High quality glass certainly makes the images clearer but does not necessarily give the best mirage image (depending on scope specifications - focal length, objective diameter (f ratios) etc. Some of the best mirage images I have seen have come from converted bushnell and kowa spotting scopes with relatively large f ratios and high quality eyepieces.
BUT not only do images differ greatly with different "F ratios" (ratio of focal length to objective diameter) and glass quality, power etc but peoples opinions on what they like to see do too. Some TR shooters used to 15-20 power, huge depths of field will not appreciate the "intense" high powered, shorter focal length images from some other scopes. Conversely F classers used to mirage images magnified at 30-50 power might not like the image seen through the lower power spotting scopes. It can be completely personal preference and what you are used to interpreting.
I am often told there is a general push from scope manufacturers to shorten the focal length to satisfy the market. Large diameter, high quality objectives (fluoride coated) with short focal lengths (for compactness) can present fantastically clear images but possibly at the sacrifice of focal length.
It would be an interesting survey (especially amongst coaches) as to personal preference.
For me it is around 32 power with long focal length and good quality glass. My current scope can be set at the 300-400 yard line and clearly see the target lines and also in focus grass back to 7-800 yards when sitting at 1000.
Bruce (interesting discussion)- of course it is also feasible for wind to be moving in opposite directions at each end of the range. A fish tailing head/tailwind will often show this in mirage as the change moves down the course. Some ranges get "back eddies" behind the tree line (or other obstructions) as well that come and go quickly and have to be watched carefully. "dust devils" and other swirling winds can all cause this effect. BUT the often discussed issue of a focal point behind or in front of the target when using the target as your backdrop for mirage. Can this reverse the image and give you a false indication??
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