Tony Northrup is the author of Stunning Digital Photography the #1 best seller in the Digital Photography category of Amazon.
In this video he goes really deep down the rabbit hole. Some important difference he discusses are the difference between the 70-200mm f2.8 and the 24-70mm f2.8 lenses on both a full frame and a crop frame camera.
he then explains why in many cases lenses like the 18-55 f3.5-5.6 and the Sigma 18-35mm f1.8 may actually outperform those lenses at less than 1/2 the price.
I am really good with math but even I had to do a double take and watch this video a couple of time to wrap my head around these concepts.