I was setting up a scope yesterday and trying to calibrate the sidewheel. Once out past 45yds I was having to deal with heat shimmer, which made it near-on impossible to discern a clear focus point. In the event I waited for a downpour and the sky to cloud over. The settings I got without the shimmer were different to the earlier settings, right down to 30yds.
That's OK when dealing with known distances but on a course the whole point is you don't know how far they are - that's why we parallax! I'm think of resorting to bracketing when the heat shimmer gets too bad, but that involves holding the image steady enough to bracket accurately. Telling the difference in size between 50 and 55 is not easy - not for a wobbly bugger like me, anyway
How then do you
deal with heat shimmer when trying to rangefind when it's hot