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05-16-2011, 10:41 AM   #1
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K-x back focusing

Hi all, I recently got myself an F50 1.7 which is undoubtedly a very nice lens. However as soon as I stuck it on my k-x I noticed that the center of the shot was always out of focus. I always use the center focus point.

I uploaded the shots onto my computer for a closer look and sure enough every single shot is focused slightly behind where I was actually focusing!

I searched here and there are plenty of threads (usually regarding front-focusing) but very few for the K-x.

All my other lenses focus perfectly so it really is only this one lens. I then tried it on my friends K20d and exactly the same thing! But alas my k-x has no focus adjustments that I can find - is there anything I can do, or is this a quirk I need to learn to live with and try and work around? I really love the lens otherwise!!!

Thanks, Az


Last edited by A-z; 05-16-2011 at 01:11 PM. Reason: typo
05-16-2011, 02:39 PM   #2
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RiceHigh's Pentax Blog: K-x Debug Mode Tutorial - AF Adjustment

Use the link and follow the directions. You will then be able to enter the debug menu of the K-x and correct the front/back focusing problems.
05-16-2011, 02:44 PM   #3
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The 50 almost certainly has shallower DOF than your other lenses, which could explain why any problems your camera might be having aren't seen with other lenses. But before assuming there really is a problem, you should do a *controlled* test with a good focus test chart, not just looking at regular pictures. Even though you are selecting the center focus "point", it isn't a point at all, but a largish blob, and the camera can legitimately focus anywhere within that area. So unless the thing you are pointing at is completely level and exactly perpendicular to the camera, you can't really ever be sure *exactly* where the focus will be. So for instance, you might aim the center point at one petal of a flower, but the petal behind that one might still be in range of that focus sensor. That much is something you have to live with. Focus test charts are designed to remove that ambiguity and determine for sure if you really have a problem or not.

Of course, if the answer is yes, the answer is likely that you will need to live with it anyhow, or else you can try sending the camera & lens in for service. Sometimes for some models there is a special undocumented "debug mode" that can be used to access lens adjustments on cameras that don't offer this feature normally, but I don't know if there are instruction floating around on ho to do this on the K-x or not.
05-16-2011, 03:10 PM   #4
A-z
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Hi, thanks for the responses, much appreciated.

That debug mode would be great if it was a general problem, but I've tried every other lens I own and they all nail the focus perfectly. As much as I love having a fast 50, it's not quite worth throwing my other lenses out for!!

Marc, I was very conscious of the DOF being so shallow, and that was my first thought. However I also have an M50 1.7 which focuses perfectly wide open, both by eye (20/20 vision me!) or using catch in focus... I've done some (uncontrolled) tests with a few lenses at once of the same scene and it's definitely just the new F 50 that misses. It is really sharp, but not in the center - always behind!

I assume that if I enter the debug mode and adjust the back focusing that it will apply to every lens I use, therefore making all my other lenses front-focusing?!?

Is there any way to adjust the lens itself to correct this, or is that a job for the service center?

05-18-2011, 12:56 PM   #5
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QuoteOriginally posted by A-z Quote
Hi all, I recently got myself an F50 1.7 which is undoubtedly a very nice lens. However as soon as I stuck it on my k-x I noticed that the center of the shot was always out of focus. I always use the center focus point.

I uploaded the shots onto my computer for a closer look and sure enough every single shot is focused slightly behind where I was actually focusing!

I searched here and there are plenty of threads (usually regarding front-focusing) but very few for the K-x.

All my other lenses focus perfectly so it really is only this one lens. I then tried it on my friends K20d and exactly the same thing! But alas my k-x has no focus adjustments that I can find - is there anything I can do, or is this a quirk I need to learn to live with and try and work around? I really love the lens otherwise!!!

Thanks, Az
I feel your pain!!!

