How can we repair a lens that is back focusing excessively?

HarisF1 · 11-12-2021, 09:32 AM

Hey guys, I have a lens that has excessive back focus. I've used it in the past with +10 but it still won't produce adequately sharp focus.

I've now tested it on numerous bodies with the same issue so I'd like to have a look at sorting it out. Has anyone ever managed to tweak the AF tuning in the lens itself?

Best Regards
Haris

Last edited by HarisF1; 11-13-2021 at 02:49 AM.

stevebrot · 11-12-2021, 10:15 AM - **1 Like**

Originally posted by HarisF1

Has anyone ever managed to tweak the AF tuning in the lens itself?

There is no AF tuning in the lens per se. Fixing the lens involves placing it on a test/repair harness and adjusting alignment and movement of the optics.

Steve

**jumbleview** · 11-12-2021, 10:48 AM

Is it focuses OK in the live view?

HarisF1 · 11-12-2021, 11:30 AM

Yes, focusing in live view is bang on.

**jumbleview** · 11-12-2021, 11:40 AM

Originally posted by HarisF1

Yes, focusing in live view is bang on.

It looks like for this lens your Pentax camera converted to mirrorless only. It is the industry trend.

**Bob 256** · 11-12-2021, 01:04 PM

Not sure how I read this thread so far??? If your lens focuses ok in live view, that would point to an AF error in the optical viewfinder of the camera but since you said this is across several cameras, that would appear not to be the case unless all of those cameras are mis-adjusted for optical AF. That points back to the lens again and the fact that it has an issue which makes the lens disagree with where a standard AF camera would expect to see the best focus (using 0 offset) and what stevebrot said. It shouldn't require that much AF offset to begin with.

Probably time to send it off.

HarisF1 · 11-12-2021, 01:34 PM

Originally posted by Bob 256

Not sure how I read this thread so far??? If your lens focuses ok in live view, that would point to an AF error in the optical viewfinder of the camera but since you said this is across several cameras, that would appear not to be the case unless all of those cameras are mis-adjusted for optical AF. That points back to the lens again and the fact that it has an issue which makes the lens disagree with where a standard AF camera would expect to see the best focus (using 0 offset) and what stevebrot said. It shouldn't require that much AF offset to begin with.

Probably time to send it off.

You've read it perfectly

It might need sending off to someone with the correct rig. The alternative is for me to use the K-1 debug menu to add an offset to the AF fine tuning adjustment which will bring this lens back to with the +/-10 that can be set within the camera body. This would mean I need to compensate by adjusting every 'good' lens so it probably isn't worth the effort.

**DWS1** · 11-12-2021, 06:58 PM

Originally posted by HarisF1

You've read it perfectly

It might need sending off to someone with the correct rig. The alternative is for me to use the K-1 debug menu to add an offset to the AF fine tuning adjustment which will bring this lens back to with the +/-10 that can be set within the camera body. This would mean I need to compensate by adjusting every 'good' lens so it probably isn't worth the effort.

Adjusting every "good" lens costs you nothing but time. Sending the FA 85 out for repair is probably at least $250 or more.

zapp · 11-13-2021, 05:38 AM

Send the lens for alignment.
AF fine tuning of the camera should stay close to zero, avoid extremes and don’t ask for some kind of debug mode. I would even send the camera for alignment if t was significantly off in one direction.

**Bob 256** · 11-13-2021, 08:06 AM

Originally posted by zapp

Send the lens for alignment.
AF fine tuning of the camera should stay close to zero, avoid extremes and don’t ask for some kind of debug mode. I would even send the camera for alignment if t was significantly off in one direction.

Ditto. If you play with the camera calibration, then along comes a lens that needs correction in the opposite direction and the camera is too far off to get there. DeBug mode is asking for trouble unless you really know what you're doing (and debugging something that isn't buggy is not the way to go).

stevebrot · 11-13-2021, 12:09 PM

Originally posted by DWS1

Adjusting every "good" lens costs you nothing but time. Sending the FA 85 out for repair is probably at least $250 or more.

Given the value, reputation, and going price of the FA* 85/2.4 (IF), having it tuned in a service harness at an authorized facility would seem highly appropriate. If ever sold, one would expect the problem would migrate to the next owner and decrease value at sale.

