I have been using a Sigma 150-600 for mainly bird photography for about a year now. When shooting mainly terrestrial birds I noticed that it is front focusing. After getting the doc it made a huge improvement but also presented me with another problem. How do I fine tune infinity settings? The focus point at infinity is so far how do I determine the accuracy of the focus when it is set for the 4 focal lengths the doc calls for; 150, 250, 400 and 600mm. The other focal points are easy once the focus chart is downloaded and set up.