Strange front focus case with K135mm/2.5

Yugow · 01-31-2010, 05:55 AM

Hi !
I've experienced a very strange front focus case with a manual lens ... and I can't find how to explain it ...

I've got a K-7 which works very well, I had not encoutered any focusing problems, until yesterday, when I received my K135 f/2.5. After a few test shots, it was obvious that it was front focusing (I made the classic test with a 45°-angled ruler).

Both my split-screen focusing screen and the AF indicator agreed to say the focus was OK in the center of the image, but when I use live-view or when I take the photo, the focus is actually far before the center.
Then I've made this test with other lenses, including a macro lens, and for all these, the focus is perfect for both the AF indicator, the focusing screen and the sensor.

Well, I don't understand anything... How can a manual lens produce a front focus issue althought my body seems perfectly alright ???

Thanks for your help ...

Last edited by Yugow; 01-31-2010 at 02:06 PM.

Marc Sabatella · 01-31-2010, 12:51 PM

Your split screen must not be installed correctly. It needs to be *precisely* the same distance from the lens mount as the sensor is. Did it come with shims to help you fine tune the positioning?

Yugow · 01-31-2010, 02:03 PM

But the autofocus isn't affected by the focusing screen, is it ? Because the AF indicates a good focus too...

I've tried to use the lens with an extension tube, there's still a front focus but it seems to be less visible.

asdf · 01-31-2010, 06:28 PM

Originally posted by Yugow

But the autofocus isn't affected by the focusing screen, is it ? Because the AF indicates a good focus too...

I've tried to use the lens with an extension tube, there's still a front focus but it seems to be less visible.

Try another relatively fast lens, as well. Maybe, the mirror isn't in the right spot wrt. the AF system and the focusing screen.

Last edited by asdf; 01-31-2010 at 06:36 PM.

Marc Sabatella · 01-31-2010, 06:36 PM

Originally posted by Yugow

But the autofocus isn't affected by the focusing screen, is it ? Because the AF indicates a good focus too...

Sure - *somewhere*. You emntioned using a ruler rather than a good focus test chart, so most likely, the AF sensor is simply confirming some other line is in focus.

It is remotely possible the problem is the mirror being out of alignment too, I suppose, but that's *much* less likely that an misalgined focus screen, especially with a third party screen.

**Lowell Goudge** · 01-31-2010, 06:49 PM

Originally posted by Yugow

Hi !
I've experienced a very strange front focus case with a manual lens ... and I can't find how to explain it ...

I've got a K-7 which works very well, I had not encoutered any focusing problems, until yesterday, when I received my K135 f/2.5. After a few test shots, it was obvious that it was front focusing (I made the classic test with a 45°-angled ruler).

Both my split-screen focusing screen and the AF indicator agreed to say the focus was OK in the center of the image, but when I use live-view or when I take the photo, the focus is actually far before the center.
Then I've made this test with other lenses, including a macro lens, and for all these, the focus is perfect for both the AF indicator, the focusing screen and the sensor.

Well, I don't understand anything... How can a manual lens produce a front focus issue althought my body seems perfectly alright ???

Thanks for your help ...

I find it hard to believe that the lens can cause an error in the optical adjustment of a split screen. Sure the AF but not a split screen.

I have a 135F2.5 and it is the easiest thing in the world to focus.

I have much more trouble with wide angles than teles due to the narrow DOF of a tele, especially a fast one.

pinholecam · 01-31-2010, 08:21 PM

Interesting.. I will be watching this post to see if there is a solution.

I am facing a similar problem with my focusing screen (from focusing screen.com) on my K7 on the exact same lens as well. I did not seem to notice this issue before when I was snapping away using most other focal length between 24-85mm. When I used the K135/2.5 it became very apparent and I subsequently found that there was indeed a focus misalignment in the screen. Perhaps it was not as obvious on the other focal lengths, so I never noticed earlier. Using AF lenses, I've confirmed that the split screen is not aligned when AF is confirmed.

The shims don't work for my case. It seems I need to reduce the distance to the viewfinder and not add to it (which is what the shim does). Took out the split screen an put back my original one.

Originally posted by Yugow

Hi !
I've experienced a very strange front focus case with a manual lens ... and I can't find how to explain it ...

I've got a K-7 which works very well, I had not encoutered any focusing problems, until yesterday, when I received my K135 f/2.5. After a few test shots, it was obvious that it was front focusing (I made the classic test with a 45°-angled ruler).

Both my split-screen focusing screen and the AF indicator agreed to say the focus was OK in the center of the image, but when I use live-view or when I take the photo, the focus is actually far before the center.
Then I've made this test with other lenses, including a macro lens, and for all these, the focus is perfect for both the AF indicator, the focusing screen and the sensor.

Well, I don't understand anything... How can a manual lens produce a front focus issue althought my body seems perfectly alright ???

Thanks for your help ...

