Pamt, I have a 50mm 1.4 M version and it took some time to 'get to know it', especially at f1.4 and f2. The depth of field can be so shallow that even when the green light gives you focus confirmation, it's still a little off. Or, for xample, you focus on someone's faces, and it gives you focus confirmation because the tip of the nose is in focus, but with the shallow field the rest of the face will already be out of focus. This effect is the stronger the closer the subject is... for certain shots the in focus field is only a few centimeters deep.

Where possible, I tend to take a few shots with slightly varied focus around the confirmed point.

Stefan, Oggy, I am not using any filters for these shots.

The file sizes are smallish for the two downscaled shots that show the full scene. All the crops are unscaled crops of 1024 pixel maximum size in the larger dimension, the biggest that flickr allows. I shot raw, imported via Adobe Camera Raw into Photoshop CS4 and saved the crops with jpeg setting 12, the highest available. The street scene in original size is a 6M jpeg. So no, no jpg losses anywhere.

And yes, I see a similar effect with the 55-300 DA-L that came with the kit. Had two reasoanble good shots out of six. One of them was not at full focal length, but at 200mm.

I also have a second Tammy 18-250mm with which I took another 4 shots of the radio tower earlier. This one had a better hit rate, with 1 sharp image, 2 reasonably well focused ones and one out of focus, all shot under the same conditions as before. Better, but not really good IMO...

Cheers Stefan. So you'd think it could be the camera?

Just to give you another idea, on the long end of the Tamron, the test scene I described in my OP. This is the test scene:
http://farm5.static.flickr.com/4035/4233985893_36b6b10e06_o.jpg

Here are four test shots with F6.3 and F8, all 1/500 or faster, all auto focus:
http://farm3.static.flickr.com/2638/4233987015_26e7493f35_o.jpg

http://farm5.static.flickr.com/4006/4233987789_4b97e4a250_o.jpg

http://farm3.static.flickr.com/2512/4234762262_0d255b37f7_o.jpg

http://farm3.static.flickr.com/2721/4233988627_924d90f5e4_o.jpg

And then suddenly, the AF decides to have mercy and show me what the lens can do on a good day:
http://farm3.static.flickr.com/2760/4234760638_e81cfc00d2_o.jpg

I thought this one shows decent centre sharpness at F6.3 and 250mm (with 50/50 ACR sharpening added to this crop which I didn't even bother to do for the blurry images above) - but one out of five is a bit poor a result

OK, here's a first try. For wide angle tests I am frequently shooting the view from my window, which is quite challenging:

http://farm3.static.flickr.com/2528/4234683190_af7275ff48_o.jpg

Here's a center crop from the very best result a got in auto focus from the Tamron 18-250 at 18mm and f3.5:
http://farm3.static.flickr.com/2641/4233881455_7a6a8fa0aa_o.jpg

And this is what I got from a manual focus shot:
http://farm3.static.flickr.com/2795/4233880737_9bf765c1e6_o.jpg

(The manual focus one is slightly larger - I must have leaned out just a little more, since the lens was locked to 18mm.)

Both shots at identical settings, added 50 sharpening and detail in ACR5 as well as slight exposure compensation, fill light and brightness increase, using the same values for both shots. So by all means they should have identical levels of sharpness.

As you can see, the AF shot is oof and looks slightly hazy - and I had worse oof results, which were completely blurred even though focus was confirmed. Shutter speed is 1/150, ISO100, so at 18mm it can't really be my handshake.

I'll post some more example, but in general it feels that at least half to two thirds of AF shots I take show this kind of OOF blur or worse, even with repeated focussing, and with varios subjects and focal lengths.

Cheers guys for all your suggestions and help. I'll try and upload some pictures asap. I have a free flickr account but have to prepare some crops at the right size so that flickr doesn't scale them.

In the meantime I'll take some more shots, with this problem being quite erratic obviously. Just looking through todays set again I find some which are horribly oof, and one which is stunningly sharp even at widest aperture... both taken with the AF.

From general experience, what kind of percentage of autofocus accuracy should one expect in good shooting conditions (e.g. a bright sunny winter day with clear air, and provided there's no blur from low shutter speed and other such factors)? 50%? 80, 90%? Less, more? Bids are accepted

Thanks Stefan... I shall try that as soon as I can. I get the asme effect, a little less promounced, at 18mm wide and 1/160s shutter - would I be right to assume that camera shake is unlikely to have an effect at that setting?

John, can/should I post on here? If so, how? I've never posted any pictures...

Oggy, great tip, thanks! That works perfectly

Does anyone know if I can extract from the Pentax DNG EXIF whether I used auto or manual focus for a certain image? Can't find it anywhere in ACR or the Windows property details...

I hope some of you with much more expertise than I have can give me some advice here...

I got my Kx last week, and only had a little time to play around with it so far, although with a sunny day - finally - I was able to do a series of test shots from my window which I frequently did with other cameras.

Right now, it appears to me that all my long-end autofocus shots with an infinity focus (on a radio tower some 500m away) seem to be out of focus, both with a Tamron 18-250mm and the 55-300mm DAL. At lower f stops (5.6 and 6.3) it seems to be worse, and way beyond any lens softness that one would expect; but even at f8 they are just not in focus. Out of 10-15 shots, I seemed to have one random in-focus shot at f8 with the 55-300mm, and one with an old manual focus Vivitar S1. All shots were taken at ISO100 and with 1/500 or faster shutter, mostly 1/640 or 1/800, and antishake activated.

I took a couple of indoor test shots with the Tamron 18-250mm with flash the first night, at ISO1600 and full zoom, over a distance of 3m. These came out tack sharp.

I also took numerous test shots with my trusty old K100D of the above radio tower with the Tamron before, most of which came out very nicely.

Could this be a camera issue? Has anyone experienced the same with a Kx or another Pentax DSLR? I am wondering if my model has a problem with the infinity focus, since I am getting the same issue with both tele zoom lenses.

In relation to this problem, IF the camera had an infinity focus issue, would this potentially add to the softness that these lenses display at long end? Or is it possible that the Kx ruthlessly exposes lens limitations fat beyond what the K100D would show?

Not from me... there is one on ebay for £269 or best offer, by Jacobs, apparently a new ex-display. I was VERY tempted but I've overstretched my X-Mas budget with a K-x bundle and a spontaneous purchase of an FA43 limited considerably

But maybe it's for someone else of here :

http://cgi.ebay.co.uk/Pentax-FA-50mm-f2-8-Macro-K-Mount-Lens-NEW-UK-Seller_W0QQi...