Steve

HarisF1 · 11-13-2021, 03:24 PM

I've been playing with the lens and two camera bodies. For some reason I had +10 adjustment on both bodies but after a reset, the lens worked well on my main body. I then tried out other lenses with the other body (new to me) and realised it was a common issue with my faster lenses on that particular body.

The problem has been resolved through the debug menu AF adjustment function for the new body.

So I think some of the information I gave earlier was incomplete due to a series of unusual issues.

stevebrot · 11-13-2021, 06:14 PM - **1 Like**

Originally posted by HarisF1

The problem has been resolved through the debug menu AF adjustment function for the new body.

You might want to try (or have done) dust removal from the PDAF auxiliary mirror and sensor surface on the new body. If it is new from the dealer, a replacement might be in order. Is it safe to assume you are limiting yourself to the center column AF points when using faster lenses? The others are inadequate for the task.

Steve

HarisF1 · 11-14-2021, 06:47 AM

These are great suggestions Steve, thank you.

The body was a second hand purchase so I gave it a good clean with a dust blower. And also I only tend to use centre AF since I never shoot moving subjects (other than the kids but we all know AF is a shot in the dark for them).

tibbitts · 11-16-2021, 09:52 PM

I have had this problem (PDAF identically incorrect with multiple bodies) with two (Pentax-brand) lenses. It would be nice if Pentax bodies provided more than the +/-10 per-lens adjustment range; both my lenses were just beyond +/-10.

11-12-2021, 09:32 AM	#1
HarisF1 Veteran Member Join Date: Apr 2016 Posts: 543	How can we repair a lens that is back focusing excessively? Hey guys, I have a lens that has excessive back focus. I've used it in the past with +10 but it still won't produce adequately sharp focus. I've now tested it on numerous bodies with the same issue so I'd like to have a look at sorting it out. Has anyone ever managed to tweak the AF tuning in the lens itself? Best Regards Haris Last edited by HarisF1; 11-13-2021 at 02:49 AM.

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11-12-2021, 10:15 AM - 1 Like	#2
stevebrot Otis Memorial Pentaxian Join Date: Mar 2007 Location: Vancouver (USA) Photos: Gallery \| Albums Posts: 42,007	Originally posted by HarisF1 Has anyone ever managed to tweak the AF tuning in the lens itself? There is no AF tuning in the lens per se. Fixing the lens involves placing it on a test/repair harness and adjusting alignment and movement of the optics. Steve

11-12-2021, 10:48 AM	#3
jumbleview Site Supporter Join Date: May 2012 Location: Concord, CA Posts: 1,069	Is it focuses OK in the live view?

11-12-2021, 11:30 AM	#4
HarisF1 Veteran Member Join Date: Apr 2016 Posts: 543 Original Poster	Yes, focusing in live view is bang on.

11-12-2021, 11:40 AM	#5
jumbleview Site Supporter Join Date: May 2012 Location: Concord, CA Posts: 1,069	Originally posted by HarisF1 Yes, focusing in live view is bang on. It looks like for this lens your Pentax camera converted to mirrorless only. It is the industry trend.

11-12-2021, 01:04 PM	#6
Bob 256 Site Supporter Join Date: Jun 2008 Location: Idaho Photos: Gallery Posts: 2,375	Not sure how I read this thread so far??? If your lens focuses ok in live view, that would point to an AF error in the optical viewfinder of the camera but since you said this is across several cameras, that would appear not to be the case unless all of those cameras are mis-adjusted for optical AF. That points back to the lens again and the fact that it has an issue which makes the lens disagree with where a standard AF camera would expect to see the best focus (using 0 offset) and what stevebrot said. It shouldn't require that much AF offset to begin with. Probably time to send it off.

11-12-2021, 01:34 PM	#7
HarisF1 Veteran Member Join Date: Apr 2016 Posts: 543 Original Poster	Originally posted by Bob 256 Not sure how I read this thread so far??? If your lens focuses ok in live view, that would point to an AF error in the optical viewfinder of the camera but since you said this is across several cameras, that would appear not to be the case unless all of those cameras are mis-adjusted for optical AF. That points back to the lens again and the fact that it has an issue which makes the lens disagree with where a standard AF camera would expect to see the best focus (using 0 offset) and what stevebrot said. It shouldn't require that much AF offset to begin with. Probably time to send it off. You've read it perfectly It might need sending off to someone with the correct rig. The alternative is for me to use the K-1 debug menu to add an offset to the AF fine tuning adjustment which will bring this lens back to with the +/-10 that can be set within the camera body. This would mean I need to compensate by adjusting every 'good' lens so it probably isn't worth the effort.

