Fine focus adjusted but still back focuses!!!

Might be my 1st post, I cant remember so apologies if I do something wrong. My issue is, I've noticed that the focusing has been off with my zoom, which is what I use most of the time, so I decided to do something about it... K5 body with the fine focus adjusted for both a zoom lens at the 70mm end & a 50mm prime. The test sheets show that the focus is about the same for both lens. Go outside & do a couple of test shots. Focusing (center point auto not manual) on the edge of the door, the 50mm is fine but the zoom at 70mm back focussed, the shed planks (3ft further back) are sharper than the door edge. What's going on??

The problem is almost certainly that your zoom is seen by the camera simply as Sigma lens so the camera is unable to store focus adjustment details for it. The camera doesn't recognise the lens! Generally third party lenses aren't supported apart from some where the lenses are the same as Pentax ones but rebadged.
Your other lens is a smc PENTAX-FA 50mm F1.4 so the camera recognizes the lens and it works for that.
Even if a zoom lens was recognised there is often some focus shift throughout the zoom range in them so it might not be possible to get it 100%.
What Sigma lens is it? (Presuming that has been reported to the camera accurately as some third party lenses fail even to do that!)

Thanks for the reply. The lens is a Sigma 24-70 F2.8 DG Macro. Bit confused though, if a pentax body didn't recognise a non-pentax lens wouldn't the world & his brother have shouted enough that it was common knowledge that there would be af issues? Maybe they have & i missed it, I don't do forums often. I did a bit of research prior to buying the lens & it's generally considered to be ok. I knew that I probably couldn't adjust for the whole range which is why I chose the 70mm end as that's what I use most. Put simply then, it's a lens issue & I should just buy a 70mm prime Pentax lens?

Could be that the AF point could not detect enough contrast at that moment.

if you did say 10 attempts at an identical scene , how many would be correctly focussed?

Af is not infallible.

ps ....are you doing an "apply all" AF/FA adjustment, or two individual ones for each lens ie "apply one"

I believe that if the lens isn't identified the only option given is 'apply all', or at least that's what the K-5 manual states.
It is said that Sigma reverse engineer the electronics for the Pentax mount so many of them report incorrectly in the Exif.
The lens sends a code to the camera and it is this that isn't being recognised. Do you have the latest K-5 firmware - if not updating it might sort it out. The last version was v1.16 http://www.ricoh-imaging.co.jp/english/support/digital/k-5_s.html

[edit: with the info found below it won't help but still worth updating if not current.)

For info that Sigma lens IS being identified as: Sigma 18-200mm F3.5-6.3 DC or Sigma DL-II 35-80mm F4-5.6 or Sigma DF EX Aspherical 28-70mm F2.8 or Sigma 70-300mm F4-5.6 Macro or Sigma 55-200mm F4-5.6 DC, Sigma used the same code for all of them so Pentax may ignore that code simply identifying it as a 'Sigma lens' which is the best they could do in the circumstances.

If the lens focussing is out ask Sigma to service it for you.

Thanks for the reply's. The fine focus is 'apply one' not 'apply all'. the two lenses needed differing values to correct things. The issue has been around a while over various scenes so I don't think it was a case of missing the focus on the door edge. I focused a few times to ensure that It was right. Firmware is V1.16 so up to date.
It's beginning to look like my K5 body doesn't like this Sigma lens for reasons known only to itself. I went for the Sigma as it was in a sale , had good reviews & my 70-300 Sigma is fine. It's not the end of the world just very weird that reputable manufacturers like Pentax & Sigma can't sort themselves out when we can put machines into the far extremes of the universe!! So unless anyone has a simple answer It'll be a bit of research into whether to go just 70mm or K3ii + 70mm.

I think your issue may be that a zoom cannot be fine-focus adjusted for all focal lengths, you can only adjust it at an 'average' setting and it may well be out for other focal lengths. Also, as others have mentioned, the AF on Pentax cameras is not infallible and you can get different results on the same settings each time you focus! I found that in particular my K-5 suffered from this trait, which my K-5iis and K-3 don't seem to. It's much easier to fine-focus adjust a prime lens than a zoom.

Regards
David

Odd that the lens can be adjusted correctly when using a focus chart but not in the real world.

The only other thing to suggest is to do your lens AF/FA in outside daylight. Some artificial light can affect the AF accuracy.

Focus shift !
https://photographylife.com/what-is-focus-shift
Also as you say the effect of different lighting / colour temperature.

i thought focus shift only affected fast lenses. I would not have thought a 2.8 lens would experience it.

To the OP, have you tried repeating the test on other outdoor subjects to see if the back focus issue remains consistently out? If it is reset the K-5 to the default setting and try again before resetting any fine AF adjustments.
If still a problem after that try the lens on a different camera body and see what happens. After that you should know where the problem lies.
I suspect it might be a case of you not being able to blame Pentax for Sigma making a duff lens and you've just been unlucky to get that one but you'll need to confirm that with some further tests.
My own experience with some Sigma lenses that they can struggle to obtain spot on focus (they just start hunting back and forwards in low light and can stop anywhere often way off!)

The test sheets were shot indoors but with natural light from windows so I don't think the issue is colour temp. It's not a one off problem, the issue has been around for some time, I was just too lazy to do anything about it till now. It's happened in a range of situations, in bright sun, cloud, shade, tungsten, flash, on a tripod etc. So I can discount colour temp, me shifting during the exposure, poor focus at that time. I've done a fine focus adjustment previously also & then again to check it. The issue is either the lens or the body. With the 50mm & the 70-300mm on a recent shoot focusing ok I'm leaning towards the lens. The k5 is oldish now, still serves me fine but an upgrade may be due. Likewise I have no issue (other than the cost) with flogging the zoom & getting a 70mm prime. I just wondered if there was an easy answer/fix, apparently there isn't. So my options seem to be: get a 70mm Pentax (and probably a 35mm), get a k3ii & see if the zoom works on that. Thanks for the thoughts so far.

Just one further thought / question on this:

When you took the images of the test chart with both lenses, were you exactly the same distance away from the central point of the test chart as you were from the edge of the door in the outside shots? If not, it is not a fair / good comparison. Your tests need to be done at exactly the same distance away from your subject. For example, it could be that the lens or camera is focusing correctly from a distance of let's say 6 feet away, but not from a distance of 12 feet away?

Regards
David

Nope, distances were about 2-3 ft with the charts & about 7-10 ft with the door. On a recent shoot the distance was about 15-20 ft. The distance doesn't seem to matter, back focusing occurs. I may be dumb here but if the focus is fine at say 6 ft but not 12 ft, am I to shoot at 6ft away at all times? I'm still swinging towards the Sigma lens being the issue with this body after the responses I've had.

I'm not suggesting you should be any particular distance away from the subject, but what I am suggesting is this will make a big difference. For example, depth of field errors become less the further you are from the subject. Also, the camera / lens combination could be accurate at one distance and not at another. The only thing you can do is to adjust the fine focus for real world usage, which for a medium length zoom would be somewhere from middle distance to infinity?

It's a compromise whatever you do and if there is a fault with either the camera or lens you may never get it right. IT's been suggested above to try the lens on a different body, I think this would be a good idea as it may rule out one or the other as being the culprit.

Regards
David