Kx infinity focus problem

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?

That could be it.

Try some shots on a tripod or place the camera on something steady with 2 second delay so that you can rule camera shake out and with the 2 second delay the Anti-Shake is inactive.

Also this just a little tip, the KX preforms better at ISO200 then at ISO100 how crazy that might sound.
The standard design ISO of the sensor is ISO200 and it gets ISO100 by some software tricks.

Suggest you post photos. Best if we can see the problem, no matter how good your explanation

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...

Assuming AF was used, I think it's possible that the camera has a problem with its autofocusing, even though close-distance subjects are OK.

If manual-focusing, bear in mind that modern lenses can be focused beyond infinity.

Yes, camera shake is unlikely at 1/160s at 18mm.

To show a picture here, first upload it to a hosting service such as Photobucket, then post a link to it here.

I will help to also post a crop to show part of the image at full size, if you have suitable software to do that with.

Are you manually turning the lens to infinity or automatically / manually focussing on the distant object?

If the latter, try comparing auto focus on the subject, manual focus on the subject through the viewfinder and manual focus using live view (so you are focussing at the sensor).

Kevin

Posting photos. Look here>

https://www.pentaxuser.com/forum/topic/linking-images-into-posts-4791

Haven't done it myself though

I've a Tamron 18-250 which is very soft/out of focus a t the long end on my K200d(I haven't tried it on my K110d yet).

I would check with your standard lens(es) as to infinity as a starter. After that do a front/back focus test and if possible adjust the camera to suit. I think this can be done on the K-x but could be wrong

No only the recent high-end have that so the K20D and the K7.

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

It depends on the contrast and size of the subject and the lighting.
If it has quite some contrast and it's during day light then I expect it to be accurate enough to rely on, so over the 90%.
Although I know the limitations so I don't rely that it would get the focus that I want most of the time.

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.

Indeed that looks like a focus problem.
Contact the shop and see what they can do for you, probably a recalibration by pentax might solve it I presume the camera is new so you might ask that they swap it for a new one.

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

Just to get it right, both your long lenses have this problem right?

I think it's an AF problem but it could be a bit of both so the lens and the camera. It could be that the camera work well with another lens but since you've two lenses I think that the camera is more likely the source.

One thing though, do you use an filter on your lens, if so try it with out it. I believed I've heard that some lenses don't like filters, not with these lenses though but it's easy to try out and test right but I don't think that's it though.