11-12-2021, 06:58 PM	#8
DWS1 Site Supporter Join Date: Aug 2018 Location: Arkansas, USA Posts: 1,169	Originally posted by HarisF1 You've read it perfectly It might need sending off to someone with the correct rig. The alternative is for me to use the K-1 debug menu to add an offset to the AF fine tuning adjustment which will bring this lens back to with the +/-10 that can be set within the camera body. This would mean I need to compensate by adjusting every 'good' lens so it probably isn't worth the effort. Adjusting every "good" lens costs you nothing but time. Sending the FA 85 out for repair is probably at least $250 or more.

11-13-2021, 05:38 AM	#9
zapp Pentaxian Join Date: Jul 2011 Posts: 2,385	Send the lens for alignment. AF fine tuning of the camera should stay close to zero, avoid extremes and don’t ask for some kind of debug mode. I would even send the camera for alignment if t was significantly off in one direction.

11-13-2021, 08:06 AM	#10
Bob 256 Site Supporter Join Date: Jun 2008 Location: Idaho Photos: Gallery Posts: 2,375	Originally posted by zapp Send the lens for alignment. AF fine tuning of the camera should stay close to zero, avoid extremes and don’t ask for some kind of debug mode. I would even send the camera for alignment if t was significantly off in one direction. Ditto. If you play with the camera calibration, then along comes a lens that needs correction in the opposite direction and the camera is too far off to get there. DeBug mode is asking for trouble unless you really know what you're doing (and debugging something that isn't buggy is not the way to go).

11-13-2021, 12:09 PM	#11
stevebrot Otis Memorial Pentaxian Join Date: Mar 2007 Location: Vancouver (USA) Photos: Gallery \| Albums Posts: 42,007	Originally posted by DWS1 Adjusting every "good" lens costs you nothing but time. Sending the FA 85 out for repair is probably at least $250 or more. Given the value, reputation, and going price of the FA* 85/2.4 (IF), having it tuned in a service harness at an authorized facility would seem highly appropriate. If ever sold, one would expect the problem would migrate to the next owner and decrease value at sale. Steve

11-13-2021, 03:24 PM	#12
HarisF1 Veteran Member Join Date: Apr 2016 Posts: 543 Original Poster	I've been playing with the lens and two camera bodies. For some reason I had +10 adjustment on both bodies but after a reset, the lens worked well on my main body. I then tried out other lenses with the other body (new to me) and realised it was a common issue with my faster lenses on that particular body. The problem has been resolved through the debug menu AF adjustment function for the new body. So I think some of the information I gave earlier was incomplete due to a series of unusual issues.

11-13-2021, 06:14 PM - 1 Like	#13
stevebrot Otis Memorial Pentaxian Join Date: Mar 2007 Location: Vancouver (USA) Photos: Gallery \| Albums Posts: 42,007	Originally posted by HarisF1 The problem has been resolved through the debug menu AF adjustment function for the new body. You might want to try (or have done) dust removal from the PDAF auxiliary mirror and sensor surface on the new body. If it is new from the dealer, a replacement might be in order. Is it safe to assume you are limiting yourself to the center column AF points when using faster lenses? The others are inadequate for the task. Steve

11-14-2021, 06:47 AM	#14
HarisF1 Veteran Member Join Date: Apr 2016 Posts: 543 Original Poster	These are great suggestions Steve, thank you. The body was a second hand purchase so I gave it a good clean with a dust blower. And also I only tend to use centre AF since I never shoot moving subjects (other than the kids but we all know AF is a shot in the dark for them).

11-16-2021, 09:52 PM	#15
tibbitts Pentaxian Join Date: Apr 2007 Location: Oklahoma USA Posts: 2,193	I have had this problem (PDAF identically incorrect with multiple bodies) with two (Pentax-brand) lenses. It would be nice if Pentax bodies provided more than the +/-10 per-lens adjustment range; both my lenses were just beyond +/-10.