50mm 1.7 autofocus inaccurate?

jremick · 12-27-2010, 03:52 PM

I bought a used 50mm 1.7 via the marketplace here in essentially mint condition and over the last few days I've noticed it's having issues focusing accurately. I assumed it was user error so I tried different things but it seems to be pretty consistently focusing slightly farther than where I have the center focus point aimed.

Here are example shots with the center-point aimed at the top corner of the center pillow.

autofocus: IMGP1450.jpg
manual focus: IMGP1451.jpg

These shots were taken in aperture priority mode (mode doesn't seem to make a difference), ISO 800, f2.2, 1/40s with the center AF point.

Any ideas as to whether I'm doing something wrong or if something is wrong with the camera/lens?

I don't have this problem with other lenses so I assume it's an issue with this particular lens. Since I haven't read about this issue with this lens, I assume it's just the copy I have. Thoughts?

**nater** · 12-28-2010, 11:18 AM

If the focus point is accurate when you take a shot at f/1.7 (rather than f/2.2), you are probably experiencing focus shift.

Pentaxor · 12-28-2010, 11:39 PM

try using your camera's AF adjustment. your lens could be backfocusing.

Jewelltrail · 12-29-2010, 12:22 AM

I imported both of your samples into a photo browser and enlarged them. What I see is the first image is back focused, but the 2nd image, taken just 9 secs later, is pretty much spot on (center is in focus better than any other part of the image).

Also, my photo browser says they were shot @ 1/100th, not 1/40th as you say. !/40th is more likely to have some shake in the shot. EXIF on my end also shows 640 ISO, not 800.

Remember, pillows are angled in their distribution, creating varying distance from camera--this, coupled with shallow DOF from 2.2 aperture is only going to have a small area of the image in focus, which is what I see in 2nd shot--1451.

jremick · 12-29-2010, 03:59 PM

Originally posted by Jewelltrail

I imported both of your samples into a photo browser and enlarged them. What I see is the first image is back focused, but the 2nd image, taken just 9 secs later, is pretty much spot on (center is in focus better than any other part of the image).

Also, my photo browser says they were shot @ 1/100th, not 1/40th as you say. !/40th is more likely to have some shake in the shot. EXIF on my end also shows 640 ISO, not 800.

Remember, pillows are angled in their distribution, creating varying distance from camera--this, coupled with shallow DOF from 2.2 aperture is only going to have a small area of the image in focus, which is what I see in 2nd shot--1451.

Oops, sorry about that. That is correct (1/100th, etc). I must've had another photo selected when I was looking at that info.

The first image is using AF, the second using manual focus. It looks like either the lens or the body has a back focus problem, but I haven't had time to figure that out just yet. As soon as I do and I (hopefully) get the issue fixed, I'll post back here again.

Thanks everyone!

jremick · 12-29-2010, 10:39 PM

Well, now I'm back to square one (or possibly zero). Before I attempted to "fix" anything, I wanted to do a lot more tests to make sure it was a back-focus issue. I was able to consistently reproduce the issue in every photo so I swapped lenses to see if it was a lens or body issue. No issues with other lenses so I switched back to the 50mm.

Now the 50mm is focusing correctly... I'm no longer able to reproduce the issues I was having before. So, now I have no idea what the problem could've been, but it seems to be gone... until it decides to come back at the most inconvenient moment to ruin more photos.

Class A · 12-30-2010, 02:30 AM

Originally posted by jremick

So, now I have no idea what the problem could've been, but it seems to be gone...

The most likely explanation to me seems that the camera had a different idea what to focus on than you did. Bear in mind that the AF "points" really are AF areas.

If you want to check any of your lenses for FF/BF again you may want to check out these AF adjustment hints.

Jewelltrail · 12-30-2010, 03:44 AM

Did you have "center focus" selected on the camera body? Low light can wreak havoc on a focus system. Good luck, hope there are no more posts here by you for focus problems.

ytterbium · 12-30-2010, 07:22 AM

I've had minor AF errors with 50/1.7 but nowhere near the difference shown in your photos.
I wonder what can actually change with simply re-mounting the lens. Only thing might be the AF point, since K-X doesn't display them.
There are no AF adjustments or other similar settings on that camera.

jremick · 12-30-2010, 12:19 PM

Originally posted by Jewelltrail

Did you have "center focus" selected on the camera body? Low light can wreak havoc on a focus system. Good luck, hope there are no more posts here by you for focus problems.

Yup, although I did test out the other modes as well.

I read on another site that the contacts between the lens and the body could've gotten a little dirty causing AF issues and I rented a lens over the Christmas holiday (when the issue showed up), so maybe that was the problem.

Thanks for the help everyone!

12-27-2010, 03:52 PM	#1
jremick Senior Member Join Date: Mar 2010 Location: Melbourne Photos: Gallery Posts: 141	50mm 1.7 autofocus inaccurate? I bought a used 50mm 1.7 via the marketplace here in essentially mint condition and over the last few days I've noticed it's having issues focusing accurately. I assumed it was user error so I tried different things but it seems to be pretty consistently focusing slightly farther than where I have the center focus point aimed. Here are example shots with the center-point aimed at the top corner of the center pillow. autofocus: IMGP1450.jpg manual focus: IMGP1451.jpg These shots were taken in aperture priority mode (mode doesn't seem to make a difference), ISO 800, f2.2, 1/40s with the center AF point. Any ideas as to whether I'm doing something wrong or if something is wrong with the camera/lens? I don't have this problem with other lenses so I assume it's an issue with this particular lens. Since I haven't read about this issue with this lens, I assume it's just the copy I have. Thoughts?

