That's not a bad supposition Dave. The page of text suggestion I made would rule out any unusual variances across the frame and may confirm if you are correct here.

Regards
David

Hi Everyone

I recently acquired a Sigma 17-70 F2.8-4 HSM OS Macro lens from another Forum Member, with the intention of keeping either this or my Pentax DA17-70 as my default walkabout lens. However, since getting the Sigma I have struggled to get sharp images from it, so today I decided to calibrate it and do some testing. In short, there's nothing wrong with the lens, it just needed a '+5' adjustment on both my K-5IIs and my K-3. After sorting out the focussing accuracy, I decided to do some comparison tests with the Pentax DA 17-70 at F4, nothing scientific here, just my own thoughts after comparing images side by side in Photoshop with no processing applied. Bear in mind, these results reflect the copies of the lenses I own and may be different for other copies. I was mainly interested in sharpness at different focal lengths, but also looked at contrast and colour. The controls applied were:

- Camera on tripod, pointed at identical subjects, sharpness assessed on a road sign approx. 70m away
- 2 second self timer used to avoid shake
- F4 used for all images on both lenses
- Centre focus point only
- Images shot at 70mm, 35mm and 17mm on each lens
- Same comparisons were shot with the K-5IIs and the K-3, all results were consistent between the two cameras
- Images were compared in Photoshop at 200%, side by side in a vertical tiling arrangement so it was easy to see the differences

Here's what I found:

Contrast
- The Pentax lens has much higher contrast than the Sigma at all focal lengths

Colour
- The Sigma is 'warmer'- most noticeable with Reds - the Siggy renders reds with more yellow / green in them, making a more 'orangey' red than the Pentax, which in turn has more magenta. The Sigma is probably marginally more accurate with reds, but the downside is that this tendency makes greens look too yellowy! No conclusive win for either here, but because of the Pentax's contrast, the colours look more 'punchy' straight out of the camera with the DA 17-70.

Sharpness
This is where it gets interesting!

70mm - The Pentax blows the Sigma away completely. At 70mm on the Siggy, there are halos around objects and a fuzziness even when accurately focussed, whereas on the Pentax everything is crisp and sharp; quite a surprising result.

35mm - The Sigma blows the Pentax away! This surprised me even more than the first result, the Siggy has better edge definition, overall a sharper image.

17mm - A closer match, but the Sigma just beats the Pentax for overall sharpness.

I would suspect that if both lenses were stopped down to F7.1 or so the results would be almost indistinguishable where sharpness is concerned, but colours and contrast would tell them apart. The other thing I noticed is that the Siggy exposes higher than the Pentax does, at the same aperture with the Siggy at 1/125, the Pentax lens produced a shutter speed of 1/250.

So, what does all of this mean? Which is the better lens? I still have no idea, it depends on how you like images to look and also what focal length you tend to use the lens at. I still don't know which one to keep and which I should sell.

I hope this is useful information, not sure how relevant it is to anyone else but thought it worth posting anyway. Both lenses are very well made and focus at about the same speed, both are silent focussing, the Siggy has a wider aperture at the wide end and also has smoother bokeh than the Pentax does. Pentax has a 67mm filter size, Sigma is 72mm.

Cheers
David

It depends on another menu setting, if using 'AF.S', item 15 in the menu allows you to specify if the shutter can be fired whilst the subject is not in focus. I just leave mine on option 2 which is 'Release Priority', i.e. the photographer has control over when the shutter fires, whether the subject is in focus or not. As an aside, I almost always use 'AF.S', I prefer to press the AF button again if I need to re-focus, rather than the camera doing it automatically.

Regards
David

I don't have this issue with mine, see other post for my thoughts. It works really well with the 150-500 and with all other lenses tried so far.

Regards
David

Another thought Dave, try taking some images of a page of small text at an angle, this would clearly show if there are areas of the images not consistently in focus across the frame and any differences in front/back focus. Ronnimac's suggestion above is a good one - if there is any smearing of anything on the sensor, no matter how small and slight, it may create issues.

Regards
David

Thanks John, I knew this would not escape your attention!

Cheers
David

Additionally Dave, I would get your K-3 checked if I were you. It could have some mis-alignment of the mount or sensor - this would possibly create the effect you are seeing. If the alignment of the sensor is not parallel, the there will be a difference in the focal plane across the frame of the image - this looks suspiciously like what you may have. I haven't noticed anything like this with my K-3.

However, you've got me thinking now and I had very similar issues with my first K-5. The damn thing was so inconsistent with focus, particularly with the DA 17-70, sometimes it back-focussed, sometimes it front focussed and occasionally it got it right! I haven't had any other bodies that exhibited this behaviour and it was only with certain lenses, also the same lens on other bodies behaves perfectly!

Regards
David

Have you calibrated to AF of the DA*300 to the K-3, using the AF Fine Adjustment Dave? If not, this would be the first thing I would do.

I have to calibrate all my lenses with each camera, the longer the lens the more critical this becomes.

As an aside, I've found the focussing on my K-3 to be very accurate and consistent, with all lenses tried so far. The Sigma 150-500 seems very good with this combination, as for the DA*300, well I don't have one yet, but hope to get one pretty soon, so I will report back then.

Regards
David

I won't tell anyone if you don't Drew. It was a Pentax outing, organised by Neil, a PU member, it might be a good comparison to see some of your shots?

Cheers
David

I like your set of images Peter, the candid one is really great and the ones of the graffiti are very striking.

Regards
David