I've have both Pentax-F 50 f1.4 and f1.7 each focuses superbly on my K-m but back focus on my K10. However, the K10 can properly focus every other AF lens I own except for these 2 and these 2 are the ones I'd like to use on my K10...
05-18-2011, 06:09 PM   #6
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After a year or so of use / 10 000 actuations,

my K-x also has gotten more and more 'back-focused' over time,

on top of that the horizon of all recorded pics is now off by 1 or so.
(recorded pic & pic review on LCD are not level to what I composed/shot thru the viewfinder.

Off to the repair center while it's still under warranty...

Michel

Last edited by mlatour; 05-18-2011 at 06:36 PM.
05-19-2011, 03:14 PM   #7
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If it's just one lens, your only option would be to have the camera serviced. What they will do is to program the body memorizing that lens for AF compensation. The DEBUG MODE will do you no good in this case.
05-19-2011, 06:13 PM   #8
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This issue of no AF fine tuning, and the issue of not being able to use a cable release (esp for HDR) are the 2 reasons pushing me to upgrade my K-x. Once you own a few lenses you are going to run into this problem where some lenses will FF and others will BF.

05-20-2011, 12:49 AM   #9
A-z
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Yep, very frustrating, I've decided to sell the lens as I cant justify a body upgrade any time soon. A real pity as I really love the lens. Back to my M-50, sharp as a razor!

I'm keeping my eye open for another copy, or I may grab a DA 40 instead...

Twitch, can't you use a remote?
05-20-2011, 01:08 AM   #10
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With the kx debug mode you can change your af adjust in the field. You just leave the modset file in the root of your sd card and boot into debug mode whenever you need to change the af adjust to suit an individual lens. You just need to note the values for each lens somewhere. This is not as convenient as the k5 which remembers the values for each lens but still allows for per lens focus adjust.
05-20-2011, 01:25 AM   #11
A-z
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So I can power up the camera in 2 different modes? Or do I need to enter debug mode every time I change between lenses and adjust the setting?
05-20-2011, 01:41 AM   #12
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Yes you will need to enter debug mode for each lens. However you will find that once you have adjusted for the fa 50 all other lenses will also be focusing accurately with the same adjustment. In all likelihood your kx focus was out all the time it's just not noticeable when the depth of field is not as shallow as the fa 50 at 1.8 or 1.4
05-20-2011, 01:42 AM   #13
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You can enter debug mode and leave it enable at all time. The downside is that the LCD will be on even when blank. Or you can leave the modset file on the SD card root directory so that the debug mode can be activated in the field. You cannot format the SD card however or the file will be lost. Either way, you have to alter the um setting whenever you switch lenses. This will probably drive me insane though in practice.

My DA14 & FA31 are giving me trouble as well and I need 3 different settings for my K-m. Eventually I had to mess with the actual AF sensor hoping the root problem could be addressed. After lengthily adjustment, it "seems" to be accurate enough with all my lenses, but I will have to do more field tests to be certain.
05-20-2011, 08:33 AM   #14
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QuoteOriginally posted by A-z Quote
Yep, very frustrating, I've decided to sell the lens as I cant justify a body upgrade any time soon. A real pity as I really love the lens. Back to my M-50, sharp as a razor!
I still say that without seeing well-controlled test images from a well-designed focus test chart - and comparison test images from other lenses with equally shallow DOF - the chances that there is an issue with your lens is pretty small. Most reported problems turn out to be due to the factors I've described - camera simply choosing to focus somewhere slightly different from where you expect by far the most common culprit, misaligned focus sensor in camera the next most likely; lens defects that somehow cause the camera to think the image is in focus when it isn't would be something you'd only consider after rigorous testing that definitely rules out the much more likely causes.
05-20-2011, 10:18 AM   #15
A-z
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Marc, I hear ya, it's just that my M50 1.7 is tack sharp in the center open wide. My F50 appears just as sharp through the viewfinder, but the shots come out back focused. Whether I use AF of manual makes no difference, so it can't be that the camera is choosing to focus elsewhere.

I'll try and muster up the patience to do one of those controlled focus tests, but I'd far rather be out shooting!
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