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12-28-2010, 11:18 AM	#2
nater Loyal Site Supporter Join Date: Aug 2009 Location: New England Photos: Albums Posts: 706	If the focus point is accurate when you take a shot at f/1.7 (rather than f/2.2), you are probably experiencing focus shift.

12-28-2010, 11:39 PM	#3
Pentaxor Veteran Member Join Date: May 2009 Location: Vancouver, B.C. Photos: Gallery Posts: 6,513	try using your camera's AF adjustment. your lens could be backfocusing.

12-29-2010, 12:22 AM	#4
Jewelltrail Veteran Member Join Date: May 2008 Location: Rhode Island Photos: Albums Posts: 4,180	I imported both of your samples into a photo browser and enlarged them. What I see is the first image is back focused, but the 2nd image, taken just 9 secs later, is pretty much spot on (center is in focus better than any other part of the image). Also, my photo browser says they were shot @ 1/100th, not 1/40th as you say. !/40th is more likely to have some shake in the shot. EXIF on my end also shows 640 ISO, not 800. Remember, pillows are angled in their distribution, creating varying distance from camera--this, coupled with shallow DOF from 2.2 aperture is only going to have a small area of the image in focus, which is what I see in 2nd shot--1451.

12-29-2010, 03:59 PM	#5
jremick Senior Member Join Date: Mar 2010 Location: Melbourne Photos: Gallery Posts: 141 Original Poster	Originally posted by Jewelltrail I imported both of your samples into a photo browser and enlarged them. What I see is the first image is back focused, but the 2nd image, taken just 9 secs later, is pretty much spot on (center is in focus better than any other part of the image). Also, my photo browser says they were shot @ 1/100th, not 1/40th as you say. !/40th is more likely to have some shake in the shot. EXIF on my end also shows 640 ISO, not 800. Remember, pillows are angled in their distribution, creating varying distance from camera--this, coupled with shallow DOF from 2.2 aperture is only going to have a small area of the image in focus, which is what I see in 2nd shot--1451. Oops, sorry about that. That is correct (1/100th, etc). I must've had another photo selected when I was looking at that info. The first image is using AF, the second using manual focus. It looks like either the lens or the body has a back focus problem, but I haven't had time to figure that out just yet. As soon as I do and I (hopefully) get the issue fixed, I'll post back here again. Thanks everyone!

12-29-2010, 10:39 PM	#6
jremick Senior Member Join Date: Mar 2010 Location: Melbourne Photos: Gallery Posts: 141 Original Poster	Well, now I'm back to square one (or possibly zero). Before I attempted to "fix" anything, I wanted to do a lot more tests to make sure it was a back-focus issue. I was able to consistently reproduce the issue in every photo so I swapped lenses to see if it was a lens or body issue. No issues with other lenses so I switched back to the 50mm. Now the 50mm is focusing correctly... I'm no longer able to reproduce the issues I was having before. So, now I have no idea what the problem could've been, but it seems to be gone... until it decides to come back at the most inconvenient moment to ruin more photos.

12-30-2010, 02:30 AM	#7
Class A Pentaxian Join Date: Aug 2008 Location: Wellington, New Zealand Posts: 11,251	Originally posted by jremick So, now I have no idea what the problem could've been, but it seems to be gone... The most likely explanation to me seems that the camera had a different idea what to focus on than you did. Bear in mind that the AF "points" really are AF areas. If you want to check any of your lenses for FF/BF again you may want to check out these AF adjustment hints.

12-30-2010, 03:44 AM	#8
Jewelltrail Veteran Member Join Date: May 2008 Location: Rhode Island Photos: Albums Posts: 4,180	Did you have "center focus" selected on the camera body? Low light can wreak havoc on a focus system. Good luck, hope there are no more posts here by you for focus problems.

12-30-2010, 07:22 AM	#9
ytterbium Veteran Member Join Date: Jan 2008 Posts: 1,076	I've had minor AF errors with 50/1.7 but nowhere near the difference shown in your photos. I wonder what can actually change with simply re-mounting the lens. Only thing might be the AF point, since K-X doesn't display them. There are no AF adjustments or other similar settings on that camera.

12-30-2010, 12:19 PM	#10
jremick Senior Member Join Date: Mar 2010 Location: Melbourne Photos: Gallery Posts: 141 Original Poster	Originally posted by Jewelltrail Did you have "center focus" selected on the camera body? Low light can wreak havoc on a focus system. Good luck, hope there are no more posts here by you for focus problems. Yup, although I did test out the other modes as well. I read on another site that the contacts between the lens and the body could've gotten a little dirty causing AF issues and I rented a lens over the Christmas holiday (when the issue showed up), so maybe that was the problem. Thanks for the help everyone!