@rnlzkbrs points to a good article at the Deon website. I would urge you to read it, maybe even a few times.
I'm an F-class shooter (boo, hiss) and one the challenges of that discipline is that which is incorrectly, but colloquially referred to as "mirage". The article explains how ED and especially Super ED glass seems to tame this mirage. In effect, this type of glass seems to retard the degradation of the IQ of a riflescope compared to non-ED/Super ED glass. In F-class, we run our riflescopes at 30X and above. I've run my March-X 5-50X56 at 40X year-round for years, regardless of mirage conditions. This SFP riflescope is a design very similar to the March-FX 5-40X56 both generations. When I upgraded this scope to the March-X 10-60X56 HM with Super ED glass, I increased my base magnification to 50X. I'm always at 50X.
Let me state right here and now that the IQ of the image at 50X is affected by mirage, but even in heavy mirage, I can still make out the rings and the aiming black is still very round. To my eye, it appears as if there's an electrical aura to the various rings, but they are distinct and they are not moving. Because of that I am able to place my aiming dot exactly where I want on the target, in a repeatable manner. The lesser riflescopes that I used before that simply could not do that, I had to dial down the magnification just to get the aiming black to be round and sit still.
What I have noticed also is that in the Super ED March-X 10-60X56, I can detect the mirage very early on and it appears like a river in the riflescope, showing direction and amplitude. This has helped me recognize switches in conditions and save many points.
The article does state that there are riflescope designs that have inherently better "shimmer protection" than others and it does on to name a few such as the aforementioned March-X 10-60X56 HM. One thing the article does not mention is the difference between non-ED glass and ED glass. They only talk about the difference between ED and Super ED, simply because there are no non-ED glass March riflescope they could use for comparison.
I have not used the March-FX 5-40X56 in an F-class match. It's an FFP design and I need a thin reticle at high magnification. Also 40X is where I start, not end. I would expect its "shimmer protection" to be similar to the 5-50X56 meaning that for me, I could use it at 40X all the time, regardless of mirage. I have never used a 5-42X56HM except on a tripod but I would think its "shimmer protection" would be similar to the 5-40X56; it will not be at the same level as the three scopes named specifically in the Deon article.
My understanding is that for improved "shimmer protection", you need to start with good quality ED glass and a certain design and you can increase this protection with higher quality ED glass and improved design.
