AF Backfocusing

23mystiq · 07-28-2015, 09:57 PM

Hi fellow Pentaxians!

I found out that my lens (SMC DA 18-55mm) are backfocusing. On no zoom images are blurry, but when zooming to infinity, images are sharp. I've played with AF finetuning in the menu but that does not solve the problem as micro adjustments will apply to all focal lenghts. I guess I need to bring the back lens closer to the sensor. I wonder if there is some screw on the back side of the lens that might adjust that. I can see a screw, than a little rod and also a piece for opening/closing aperture. Could this be done manually or if not is it serviceable?

Sandy Hancock · 07-28-2015, 10:13 PM

Post some images comparing phase-detect ("normal") AF with contrast-detect AF in live view, with the camera on a tripod at a variety of focal lengths.
Zooms are tricky. Sometimes focus accuracy varies from the wide end to the long of its focal length range, so any fine adjustments made in camera might have to be a compromise.
It is worth remembering this is a very cheap kit lens, so it might be better to stop it down a little rather than spending time and money getting it serviced.

Adam · 07-29-2015, 01:17 AM

Out of focus or just soft? The original 18-55mm isn't exactly stellar at the wide end. Would be hard to imagine FF or BF causing issues with such a wide depth of field, either.

kh1234567890 · 07-29-2015, 02:44 AM

Before taking the lens to bits (the screws that you can see simply attach the lens mount, there is nothing accessible to adjust) check that you can get it to focus properly in live view (both AF and manually). You don't say which version of the DA18-55 you have, but the WR version can be acceptably sharp even at 18mm, if it is working properly.

stevebrot · 07-29-2015, 08:19 PM

Originally posted by Adam

he original 18-55mm isn't exactly stellar at the wide end.

...and not so great at 55mm either.

Steve

drypenn · 07-29-2015, 09:11 PM

Originally posted by stevebrot

...and not so great at 55mm either.

Steve

...and mediocre inbetween.

23mystiq · 07-30-2015, 06:45 PM

Thanks for answers. Here are some pics. I don't have a tripod with me, so I had to put it on the car's roof - that's why these images are not taken from exactly same position.
I forgot to reset AF fine adjustment, so the difference is not so apparent, but you can still notice that 18 mm photos are not so sharp as those 55mm ones. I was focusing the tree.

cropped actual size LV 18mm

cropped actual size LV 55mm

cropped actual size Phase detect 18mm

cropped actual size Phase detect 55mm

The last one is just as sharp as it can get and the whole picture for reference
Whole image Pixel Shift + MF 18mm

Last edited by 23mystiq; 07-30-2015 at 06:53 PM.

aleonx3 · 07-30-2015, 07:38 PM

It appears more likely to be front focus than back focus to me... did you try to take a picture of an object closer to you? so it can show a bit more definitively if there is indeed a problem as you described?

drypenn · 07-30-2015, 11:13 PM

At that distance both 18 and 55 should already be at "infinity". The 18-55 is not a good performer, but it should not be that "bad". I suspect that your lens has already developed a "physical" problem. And based on your pictures, I don't think that it's a simple FF/BF problem.

Has it been dropped? Do you have any focus problem with other AF lens?

bassek · 07-30-2015, 11:33 PM

Try to repeat this with a wall shot from 15 feet, both wide and tele. And with an angle as well if possible.
Things don't look good, misaligned glass comes to my mind as drypenn hinted.

Seb

kh1234567890 · 07-31-2015, 12:46 AM

Originally posted by aleonx3

It appears more likely to be front focus than back focus to me...

The problem of front/back focus should not arise in live view. Something is broken.

Sandy Hancock · 07-31-2015, 12:54 AM

I suspect something is seriously amiss. If it is still under warranty get it repaired, if not it might not be worth bothering.

stevebrot · 07-31-2015, 12:03 PM

Originally posted by drypenn

At that distance both 18 and 55 should already be at "infinity". The 18-55 is not a good performer, but it should not be that "bad". I suspect that your lens has already developed a "physical" problem. And based on your pictures, I don't think that it's a simple FF/BF problem.

