted by Marc Sabatella Quote
That's no backfocusing. Backfocusing is the camera missing focus by a few millimeters on the target it chose. The discrepancy here is one of many meters. That's simply a case of the camera *choosing* to focus somewhere other than where you intended. It either saw right through the fence to the background and choose to focus there (a legitimate choice), or it is not using the center point even thoug you selected it (if so, a camera problem).
If that's back focusing, it's back focusing to infinity. Frankly, I think he's conducting the test very poorly.

Ubuntu, please use a simple flat surface, not a chain link fence with holes in it. Try a wooden one or a wall with decorative wallpaper. Even better, print out a focus distance chart and use that. Focus on closer distances, like 2-3 feet away, not 15 feet away.
07-04-2010, 06:44 PM   #27
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QuoteOriginally posted by hangu Quote
If that's back focusing, it's back focusing to infinity. Frankly, I think he's conducting the test very poorly.

Ubuntu, please use a simple flat surface, not a chain link fence with holes in it. Try a wooden one or a wall with decorative wallpaper. Even better, print out a focus distance chart and use that. Focus on closer distances, like 2-3 feet away, not 15 feet away.
I know... I don't have a good place to test it on. Here's a side of my neighbors house. Again, f/2.8 center point focus. First AF, second MF.

Photoshop.com - IMGP4675.JPG

Photoshop.com - IMGP4676.JPG
07-04-2010, 06:56 PM   #28
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Oh here's a photo from a wedding I attended yesterday... Center point focus, f/5.6 I put center focus point on the brides face, locked the focus, then reframed the image. What do you think?

Photoshop.com - IMGP4663.JPG
07-04-2010, 07:00 PM   #29
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QuoteOriginally posted by Ubuntu_user Quote
I know... I don't have a good place to test it on. Here's a side of my neighbors house. Again, f/2.8 center point focus. First AF, second MF.

Photoshop.com - IMGP4675.JPG

Photoshop.com - IMGP4676.JPG
That is literally the worst possible test subject I've ever seen.

Again, please use an object with some sort of discernible pattern on it (I use my keyboard in a pinch and focus on the letter H key). Please stand at a 45 degree angle to it. Try to get as close as you can to the object for the narrow depth of field but still have most of the object in your viewfinder. Don't forget to switch it to the longest reach possible (50mm).
07-04-2010, 07:08 PM   #30
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QuoteOriginally posted by Ubuntu_user Quote
Oh here's a photo from a wedding I attended yesterday... Center point focus, f/5.6 I put center focus point on the brides face, locked the focus, then reframed the image. What do you think?

Photoshop.com - IMGP4663.JPG
Can't tell because:
A) You're probably using the wide end of it where the depth of field is humongous, pretty much everything here is in focus.
B) It's not cropped so no pixel peeping here.
C) Based on the poor ambient lighting and the absence of flash, my guess is you handheld it and used a slow shutter speed, which would contribute to the softness.

Until you do an adequate focus test, my guess is user error.
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