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01-31-2010, 05:55 AM   #1
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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.
01-31-2010, 12:51 PM   #2
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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
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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
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QuoteOriginally posted by Yugow Quote
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
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QuoteOriginally posted by Yugow Quote
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
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QuoteOriginally posted by Yugow Quote
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
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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.


QuoteOriginally posted by Yugow Quote
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
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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
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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
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QuoteOriginally posted by Lowell Goudge Quote
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
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QuoteOriginally posted by Yugow Quote
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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