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12-14-2009, 09:43 PM   #1
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k-x back focus problem?

I just plunged into the DSLR world from a Panasonic fz8 so bear with me. I suspected some focusing issues with the 18-55mm kit lens with my new k-x so I printed out a test chart and followed the instructions for conducted a controlled focus test (camera mounted to tripod with MLU enabled). Below is the result. Back focus problem? Should I send the camera back to Newegg? I'm running out of my 30 day window in just a few days.

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12-15-2009, 01:06 AM   #2
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its probably the lens, id be willing to bet it goes from front to back focus depending on the zoom level.

you can adjust the focus with a hack. in any case it does not look very bad... the 4mm looks to be best, my kx with a 50mm f1.4 that i just tested then it 4mm front focused.

So im about to see if the adjustment fixes it. I've noticed before with my k20d that there is many many variables involved. Such as lighting conditions you took the test under, zoom levels, subject distance.

you might well 'fix' a back focus issue only to create a front focus issue in different circumstances. So unless you are experiencing a problem in the real world id be careful.
12-15-2009, 05:58 AM   #3
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Thank you Werticus. I had noticed what I thought to be some soft images primarily of my kids so it may be a combination of slight back focus and kids moving. I want it to be perfect, but I don't know if it warrants a hack at this time. Is front focus & back focus problems common to other DSLRs & lenses?
12-15-2009, 08:04 AM   #4
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Try with more lenses. If all lenses miss the focus in the same direction, you can use the debug mode to fine tune autofocus. I just did that for my K-x....satisfied now.

If different lenses miss focus in different ways, then K-x cannot handle this case. Only K20d and K-7 will do.

12-15-2009, 08:53 AM   #5
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I assume that is a crop from the full image? Could you post the full image, to better judge if you actually performed the test correctly?
12-15-2009, 09:23 AM   #6
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Yes, that is a crop from the test chart image. I will try to post the full image later today. The only other lens that I currently have is the DA L 55-300mm. Can I use the same standard lens test chart to check focus with this telephoto lens? I downloaded the chart from pentaxdslrs blog.
12-15-2009, 10:03 AM   #7
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you can use the same chart, at least when they are both around 50mm mark.
12-15-2009, 10:37 AM   #8
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QuoteOriginally posted by pexley Quote
I just plunged into the DSLR world from a Panasonic fz8 so bear with me. I suspected some focusing issues with the 18-55mm kit lens with my new k-x so I printed out a test chart and followed the instructions for conducted a controlled focus test (camera mounted to tripod with MLU enabled). Below is the result. Back focus problem? Should I send the camera back to Newegg? I'm running out of my 30 day window in just a few days.

Attachment 49573
BF/FF = sad reality of PDAF in dSLR... use the undocumented Debug Mode feature to correct that... because it can happen w/ another Kx body and another lens... or you can send Kx and lens to Pentax (is it sill up to 4 lens if tuning done by Pentax Service Center) and they should be able to fix the issue for your specific body/lens combo

PS: what I really do not understand is why Pentax does not provide an option to do an automatic BF/FF correction now that we have CDAF available too ? it seems so easy to implement - put a camera on a tripod w/ some high contrast target suitable for both PDAF and CDAF focusing, select some item in the camera menu and camera will do PFAF focusing on a target, raise the mirror, do CDAF focusing, lower the mirror, do PDAF focusing again - record the necessary correction for PDAF in camera's memory... that can be done even w/o tripod - just put a camera on some solid surface... and that option can be provided along w/ the option to enter manual corrections if you do not wish to use the automated procedure... come on Pentax, somebody kick their firmware engineers in Japan finally !

12-15-2009, 05:01 PM   #9
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Full-sized images

Ok...below is a link to a public Picasa gallery showing full-sized test chart images of both the DA L 18-55mm & DA L 55-300mm lenses. I'm not sure that I conducted the tests correctly especially with the telephoto lens. It's also not the best lighting, but it's all I could do at the moment. Please let me know if you see any back or front focusing issues.

Picasa Web Albums - Patrick - k-x test char...
12-15-2009, 05:27 PM   #10
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To start with, are you having any focus issues with your regular shots that lead you to check the focus?

Second, don't try the focus test under bad lighting, especially tungsten. Tungsten light can be a cause of focus issues.
12-15-2009, 08:24 PM   #11
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Is it shot at 45 degree's to the camera?, so any reading off it will not be exaggerated or understated.
12-15-2009, 09:17 PM   #12
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I've uploaded 2 more pictures to the Picasa k-x test chart images album. One is an updated telephoto test chart at 45 degrees. The other is the picture that I took of my soon, which is where I first became suspicious of a focusing problem. Perhaps I'm just expecting too much from the camera. When I was comparing DSLRs my "runner up" camera was the Olympus e-620. I saw some very sharp pictures from users of this camera, and I guess I was hoping to get similar results with my k-x. I think that I can, but I guess I still have a lot of learning to do, and I wanted to make sure that there wasn't a problem with the camera or lenses.
12-16-2009, 10:30 PM   #13
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The focus test chart pictures you posted are kind of all over the map. The chart has to be *flat*, you have to be shooting it *straight on* (not from off to the side, and it has to be on angle, not from directly above.

Anyhow, in the picture of your son does appear the camera chose to focus behidn him. No camera can read you mind. If you did not specifically choose the center focus point, the camera might well have picked a different one. And even if you did, the center focus point is larger than your son's head appears in the viewfinder, so it's impossible to force the camera to focus there is something in the background is begging for the camera's attention as is the case here. In situations like this, you should shoot from close enough that the subject fills the entire center brakceted area of the viewfinder, and be sure to tell the camera to use the center focus point. or simply focus manually.
12-17-2009, 12:04 AM   #14
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Looking at your son photo id say the camera chose to focus on the flowers, it even looks like your aiming at them not the boy.

Or at least your aiming at his black head which offers nothing to lock focus on so it probably chose to focus above that.

Operator error on that one, but your test charts do show slight BF assuming you've done them correctly.
12-17-2009, 12:36 AM   #15
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Try the focus charts on this page: Jeffrey Friedl’s Blog Jeffrey’s Autofocus Test Chart

The gradations are very fine so you can clearly see how the focus is doing. The page also has great info on how to setup the camera and interpret the results.

- Jason
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