Front/Back focus: how often do it happens?

Exhale · 03-09-2017, 02:48 AM

Hello, after having sent back to amazon for the second time the 35mm "Plastic Fantastic" for unsolvable back focus problems, and after having spent an hour on the kit 18-55 to fix what I recognized to be front focus, i'm wondering: how often does it happens to you to have the front / back focus problems?

I have a K3 bought from amazon wharehouse deals with 6000 shots, and now I got the paranoia that it can be a problem of the body camera.
But I ask, theoretically if a body has a sensor focus problem, it should be always front or always back, right?

This problem is also keeping me from buying other lenses, I tried a Sigma 17-70 (old model f2.8 / f4.5) that i could get for a real bargain price, but at 17 wide open i found it was really too soft (and I read that it is sharp already at f2.8) so i assumed that that lens had F/B focus problems too and I gave up.

I'm getting / have -i become too paranoid?

**clackers** · 03-09-2017, 02:56 AM

Yes, it may be your body.

If you find you that +10 or -10 isn't enough for most lenses, its AF module alignment is what needs to be fixed.

Reed · 03-09-2017, 03:38 AM

I had similar case during my time in service centre. Auto focus sensor plane in DLSR was miss aligned to auto focus support mirror and camera image matrix. It was caused because the camera got dropped and mirror hinge got a bit shifted to front of the camera. Still that camera behaved like that with every lens, even focus confirmation in EVF with old manuals was off.

pschlute · 03-09-2017, 05:55 AM

Originally posted by Exhale

But I ask, theoretically if a body has a sensor focus problem, it should be always front or always back, right?

It will perform differently with different lenses, but generally you would expect a pattern. When you test you need to use tripod and do multiple tests of each lens, defocussing between each shot/AF lock. You will be surprised at the variation you will get even with a "perfect" body and lens

JohnX · 03-09-2017, 10:46 AM

Both the bodies and lenses are made to certain tolerances.

It's the combination of the tolerances that results in either front or back focus, so you can get all sorts of strange results just with different combinations of bodies and lenses, i.e. a lens might bf on body A but be perfect on body B.

Zooms might give different results at each end of it's range, etc.

03-09-2017, 02:48 AM	#1
Exhale New Member Join Date: Feb 2017 Posts: 18	Front/Back focus: how often do it happens? Hello, after having sent back to amazon for the second time the 35mm "Plastic Fantastic" for unsolvable back focus problems, and after having spent an hour on the kit 18-55 to fix what I recognized to be front focus, i'm wondering: how often does it happens to you to have the front / back focus problems? I have a K3 bought from amazon wharehouse deals with 6000 shots, and now I got the paranoia that it can be a problem of the body camera. But I ask, theoretically if a body has a sensor focus problem, it should be always front or always back, right? This problem is also keeping me from buying other lenses, I tried a Sigma 17-70 (old model f2.8 / f4.5) that i could get for a real bargain price, but at 17 wide open i found it was really too soft (and I read that it is sharp already at f2.8) so i assumed that that lens had F/B focus problems too and I gave up. I'm getting / have -i become too paranoid?

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03-09-2017, 02:56 AM	#2
clackers Otis Memorial Pentaxian Join Date: Jul 2013 Location: Melbourne Photos: Albums Posts: 16,397	Yes, it may be your body. If you find you that +10 or -10 isn't enough for most lenses, its AF module alignment is what needs to be fixed.

03-09-2017, 03:38 AM	#3
Reed Pentaxian Join Date: Mar 2015 Location: Warsaw Photos: Gallery Posts: 638	I had similar case during my time in service centre. Auto focus sensor plane in DLSR was miss aligned to auto focus support mirror and camera image matrix. It was caused because the camera got dropped and mirror hinge got a bit shifted to front of the camera. Still that camera behaved like that with every lens, even focus confirmation in EVF with old manuals was off.

03-09-2017, 05:55 AM	#4
pschlute Moderator Loyal Site Supporter Join Date: Mar 2007 Location: Surrey, UK Photos: Gallery Posts: 8,217	Originally posted by Exhale But I ask, theoretically if a body has a sensor focus problem, it should be always front or always back, right? It will perform differently with different lenses, but generally you would expect a pattern. When you test you need to use tripod and do multiple tests of each lens, defocussing between each shot/AF lock. You will be surprised at the variation you will get even with a "perfect" body and lens

03-09-2017, 10:46 AM	#5
JohnX Pentaxian Join Date: Jul 2009 Location: London Posts: 1,116	Both the bodies and lenses are made to certain tolerances. It's the combination of the tolerances that results in either front or back focus, so you can get all sorts of strange results just with different combinations of bodies and lenses, i.e. a lens might bf on body A but be perfect on body B. Zooms might give different results at each end of it's range, etc.