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05-07-2016, 02:09 AM   #1
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K50 Front focus issue (presumably at lower focal lenghts (under 50mm).

Hey guys,

running a K50 here and struggling sort of from day one with this ... (sort of).

I started with a manual lens wasn't really an issue here.

Then I had some issues with my 35 2.4 that wouldn't properly focus to infinity at 2.4 (live focus worked just fine). Sent in both to service - but all they seem to have done is dial in -3 in the AF adjustment - it was sort of acceptable so I figured - ok, maybe the lens is sort of soft for this (closer ranges were focusing better).

Then I bought a DA50 and with a -4 AF adjustment I figured at home with some tests it nails the focus pretty much every time.

HOWEVER:
SIGMA 28-105 2.8-4 even when stopped down sort of front focuses. Now at -10 to give some better photos at 5.6.

I recently DID NOT buy a Tamron 18-250 which was in immaculate condition from a fellow ex pentaxian as the focus was always spot on when using 150+ focal lengths - but was terribly front focusing at lower focal lenghts (18 for example gave about 2-3-4 meters of front focus (when trying to focus to infinity)).
We tried setting the AF ADjustment to (debug mode -20) and -10 for the individual setting to try and compensate - alas no success. Still focused perfectly with the long range - but lower focal lengths were eerm ..

Then what came to my mind is - that maybe my camera has problems with front focus at lower focal lenghts in general - ergo the 35mm infinity problem, the SIGMA softish focus and the Tamron huge front focus issue.

Mind you the 50mm is spot on at all settings and distances to focus (tested at 1.8 under artificial and normal light).

I doubt that 3 lenses fail and just 1 works ...

What do you guys think of this?

05-07-2016, 02:27 AM   #2
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Look, it's possible that the AF module isn't perfectly aligned - they have to get this right at the factory. You might send it in for a service at somewhere other than the first company you used.

05-07-2016, 02:35 AM   #3
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Front focusing can often by mistaken for mis-focus or simply a soft lens. Have you tested both zooms using an alignment tool such as a LensAlign?

Mind you, those zooms aren't the sharpest lenses around, and it is actually normal for the degree of AF inaccuracy to vary depending on the focal length. It's hard to say more without controlled tests, however.

Adam
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05-07-2016, 02:44 AM   #4
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Well Live view is spot on for both and they give somewhat sharp results (as much as a super zoom is expected to give)..

I haven't tested the Tamron with a tripod but the Sigma I've tried with a lens alignment test and managed to match the live view result with the pdaf focus using a - 10 adjustment.. Mind you this was at about a meter distance to target..

The DA 35mm is spot on like that, the DA50 also.. The 35mm sometimes gets proper infinity af, the 50mm always..

Could it be a mismatch of lens / body?

05-07-2016, 03:11 AM   #5
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Can you upload some photos?
05-07-2016, 03:41 AM   #6
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Infinity test + Adjustments.
Setting:
K50
Tripod
Sigma 28-105 2.8-4
DA 35
DA 50

Target is the Billboard in the middle.
Using center focus point only.
Approximately 45 angle degrees from shooting position and 20 meters away.
All open at their widest apertures.
Stabilization off.
Remote shutter with 3 secs delay to prevent hand shutter.
All images resized to 1000px. No PP applied.

Debug menu (global AF adjustment -10).

1. Sigma @ 28mm :
Sigma28LV vs Sigma28PDAF
Adjustment set to 0 (global applied -10).

2. Sigma @ 105mm:
Sigma105LV vs Sigma105PDAF
Adjustment set to 0 (global applied -10).

3. DA 35
DA35LV vs DA35PDAF
Adjustment set to 0 (global applied -10).

4. DA50
DA50LV vs DA50PDAF
Adjustment set to -8 (global applied -10 = effective -18?!)

So problem is that ... these settings are technically all at -10 AF Adjust to start off and in addition this was adjust slightly so that it gives equal focus @ infinity to match the live view ... if I do the same test at a closer range and the same settings - it's bye bye focus again ...
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05-07-2016, 05:51 AM   #7
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2nd test and adjustment:

Tripod.
SR off.
Base Iso.
Target: Matchbox + 2 batteries on each side.
Center focus only.
Shutter delay 2 seconds.
All shots at widest aperture possible.

1. Sigma - with 0 personal and -10 global correction
@ 28mm now perfect at close
@28mm now perfect at distant target (handheld w/ SR).
@105mm now perfect at close
@105 now perfect at distant target (handheld again w/ SR)

2. DA35mm
+3 individual AF added to -10 Global = -7 adjustment
Close target - dead on
Distant target handheld - on target (maybe slightly better with Live view - but could be just me - can't bother again).

3. DA50mm
individual 0, global -10.
Close target - dead on
Distant target - dead on.

There seems to be lots of fluctuation when doing these tests ... previous time at infinity I got certain settings, before this I had completely different settings for the DA50 for example ... now these ... not sure what I will get when I wake up tomorrow ...

Not sure what to make of it...
05-07-2016, 05:32 PM   #8
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I think your method (with regard the photos posted) is unlikely to give you good results. Your second test is, no doubt, a better way of doing it. That's why you're seeing different results this time round. A good (simple) test is to take a rigid ruler, mount to a second tripod, at a 45 degree angle facing the camera. Then, with the camera on a tripod, focus on the centre of the ruler. This is a bit hard to do. You will get the best results if you have a vertical target next to the centre point of the sloping ruler. You can then focus on the vertical target, and at the same time review what part of the angled ruler is in focus. It's then easy to clearly see if you have front focus or back focus.

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