Has it been dropped? Do you have any focus problem with other AF lens?

My thoughts as well. Missed focus does not look like that. That being said...the DA 18-55 (any version, even a good copy) is out of its depth on the K-3.

One other comment...Pixel Shift does not make soft lenses sharp or bad focus good. What it does do is simply collect more data from the projected image such that more available detail is retained.

Steve

ZombieArmy · 07-31-2015, 04:09 PM

Have you tried setting it to center focusing and pointing it exactly what you want it to focus on?

Sounds basic sure, but it would clear whether or not it's just focusing on the wrong subject...

kh1234567890 · 08-01-2015, 04:03 PM

Originally posted by stevebrot

That being said...the DA 18-55 (any version, even a good copy) is out of its depth on the K-3.

Just as well that I only have a battered K-5ii with some 20% lower linear resolution than the K-3 and a nasty AA filter in the way. Otherwise I'd have to bin mine ...

07-28-2015, 09:57 PM	#1
23mystiq New Member Join Date: Jun 2015 Posts: 11	AF Backfocusing Hi fellow Pentaxians! I found out that my lens (SMC DA 18-55mm) are backfocusing. On no zoom images are blurry, but when zooming to infinity, images are sharp. I've played with AF finetuning in the menu but that does not solve the problem as micro adjustments will apply to all focal lenghts. I guess I need to bring the back lens closer to the sensor. I wonder if there is some screw on the back side of the lens that might adjust that. I can see a screw, than a little rod and also a piece for opening/closing aperture. Could this be done manually or if not is it serviceable?

07-29-2015, 01:17 AM	#3
Adam Administrator Site Webmaster Join Date: Sep 2006 Location: Arizona Photos: Gallery \| Albums Posts: 51,597	Out of focus or just soft? The original 18-55mm isn't exactly stellar at the wide end. Would be hard to imagine FF or BF causing issues with such a wide depth of field, either. Adam PentaxForums.com Webmaster (Site Usage Guide \| Site Help \| My Photography) PentaxForums.com server and development costs are user-supported. You can help cover these costs by donating or purchasing one of our Pentax eBooks. Or, buy your photo gear from our affiliates, Adorama, B&H Photo, KEH, or Topaz Labs, and get FREE Marketplace access - click here to see how! Trusted Pentax retailers:

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07-28-2015, 10:13 PM	#2
Sandy Hancock Moderator Site Supporter Join Date: Feb 2012 Location: Adelaide Hills, South Australia Photos: Gallery \| Albums Posts: 11,275	Post some images comparing phase-detect ("normal") AF with contrast-detect AF in live view, with the camera on a tripod at a variety of focal lengths. Zooms are tricky. Sometimes focus accuracy varies from the wide end to the long of its focal length range, so any fine adjustments made in camera might have to be a compromise. It is worth remembering this is a very cheap kit lens, so it might be better to stop it down a little rather than spending time and money getting it serviced.

07-29-2015, 02:44 AM	#4
kh1234567890 Veteran Member Join Date: Sep 2010 Location: Manchester, UK Photos: Gallery Posts: 2,653	Before taking the lens to bits (the screws that you can see simply attach the lens mount, there is nothing accessible to adjust) check that you can get it to focus properly in live view (both AF and manually). You don't say which version of the DA18-55 you have, but the WR version can be acceptably sharp even at 18mm, if it is working properly.

07-29-2015, 08:19 PM	#5
stevebrot Otis Memorial Pentaxian Join Date: Mar 2007 Location: Vancouver (USA) Photos: Gallery \| Albums Posts: 42,007	Originally posted by Adam he original 18-55mm isn't exactly stellar at the wide end. ...and not so great at 55mm either. Steve

07-29-2015, 09:11 PM	#6
drypenn Veteran Member Join Date: Nov 2012 Photos: Albums Posts: 963	Originally posted by stevebrot ...and not so great at 55mm either. Steve ...and mediocre inbetween.

