[Spodeworld]

I just picked up a Sigma 17-70 f2.8-4.5 (screw mount) lens for my K-01, which I have still to put through its paces. I was shocked at how poor the autofocus was as you approached 70mm. More often than not it missed.

I then swapped it with the Tamron 17-50mm f2.8, and with the few pictures I took, each one was in sharp focus (tested at long end where I had the problems in the Sigma).

I thought perhaps there was a problem with the lens, but given that it is a screw mount, the focus is controlled in camera and contrast focus is supposed to be quite accurate once locked. I then put it on my K20D and it seems to be behaving much better. I only took a handful of images, but there seemed to be no issues with the focus at the long end.

So, unless I missed something, it seems that the Sigma 17-70 f2.8-4.5 does not appear to be a good match for the K-01. It seemed to do just fine at the wide end, but at the other end, it really struggles to achieve focus. The Tamron 17-50 f2.8 on the other hand appears to be a good match (as I assume the Sigma 17-50 f2.8 is too).

Perhaps the Sigma 17-70 with internal motor is a better bet than the screw mount for the K-01.

[GXRUser]

I just picked up a Sigma 17-70 f2.8-4.5 (screw mount) lens for my K-01, which I have still to put through its paces. I was shocked at how poor the autofocus was as you approached 70mm. More often than not it missed.

I then swapped it with the Tamron 17-50mm f2.8, and with the few pictures I took, each one was in sharp focus (tested at long end where I had the problems in the Sigma).

I thought perhaps there was a problem with the lens, but given that it is a screw mount, the focus is controlled in camera and contrast focus is supposed to be quite accurate once locked. I then put it on my K20D and it seems to be behaving much better. I only took a handful of images, but there seemed to be no issues with the focus at the long end.

So, unless I missed something, it seems that the Sigma 17-70 f2.8-4.5 does not appear to be a good match for the K-01. It seemed to do just fine at the wide end, but at the other end, it really struggles to achieve focus. The Tamron 17-50 f2.8 on the other hand appears to be a good match (as I assume the Sigma 17-50 f2.8 is too).

Perhaps the Sigma 17-70 with internal motor is a better bet than the screw mount for the K-01.

For clarification, did you mean screw "focus".

[Spodeworld]

Yes, controlled by the in-body motor.

[K(s)evin]

Nope. I have the HSM version and it won't focus on tho K-01 at all. I also have the Sigma 18-250 HSM non -macro and it does not focus either.

[jatrax]

Perhaps the Sigma 17-70 with internal motor is a better bet than the screw mount for the K-01.

Should not matter, the focus is not 'controlled' by the motor but by the camera. If using screw drive then the motor is in the camera if using HSM then the motor is in the lens, but the focus is controlled by the camera.

Sigma 17-70 f2.8-4.5 (screw mount) lens

Just FYI but 'screw mount' usually refers to M42 screw mount lenses. I got real excited for second thinking you had an AF screw mount. But I have not had my coffee yet.....

I did read someplace that later firmware updates improved the autofocus system. You might check to see which firmware version you have installed.

[Spodeworld]

You're right Jatrax, I have enough M42 lenses to know better.

I do have the latest firmware, but it seems that that lens is just not a good pairing with the K-01 once you go beyond 50mm or so.

[vonBaloney]

Use the latest firmware.

But the difference probably has more to do with light loss than with the screw-drive -- the Sigma is at f/4.5 at the long end while the Tamron is at f/2.8 throughout. The CDAF usually gets flakey above 5.6, so it still should be work OK at f/4.5, but in low-light maybe not.

[ryan s]

My Pentax 17-70/4, which is SDM only, shows focusing "faults" at the wide and tele ends which it's known for...but on the K-01, it usually ends up with correct focus. It may not be super quick, but it gets there.

The DA* 55 is a great pairing, but that's because it's an f/1.4 lens. It has less to do, I think, with AF drive technology--rather, more with max aperture and lens design (zooms have more elements to move, etc).

At the far end of the spectrum, the screw drive FA* 300/4.5 barely moves on the K-01.

[Spodeworld]

Put the lens on a K20D and it works fine. Probably not a good pairing with the K01. Assume the K01 needs a faster lens.

[vonBaloney]

Pit the lens on a K20D and it works fine. Probably not a good pairing with the K01. Assume the K01 needs a faster lens.

