It is a quality pinnacle in the overwhelming wash of low-quality rubbish internet sites dealing with photographic lenses. I do wish reading the stuff was mandatory for any review site on lenses.
As already mentioned, the illustrations (with ray tracing) are extremely nice, and the worked-through examples facilitate the understanding of the underlying principles. The author invites the audience to learn and understand, it's as simple as that. Nothing is dumbed down, rather it is explained.