07-30-2015, 06:45 PM	#7
23mystiq New Member Join Date: Jun 2015 Posts: 11 Original Poster	Thanks for answers. Here are some pics. I don't have a tripod with me, so I had to put it on the car's roof - that's why these images are not taken from exactly same position. I forgot to reset AF fine adjustment, so the difference is not so apparent, but you can still notice that 18 mm photos are not so sharp as those 55mm ones. I was focusing the tree. cropped actual size LV 18mm cropped actual size LV 55mm cropped actual size Phase detect 18mm cropped actual size Phase detect 55mm The last one is just as sharp as it can get and the whole picture for reference Whole image Pixel Shift + MF 18mm Last edited by 23mystiq; 07-30-2015 at 06:53 PM.

07-30-2015, 07:38 PM	#8
aleonx3 Veteran Member Join Date: Jul 2007 Location: Brampton, Ontario Photos: Gallery \| Albums Posts: 3,996	It appears more likely to be front focus than back focus to me... did you try to take a picture of an object closer to you? so it can show a bit more definitively if there is indeed a problem as you described?

07-30-2015, 11:13 PM	#9
drypenn Veteran Member Join Date: Nov 2012 Photos: Albums Posts: 963	At that distance both 18 and 55 should already be at "infinity". The 18-55 is not a good performer, but it should not be that "bad". I suspect that your lens has already developed a "physical" problem. And based on your pictures, I don't think that it's a simple FF/BF problem. Has it been dropped? Do you have any focus problem with other AF lens?

07-30-2015, 11:33 PM	#10
bassek Pentaxian Join Date: Feb 2011 Photos: Gallery Posts: 706	Try to repeat this with a wall shot from 15 feet, both wide and tele. And with an angle as well if possible. Things don't look good, misaligned glass comes to my mind as drypenn hinted. Seb

07-31-2015, 12:46 AM	#11
kh1234567890 Veteran Member Join Date: Sep 2010 Location: Manchester, UK Photos: Gallery Posts: 2,653	Originally posted by aleonx3 It appears more likely to be front focus than back focus to me... The problem of front/back focus should not arise in live view. Something is broken.

07-31-2015, 12:54 AM	#12
Sandy Hancock Moderator Site Supporter Join Date: Feb 2012 Location: Adelaide Hills, South Australia Photos: Gallery \| Albums Posts: 11,275	I suspect something is seriously amiss. If it is still under warranty get it repaired, if not it might not be worth bothering.

07-31-2015, 12:03 PM	#13
stevebrot Otis Memorial Pentaxian Join Date: Mar 2007 Location: Vancouver (USA) Photos: Gallery \| Albums Posts: 42,007	Originally posted by drypenn At that distance both 18 and 55 should already be at "infinity". The 18-55 is not a good performer, but it should not be that "bad". I suspect that your lens has already developed a "physical" problem. And based on your pictures, I don't think that it's a simple FF/BF problem. Has it been dropped? Do you have any focus problem with other AF lens? My thoughts as well. Missed focus does not look like that. That being said...the DA 18-55 (any version, even a good copy) is out of its depth on the K-3. One other comment...Pixel Shift does not make soft lenses sharp or bad focus good. What it does do is simply collect more data from the projected image such that more available detail is retained. Steve

07-31-2015, 04:09 PM	#14
ZombieArmy Pentaxian Join Date: Jun 2014 Location: Florida Posts: 3,210	Have you tried setting it to center focusing and pointing it exactly what you want it to focus on? Sounds basic sure, but it would clear whether or not it's just focusing on the wrong subject...

08-01-2015, 04:03 PM	#15
kh1234567890 Veteran Member Join Date: Sep 2010 Location: Manchester, UK Photos: Gallery Posts: 2,653	Originally posted by stevebrot That being said...the DA 18-55 (any version, even a good copy) is out of its depth on the K-3. Just as well that I only have a battered K-5ii with some 20% lower linear resolution than the K-3 and a nasty AA filter in the way. Otherwise I'd have to bin mine ...