Take a look at these produits from a dutch company named Redged, i think i Will buy the TSC 428
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So you want to have a lens which is 100 more in reach than the 200-400, and has the same max aperture of F4
I'd avoid using the 100 for snakes... its not quite as tight a crop as you'd think...
Just add in at least once thing with the new 100-400mm design and I'll be a happy camper: