Review: Canon EF 600mm f/4L IS III USM

I own the Canon 600mm f/4L II and marvel at it. I really like some of the improvements to this lens with version III—the biggest being the considerable drop in weight. However, I find the loss of image sharpness compared to my version II 600mm a deal-breaker. It looks like more than a "slight" decrease to me. As Bryan mentioned, this loss of sharpness is more pronounced with extenders added, and I use a 1.4x III extender often enough that this is even more concerning. The excellent and useful image comparison tool on Bryan's website confirms this fact. I am sure for many photographers, the saving in weight and improved balance on this new version III will outweigh diminished sharpness, but I don't think I could ever be happy with less sharpness after seeing the impressive results I get in that regard with my current version II lens. I was hoping for decreased weight and equal or better sharpness with the new 600, but it appears there are tradeoffs for going lighter.
 
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