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11-17-2015, 11:48 AM   #1
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Back-focus problem on k-x with new 50mm prime

I bought a new Pentax DA 50mm 1.8 prime lens for my k-x and immediately noticed a back-focus problem. So I returned the lens back to Amazon and got a replacement. The second lens too shows the same problem (probably to a lesser degree, but I could be mistaken). To be double sure, I tested the second lens on my cousin's k-x and found it to be working fine.

So the culprit looks to be my camera. But then the same camera focuses flawlessly with my two kit lenses, 18-55 and 50-200. Can this data be enough to say for sure the problem is only with the camera?

I am aware of dialing-in focus correction through the debug menu, but from whatever I have read, it seems the correction applies to ALL lenses on the k-x. Is there anything else I can try? Or getting the camera serviced is my only option? Please advise.

Thanks,
Sushant

11-17-2015, 12:43 PM - 1 Like   #2
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For one thing, front/back focus problems become more evident the larger the aperture is. So, the 50mm 1.8 can show you problems that are hidden with your two other lenses with relatively small aperture. Your back focus problem could be from the Kx but is not important enough to be visible unless you're using a large aperture lens.

So, I would try to correct the back focus from the body. Then look how things work with the two other lenses. If it looks as good or better than before, you're all set. If the DA50 works fine but not the others, I fear it will need servicing...
11-17-2015, 01:24 PM   #3
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Back or front focus problems are usually because the factory that makes the camera and lens has some small tolerances. But if both lens and camera are on other side of the tolerance, then you might get back or front focusing. This is usually a specific lens and camera combination. Your camera might work great with some lenses, but not with others. And those lenses might work great with some cameras, but not this one. It happens with all brands, except maybe with brands like Leica (but they cost 15x as much as Pentax). This is why all the cameras have focus correction.
QuoteOriginally posted by sushantji Quote
I am aware of dialing-in focus correction through the debug menu, but from whatever I have read, it seems the correction applies to ALL lenses on the k-x. Is there anything else I can try?
Correct, this is the problem for you, because your other lenses work well. Here are your options:
a) You can try buying another camera like K-x or K-r and hope that it will work well with all your lenses.
b) You can buy another camera like K-5 or newer cameras, because they can remember 10 different lens AF settings; maybe you will not even need it.
c) You can send the camera and lenses for calibration. If you do this, a technician would make them all work together. But this is not free. Especially if this service is not done locally (some shops ship this to another country, shipping is risky and slow)
d) Taking the lens apart yourself and adjusting focus yourself might be too risky, because you lose warranty. But if you have knowledge and tools, maybe you want to try. I would not recommend it.
e) You can add an AF adjustment for the 50mm or something between the 50mm and your other lenses. This is a compromise, where the AF might not be perfect, but might still be better. (CarlJF explained this above)

I don't know of any other options for you, sorry. You can try option e) first, because it costs you nothing. Also, make sure you have the latest camera firmware on your camera, as it might contain improved lens profiles for the DA 50mm. There are many threads and blog posts and youtube videos about firmware updating.
If I were you, I would first check the prices of option c) and maybe the warranty can cover some of this cost. You can try looking online for instructions for option d). Your next best option is b), where you get the bonus of newer, better camera (depending on camera: you can get 16, 20, 24MP, faster AF, better video options, WR, focus peaking and other features like that). You can find a classic K-5 or a K-5II for a good price these days, and it is better than K-x. Or a K-S2, which has all newest technology.
Good luck, and report back how it goes

Edit: Oh, one more thing! DA 50mm f1.8 is has much much more shallow DoF than the other lenses you mentioned. It is possible that you got some blurry photos because of technique or because DoF is so shallow that at f8, the photo would look okay, but at f1.8, it looks OoF. Lenses faster than f2.8 are much more difficult to focus, so give it a chance and make sure that the focusing is wrong because of the machine, not any other reason (like camera choosing other focus point than you wanted).

Last edited by Na Horuk; 11-17-2015 at 01:36 PM.
11-18-2015, 11:13 AM - 1 Like   #4
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Thank you Na Horuk for the detailed reply. I will try these options and update.

Best Regards.

Edit: Forgot to mention, I'm already running latest available firmware on my camera, v1.03. Regarding the focus issue, I've fully ensured that the problem is not because of camera selecting wrong focus points, shake, or the almost inevitable (slight) softness at widest open aperture.


Last edited by sushantji; 11-18-2015 at 11:20 AM.
11-20-2015, 09:55 PM   #5
osv
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dslr/dslt cameras sometimes need in-camera focus correction because of focusing errors that are caused by their mirrors.

it looks like your camera will focus in liveview, which removes the mirror as a factor in the af process: Autofocus in Live View (K-7 and K-x) | Ricoh Imaging Support

1)i'm wondering if it focuses accurately in live view mode, with no mirror, you know that it's not a lens hard stop adjustment problem?
2)check the infinity hard stop setting on the lens by using live view with magnification, and manual focusing.
11-22-2015, 12:25 PM   #6
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QuoteOriginally posted by osv Quote
1)i'm wondering if it focuses accurately in live view mode, with no mirror, you know that it's not a lens hard stop adjustment problem?
Yes, it focuses fine in the live view mode.

QuoteOriginally posted by osv Quote
2)check the infinity hard stop setting on the lens by using live view with magnification, and manual focusing.
Thanks for the tip, should surely help troubleshooting and fine tuning. Couldn't do much testing this weekend, will try this shortly and update.

Last edited by sushantji; 11-22-2015 at 12:26 PM. Reason: removed full reply quote
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