Fine tuning Sigma 150-600 C requires a lot of patience. For the 15-m distance I used a plastic bottle and a ruler.
I placed it in my back garden exactly 15 m from the back of the camera. I focused on the bottle label and marked the position of the label with a piece of wire on the ruler. This was my reference point. It was extremely primitive, but worked.
I started with the dock fine-tuning values of zero. I took pictures at 4 different focal lengths (150, 250, 400 and 600 mm), three pictures at each focal length. It is important to have replicated pictures, as--despite using a tripod, mirror lock up and remote control--some of them are slightly out of focus.
I looked at these pictures on my laptop and read from the ruler where the sharpest point was. This was really tricky, as you can see in the picture above. Then, I changed tuning settings, using the dock, from zero to 5 (at 15 m, for all focal lengths). I repeated the process. And again, after setting the dock tuning to 10, 15 and 20. Mind you, you have to take the lens off the camera and put in the dock each time.
This gave me a big table with numbers: position of the best focus versus tuning dock value. I interpolated between these values and guesstimated the best dock setting. For example, at 150 mm I got the following positions of the best focal point, as a function of the fine-tuning parameter:
at 0: -3 inches
at 5: -2, -5
at 10: -5, +2, +2
at 15: ~10, ~10, ~10
at 20: ~25, ~25, ~25
As you can see I started doing replicates only later, initially I took only one shot at each focal length and that was a mistake. At settings of 15 and 20 it was very difficult to read the distance from the ruler, so I had only approximate values. From all these results I estimated the best focal point to be at the fine-tune value of 8.
At infinity, I simply took pictures of a building far away at various fine-tune settings and then decided which one was the sharpest.
I did not tune it at shorter distances.