Yugow · 02-01-2010, 01:40 AM

I've done some precise tests with a tripod, a chart and a remote control.
Here are the results :

With K135 : f2.5 focusing with the AF indicator and the focusing screen (they always indicate the same thing) : front focus

The same but focusing in live-view mode :

K55 : f1.8 focusing with the AF indicator and the focusing screen : a bit of FF too

Focusing in live-view mode :

A50 : f1.7 focusing with the AF indicator and the focusing screen : very light FF

Focusing in live-view mode :

sigma 70-300 : 300mm f5.6 : no FF on this one

panagor macro 90mm : f2.8 : no FF on this one

What do you think of these results ? Is my mirror misaligned, or does the problem come from the lense(s) ?

**Lowell Goudge** · 02-01-2010, 08:42 AM

Not to be too critical of the OP who appears to have a problem, but the focusing tests are by no means precise.

aside from all other issues, the OP is not even square on the target (i.e. equidistant right to left)

These tests can be an issue but if focusing is in fact off, then it may be as others have posted the split image screen could be wrong. I have not used my MF lenses significantly on my K7 because i have a split image in my K10D there, the split image is right on the money.

asdf · 02-01-2010, 08:48 AM

Originally posted by Lowell Goudge

I have not used my MF lenses significantly on my K7 because i have a split image in my K10D there, the split image is right on the money.

Count yourself lucky. I had to tinker with shims.

asdf · 02-01-2010, 09:01 AM

Originally posted by Yugow

What do you think of these results ? Is my mirror misaligned, or does the problem come from the lense(s) ?

The only real focusing issue caused by lenses is focus shift when not shooting wide open (http://diglloyd.com/articles/Focus/FocusShift.html). This is due to spherical aberration and is constant given an optical design--it's not a quality control issue. Since you're focusing and shooting wide open, the problem is with the distance of the mirror(s) and the focusing screen and AF system. You can calibrate AF in one of the camera menus. As for the focusing screen, you may have to change/add a shim.

01-31-2010, 05:55 AM	#1
Yugow New Member Join Date: Dec 2009 Location: Paris Photos: Gallery Posts: 7	Strange front focus case with K135mm/2.5 Hi ! I've experienced a very strange front focus case with a manual lens ... and I can't find how to explain it ... I've got a K-7 which works very well, I had not encoutered any focusing problems, until yesterday, when I received my K135 f/2.5. After a few test shots, it was obvious that it was front focusing (I made the classic test with a 45°-angled ruler). Both my split-screen focusing screen and the AF indicator agreed to say the focus was OK in the center of the image, but when I use live-view or when I take the photo, the focus is actually far before the center. Then I've made this test with other lenses, including a macro lens, and for all these, the focus is perfect for both the AF indicator, the focusing screen and the sensor. Well, I don't understand anything... How can a manual lens produce a front focus issue althought my body seems perfectly alright ??? Thanks for your help ... Last edited by Yugow; 01-31-2010 at 02:06 PM.

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01-31-2010, 12:51 PM	#2
Marc Sabatella Veteran Member Join Date: Sep 2006 Location: Denver, CO Photos: Gallery Posts: 10,685	Your split screen must not be installed correctly. It needs to be precisely the same distance from the lens mount as the sensor is. Did it come with shims to help you fine tune the positioning?

01-31-2010, 02:03 PM	#3
Yugow New Member Join Date: Dec 2009 Location: Paris Photos: Gallery Posts: 7 Original Poster	But the autofocus isn't affected by the focusing screen, is it ? Because the AF indicates a good focus too... I've tried to use the lens with an extension tube, there's still a front focus but it seems to be less visible.

01-31-2010, 06:28 PM	#4
asdf Veteran Member Join Date: Jan 2009 Location: Canada Posts: 1,252	Originally posted by Yugow But the autofocus isn't affected by the focusing screen, is it ? Because the AF indicates a good focus too... I've tried to use the lens with an extension tube, there's still a front focus but it seems to be less visible. Try another relatively fast lens, as well. Maybe, the mirror isn't in the right spot wrt. the AF system and the focusing screen. Last edited by asdf; 01-31-2010 at 06:36 PM.

01-31-2010, 06:36 PM	#5
Marc Sabatella Veteran Member Join Date: Sep 2006 Location: Denver, CO Photos: Gallery Posts: 10,685	Originally posted by Yugow But the autofocus isn't affected by the focusing screen, is it ? Because the AF indicates a good focus too... Sure - somewhere. You emntioned using a ruler rather than a good focus test chart, so most likely, the AF sensor is simply confirming some other line is in focus. It is remotely possible the problem is the mirror being out of alignment too, I suppose, but that's much less likely that an misalgined focus screen, especially with a third party screen.

