Sigma 18-35 F1.8 "ART" focus issues

Some years ago, I bought a Sigma 18-35 F1.8 "Art" lens for my Pentax K-5II and, besides its huge size and weight, I liked it very much.

It's a very high optical quality lens for an unbeatable price. The image is tack sharp from edge to edge even at maximum aperture. At least on the K5-II.

Things started to go strange when I got my K3-II and I noticed that the lens was not sharp in any aperture larger than F4 no matter about the focal length. Wow. Something was smelling bad.

I put the lens back on the K5-II and it worked without any problem. Was my K3-II faulty? I checked it with three Pentax DA lenses (40, 50 and 70 mm) and besides a very slight front focus in the 70mm that I corrected by the in-camera AF micro adjustment, everything was tack sharp and the focus dead-on. So the problem was, at first, not the camera itself.

Then I decided to use the Sigma USB dock to fix the focus issues. Camera on a sturdy tripod, a measuring tape, a focus target and everything else needed.

I used the live view and the central AF point for the adjustments and to my surprise, all pictures came in perfect focus! Strange. I took the camera and shot some random pictures at maximum aperture and... Lots of them came out of focus.

To keep things short, the camera was missing the focus when the optical viewfinder was in use and making it dead on when using the live view.

This, of course, has to be something to do with the contrast and phase focus detection system.

After some DAYS wrestling with this lens and the USB dock, I managed to get it working in a decent way, but not perfect with the K3-II. Actually, I wouldn't recommend this lens and camera combination unless you plan to use it on manual focus using a decent focusing screen.

There are lots of reports about this lens causing issues on Canon and Nikon bodies with the lateral focus points.

There is no real information from Sigma about what's the cause or if is it fixable by a lens or camera firmware update.

It's a shame because the optical quality of this lens is nothing less than stunning.

My advice is to TEST this lens in your camera before buying it. If it works well, you'll get by far the best lens on its class. If not you'll get a very frustrating and expensive paperweight.

I found a good article here about this.