Using CDAF (liveview), or PDAF through the viewfinder? PDAF not really comparable...

[Spodeworld]

Was making the point that the K-01 could only occasionally find focus @ 70mm, while the K20D found it without a problem. Bottom line, if this is representative, then this lens is not good with the K-01.

Using CDAF (liveview), or PDAF through the viewfinder? PDAF not really comparable...

[vonBaloney]

Was making the point that the K-01 could only occasionally find focus @ 70mm, while the K20D found it without a problem. Bottom line, if this is representative, then this lens is not good with the K-01.

But what I'm wondering is if it can find focus in the K20D using CDAF w/ LiveView, i.e. the same type of focusing system as the K-01. (Totally different AF system when using viewfinder.) If the K-01 can't do it then neither should the older K20D, or the K-7, or the K-5 even. I think the K20D was the first model with LiveView, and so also has the most primitive CDAF system. That would be interesting information.

As noted above, when testing the K-01, make sure you have the newest firmware, as the AF was improved.

[ripit]

Use the latest firmware.

But the difference probably has more to do with light loss than with the screw-drive -- the Sigma is at f/4.5 at the long end while the Tamron is at f/2.8 throughout. The CDAF usually gets flakey above 5.6, so it still should be work OK at f/4.5, but in low-light maybe not.

That would be my guess. Mine works great with zoom lenses as long as there is enough light. In lower light, especially at longer focal lengths where the aperture is smaller.

@Spodeworld, easy way to test it. Try the lens in daylight and see if it still has problems.

[houseofstyles]

I have the Sigma 17-70/ 2.8-4.5 with screw driven focus and it behaves well for me right across the focal length range on my K01 (just with firmware 1.01, I haven't bothered to update beyond this); this includes in dim light performance. I find this lens superior to the Pentax 17-70/ 4 which I sold after I got the Sigma.

I use small central area focus, and have the AF/AE-L button set to AF2 so you must use it for focussing, the shutter release then fires uninhibited by any attempt to refocus. I just tried it in a dim room where under P mode with f=70 mm, it returned a setting of 1/10 sec, F4.5, ISO 2200 and did a good job of focussing (it did use the focus assist light).

[ripit]

I have the Sigma 17-70/ 2.8-4.5 with screw driven focus and it behaves well for me right across the focal length range on my K01 (just with firmware 1.01, I haven't bothered to update beyond this); this includes in dim light performance. I find this lens superior to the Pentax 17-70/ 4 which I sold after I got the Sigma.

I use small central area focus, and have the AF/AE-L button set to AF2 so you must use it for focussing, the shutter release then fires uninhibited by any attempt to refocus. I just tried it in a dim room where under P mode with f=70 mm, it returned a setting of 1/10 sec, F4.5, ISO 2200 and did a good job of focussing (it did use the focus assist light).

I still think it might be a low light performance issue (in part). When I first got mine, one of the first shots I tried was about 25 feet away indoors in lower light, on a dark subject (outside of the range of the af assist light). Performance was bad. I tried increasing lights a bit (brighter and more light bulbs) which helped some but not enough. I realized a few things. It seems to like lighter objects even if they have less contrast. It just seems quirky. I learned to calculate exposure values and realized that lighter objects gave a higher ev measured through the camera which seemed to matter. A white piece of paper had less focus issues and a higher ev value than a dark brown couch, even though the couch had a lot more variation in surface, light and dark areas and such. It liked the smooth, contrast free paper better than the couch. A white face with dark hair didn't work well but the white curtain with same lighting worked great. At closer range the af assist light helped a lot. Something I noticed tonight. Indoor lighting, a couple of feet away it could not focus on my keyboard (with plenty of light from the af assist light). I moved the camera up a little so it got the desk to and it was fine.

I went shooting at a later date at a naval museum inside a battle ship and a sub with light levels relatively close to the same. It worked near perfect with the same lenses that were having so many problems in my house. Perhaps it has some sort of issue focusing on certain things or some other issue but it seems brighter light and or lighter subjects fixes it. I'm thinking the lens is fine and its shooting conditions that are causing it (with lower light/smaller aperture being part of the problem but perhaps not the whole problem). It seems the more I learn the camera though, the less problems I have.

I was thinking trying it in daylight would rule out a lens compatibility issue (if good light fixes it then light is at least part of the issue).