01-31-2010, 06:49 PM	#6
Lowell Goudge Site Supporter Join Date: Jan 2007 Location: Toronto Photos: Gallery \| Albums Posts: 17,886	Originally posted by Yugow Hi ! I've experienced a very strange front focus case with a manual lens ... and I can't find how to explain it ... I've got a K-7 which works very well, I had not encoutered any focusing problems, until yesterday, when I received my K135 f/2.5. After a few test shots, it was obvious that it was front focusing (I made the classic test with a 45°-angled ruler). Both my split-screen focusing screen and the AF indicator agreed to say the focus was OK in the center of the image, but when I use live-view or when I take the photo, the focus is actually far before the center. Then I've made this test with other lenses, including a macro lens, and for all these, the focus is perfect for both the AF indicator, the focusing screen and the sensor. Well, I don't understand anything... How can a manual lens produce a front focus issue althought my body seems perfectly alright ??? Thanks for your help ... I find it hard to believe that the lens can cause an error in the optical adjustment of a split screen. Sure the AF but not a split screen. I have a 135F2.5 and it is the easiest thing in the world to focus. I have much more trouble with wide angles than teles due to the narrow DOF of a tele, especially a fast one.

01-31-2010, 08:21 PM	#7
pinholecam Pentaxian Join Date: Oct 2009 Location: North Photos: Gallery Posts: 4,706	Interesting.. I will be watching this post to see if there is a solution. I am facing a similar problem with my focusing screen (from focusing screen.com) on my K7 on the exact same lens as well. I did not seem to notice this issue before when I was snapping away using most other focal length between 24-85mm. When I used the K135/2.5 it became very apparent and I subsequently found that there was indeed a focus misalignment in the screen. Perhaps it was not as obvious on the other focal lengths, so I never noticed earlier. Using AF lenses, I've confirmed that the split screen is not aligned when AF is confirmed. The shims don't work for my case. It seems I need to reduce the distance to the viewfinder and not add to it (which is what the shim does). Took out the split screen an put back my original one. Originally posted by Yugow Hi ! I've experienced a very strange front focus case with a manual lens ... and I can't find how to explain it ... I've got a K-7 which works very well, I had not encoutered any focusing problems, until yesterday, when I received my K135 f/2.5. After a few test shots, it was obvious that it was front focusing (I made the classic test with a 45°-angled ruler). Both my split-screen focusing screen and the AF indicator agreed to say the focus was OK in the center of the image, but when I use live-view or when I take the photo, the focus is actually far before the center. Then I've made this test with other lenses, including a macro lens, and for all these, the focus is perfect for both the AF indicator, the focusing screen and the sensor. Well, I don't understand anything... How can a manual lens produce a front focus issue althought my body seems perfectly alright ??? Thanks for your help ...

02-01-2010, 01:40 AM	#8
Yugow New Member Join Date: Dec 2009 Location: Paris Photos: Gallery Posts: 7 Original Poster	I've done some precise tests with a tripod, a chart and a remote control. Here are the results : With K135 : f2.5 focusing with the AF indicator and the focusing screen (they always indicate the same thing) : front focus The same but focusing in live-view mode : K55 : f1.8 focusing with the AF indicator and the focusing screen : a bit of FF too Focusing in live-view mode : A50 : f1.7 focusing with the AF indicator and the focusing screen : very light FF Focusing in live-view mode : sigma 70-300 : 300mm f5.6 : no FF on this one panagor macro 90mm : f2.8 : no FF on this one What do you think of these results ? Is my mirror misaligned, or does the problem come from the lense(s) ?

02-01-2010, 08:42 AM	#9
Lowell Goudge Site Supporter Join Date: Jan 2007 Location: Toronto Photos: Gallery \| Albums Posts: 17,886	Not to be too critical of the OP who appears to have a problem, but the focusing tests are by no means precise. aside from all other issues, the OP is not even square on the target (i.e. equidistant right to left) These tests can be an issue but if focusing is in fact off, then it may be as others have posted the split image screen could be wrong. I have not used my MF lenses significantly on my K7 because i have a split image in my K10D there, the split image is right on the money.

02-01-2010, 08:48 AM	#10
asdf Veteran Member Join Date: Jan 2009 Location: Canada Posts: 1,252	Originally posted by Lowell Goudge I have not used my MF lenses significantly on my K7 because i have a split image in my K10D there, the split image is right on the money. Count yourself lucky. I had to tinker with shims.

02-01-2010, 09:01 AM	#11
asdf Veteran Member Join Date: Jan 2009 Location: Canada Posts: 1,252	Originally posted by Yugow What do you think of these results ? Is my mirror misaligned, or does the problem come from the lense(s) ? The only real focusing issue caused by lenses is focus shift when not shooting wide open (http://diglloyd.com/articles/Focus/FocusShift.html). This is due to spherical aberration and is constant given an optical design--it's not a quality control issue. Since you're focusing and shooting wide open, the problem is with the distance of the mirror(s) and the focusing screen and AF system. You can calibrate AF in one of the camera menus. As for the focusing screen, you may have to change/add a shim.

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