Am I expecting too much? or is the lens auto focus whack?

GeoffMoorePhotography

GeoffMoorePhotography

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Hello folks,
I'm hoping you can help me out out, I have noticed at f4 my lens looks very soft almost out of focus, when compared to f11.
The lens was recently purchased so comes with a 30 day guarantee, so I can send it back, but am i just expecting too much? - Here is a side by side. Any feedback greatly appreciated.

Details.
Camer: Pentax K1
Lens: Pentax 60-250mm f4 SMC DA* 250mm @ 170mm
Auto Focus: Centre point / bark of tree
Distance to subject 8-10m

Left Image: F4 / 1/40th Sec - Tripod - Auto focus
Right Image F11 / 1/6th Sec - Tripod - Auto Focus

* Images cropped to centre to illustrate point,

Only Lens Corrections applied, RAW > JPG - No other processing to the image done so far.

Unfortunately this isn't an isolated case I have perhaps 20-30 shots from this morning at F4 regardless of focal distance the image appears very soft.
Is this a back/forward focusing issue? I dont think the elements are decentred as the image 'sharpness' is consistent across the frame.

Full size image can be downloaded from here > http://moore.photos/temp/f4-f11-comparision.jpg

f4-f11-comparision.jpg
 
Thanks for the comments @GreenNinja67 - perhaps its my understanding that is wrong! :) It's more than possible --- I would have expected @F4 that the subject, that being the grey tree in the middle should be sharp as that's the focus point, with the rest of the image falling off in sharpness due to DOF,
 
Which tree did you focus on? I can't really tell from one photo, but it seems to be focused somewhere on the brambles behind the centre tree on both those shots, with the depth of field making up the difference in the second shot?

Perhaps take a photo of a brick wall at an angle of around 30 degrees so you are kind of looking along it. Put an object like a box or a beer crate on the ground half way along the wall and focus on that? It should be easy to see if the focus point is on the object, in front, or behind it by looking which patch of bricks are in focus.

Something a bit like this chap is doing to demonstrate depth of field, only further back distance wise from the wall:

View: https://www.youtube.com/watch?v=98-WvzWvt7M
 
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Thanks Mr Badger :) Thanks for the video, I don't think this is a only a DoF issue though, I leaning towards this being in part an auto focus issue as per JohnX suggestion, as you will see from below... btw I was focused on the grey tree centre frame of each shot, the image provided is 2 images left side at f4 right side f11 same subject.

@JohnX Live view AF I have worked the full range following AF fine tune method, -10 to +10, however regardless of adjustment made the image still requires manual adjustment of the focus, around a mm of a turn when in live view to get a sharp image at f4 infuriating.
 
Geoff Moore said:
Thanks Mr Badger :) Thanks for the video, I don't think this is a only a DoF issue though, I leaning towards this being in part an auto focus issue as per JohnX suggestion, as you will see from below... btw I was focused on the grey tree centre frame of each shot, the image provided is 2 images left side at f4 right side f11 same subject.

@JohnX Live view AF I have worked the full range following AF fine tune method, -10 to +10, however regardless of adjustment made the image still requires manual adjustment of the focus, around a mm of a turn when in live view to get a sharp image at f4 infuriating.
Click to expand...
I wasn't referring to the depth of field aspect, I was suggesting you tried a similar target test method to see exactly what your camera and lens combination was actually focussing on when you aimed it at a specific point. That way you could find out if your camera/lens combination was spot on, or focusing behind or in front of that point.
 
Geoff Moore said:
Thanks Mr Badger :) Thanks for the video, I don't think this is a only a DoF issue though, I leaning towards this being in part an auto focus issue as per JohnX suggestion, as you will see from below... btw I was focused on the grey tree centre frame of each shot, the image provided is 2 images left side at f4 right side f11 same subject.

@JohnX Live view AF I have worked the full range following AF fine tune method, -10 to +10, however regardless of adjustment made the image still requires manual adjustment of the focus, around a mm of a turn when in live view to get a sharp image at f4 infuriating.
Click to expand...
It may be an inevitable feature of your lens that it has aperture-related focus drift, and it may also have distance-related focus drift. In other words if that's the case for best AF results at say f4 and distances of around say 20m you'll get sharpest AF results in you fine tune your AF specifically at that aperture and distance. This may mean that at wider apertures and closer distances you won't get best AF results, and will need some manual tweaking of focus. Of course since it's a zoom lens, and a wide range zoom to boot, your fine tuning may also be rather specific to the focal length at which you did the fine tuning.

What you're finding out is that phase-based AF in a DSLR with a mirror is an imperfect compromise, especially at wide apertures and especially with zooms, especially if they're wide range zooms. Some modern expensive lenses have managed to tame some of the larger AF imperfections better than older cheaper designs.
 
Thanks for the comments and help all, very much appreciated. Unfortunately the lens is a duff. Tried a 2nd copy, and it was identical to Mike's offerings, my version simply is unable to gain acceptable focus at f4 . So the retailer has been spot on, when advised, they have offered a no quibble return/refund. The lens is already winging its way to them now.
This now leaves me in a little dilemma, I can wait till the x-mas period and finger's crossed pick up a 70-200 f2.8, or wait out till 2019 for the upcoming new f4 version that is road mapped.Till then i'm limited to a 105mm or picking up a 2nd 60-250
 
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