Looks like we're thinking along the same lines Matt but you're more technically knowledgable than myself.

BTW, it's a Tokina lens not a Tamron!

This is very interesting and certainly something that I've never heard of before. Basically I think you're saying that the autofocus system is choosing a point slightly behind the subject, do you get focus confirmation (correctly on the subject) when you focus manually?
If you don't, then I suspect that the electronic contacts between camera and lens (or more specifically the chip within the lens) is somehow confusing the camera's AF system. I don't see how this can be, if you are using single-point AF with the subject perfectly positioned within the AF frame but I wonder if the lens is initially focusing correctly and then a pulse is being sent to the motor which causes the lens to shift slightly as you press the shutter release.
If this is the case, then it looks as though you've identified another example of an independant make of lens that is not quite 100% compatible.
As you don't have this problem with a Pentax lens, I suppose the only way to be sure would be to try another Tokina of the same generation but perhaps a different model.
I suggest a different model from the same range because a) they should communicate with the camera in the same way and b) the problem could be unique to your model because of it's construction (zoom/focus mechanism).
On the other hand, trying another identical lens could determine whether or not your particular lens has a fault and you've just been unlucky!

Do you have another AF body that you can try the lens on - perhaps a 35mm Z or MZ series, for example?

If the lens is a current model, perhaps a local dealer will allow you to try a few others from the range to see if it's a generic fault

Now that sounds more like it!

All you need to do now is start collecting M series cameras, then A's, Z's, MZ's.........have you considered going digital?

Just kidding but I half suspect that you'll do that anyway ......and I couldn't fault you if you did

I'm with John on this one, I'm sure you'll love the lens but that price is about what I'd expect from a high street shop and I'd see if I could knock it down a bit or else check on eBay

My first Pentax, an ME Super, came with the M series 50mm F2 and it was a gorgeous lens. Optically superb and with a beautifully smooth focusing action - many users prefer the M series above all others but there's not much to choose between them and later versions optically.
A series and later have the additional electronic contacts required for full program operation and multi-pattern metering on suitable cameras but the M series are still usable thanks to the K-mount.

I think the M 50mm 1.7 would be a very fine lens indeed and that price sounds about right but you might prefer to look for an A or F series alternative, should you be considering a more modern camera system.

Seems like no one else did very much John

Sounds like you had a good time, must admit that I didn't do anything photographic but we did go out for tea............and it was very nice

I recently made a comment in another thread that prompted this response:

" Have Contax produced a digital back, too? Certainly Leica has, but it doesn't represent new technology. Just its application to a film body".

It was actually the Leica that I was thinking of but convinced myself that it was a Contax, apologies for any confusion

Oops, that was me............dunno why I wasn't logged in

I've a number of Tamron lenses but they're all the older Adaptall2 manual focus models but I think they're great and I'm sure you'll be pleased with your purchase.
Do let us know how you get on and what camera you'll be using it with.

Thanks for the comments folks, I shall bear them in mind.
For me, 24mm is ideal for film but only equates to approx 35mm for digital work - which is hardly wide-angle at all and that is why I thought of the 18-35.
I'm glad to hear that it's a decent performer, although I expected nothing less esp. as it features AL glass.

I'll snap one up if the price is right!