Philip, my experience with Sightron III 10X50 FT scopes has been its a continuous change not a sudden switch at a threshold temp. I had mine calibrated for 51, 69 and 79 degrees and found at the last end of season Monster BAGA WI shoot at 38deg I was ranging 5 yds short at longer range in the cold temperature. Once I started adding 5 yds on long shots targets started falling. Tested rifle at 40 deg vs 68 deg and found change in rifle velocity change (slower) only accounted for 1/4 in low at 55 yds. Change in pressure for regulated volume as indicated on my dual pressure gage Daystate Wolverive, followed the ideal gas law precisely. The observed temperature change from 68 deg to 38 degrees when converted to proper absolute temp range for use in equation agreed with my rifle gage observations. The rest of the large drop in POI I had was scope ranging erroe on longer ranges. Most materials exhibit a temperature coefficient where their dimensions get smaller with colder temperatures. This is a x inch per degree shift, not a sudden change at a given temperature. I would expect this to to be a continuous function not a step change behavior. I will be setting up my scopes with marks at 4 temperatures about 15 to 20 degrees apart in future to cover likely range of temperatures encountered. Maybe 40, 60, 80 and 100 or as high as I can observe in WI. I also noted that close distances did not show the large change with temperature that the longer yards do. FYI, YMMV.
