This would be the lens of my dreams if functioning properly.
After trying 2 different new lenses on 2 different Nikon D70 cameras, focus is in 95 % of images 1-3 dm (4-12 inches) behind the subject I aim for when shooting from 10 m distance. One of the lenses was sent to authorized Sigma service for check between tests, one of the cameras was sent to Nikon for focus adjustment. Problem is still remaining afterwards. Several hundreds of pictures taken, subject NOT moving.
It is very evident shooting at F2.8 from 10 m and zoomed to 300 mm, but at closer distance and / or zoomed out it seems to be OK. Also, first focusing 2 m behind subject and then refocusing on my main subject when taking the picture seems to give worse result than first focusing at closer distance.
My 15 years old AF Nikkor 300/4 provide 100 % images with correct focus under same conditions (same subject, distance, light, camera settings, focus procedure etc.)
On my Nikon F4, I have not observed the problem, although only one roll of film taken and not exactly same distances.
Another odd observation regarding this lens is the focal length when set to 300 mm. I did observe a significant difference to my old Nikkor, which brought me closer to the subject. I took some images of a long ruler from 8 m distance and then calculated the focal length. The Nikkor was 305 mm and the Sigma 270 mm. Sigma Corporation, Tokyo head office, informed me that focal length should be measured focused at infinity. In that case focal length is less than 270 mm since it decreases when focusing to infinity (same result for both 2 Sigma lenses tested). This is not a major problem though.
Sigma corp. answered to my questions regarding focus problem with providing info of the lens optical performance, but did not comment what could cause the focus problem and not what I could do about it.
I have been testing and discussing this issue with my local retailer and Sigma for more than one month now. If they can provide a lens working properly on my D70, I will highly recommend the lens. Now it is worthless.