A tale of microadjustment...
11 months ago
My Sigma 300mm f2.8 was sharp wide open at a ditance but close up was a bit hit and miss. I'd had it adjusted initially by Sigma and the technician told me that he had adjusted it the recommended amount. It seemed fine but up close wasn't as sharp as I'd like and I had focussing problems.
I checked with the image pattern in this
and all seemed well. In that you focus in live view producing an interference pattern and when you change to normal view and AF, the distance scale shouldn't budge.
Today in a moment of boredom I decided that I would approach it in a Germanic fashion.
First I photographed targets at stepped distances.
Seemed that it was front focussing a little.
Then I got one target, and photographed it, adjusting the AF from -20 to +20 in steps of five.
+ 15 was sharpest.
I then tested that in closeups of my wife's eyes and it was spot on.
Then I got the test pattern in the above link and to my amazement it could not distinguish between 0 and + 15. No difference in behaviour. AF locked on and there was no shift between live view and normal view focus. In other words that pattern test is dodgy.
Do the laborious test I did. It works.
Also, I tested my AF at distance and it's perfect also.