Is the canon ef 400mm f 5 6l usm worth it?

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From Canon EF 400mm f/5.6L USM: Canon EF 400mm f/5.6L USM: A prime lens by Canon for Full frame cameras, 400mm f/5.6. A light, fast-focusing birding prime without stabilisation. Specs - Maker: Canon - Focal length: 400mm (prime) - Maximum aperture: f/5.6 - Made for: Full frame sensors - Mounts: Canon EF - Stabilised: no - Autofocus: yes What it frames like - On full frame (Canon EF): as marked, 400mm. - On APS-C (Canon) (Canon EF): like 640mm on full ...

From Canon EF 100-400mm f/4.5-5.6L IS II USM: Canon EF 100-400mm f/4.5-5.6L IS II USM: A zoom lens by Canon for Full frame cameras, 100-400mm f/4.5-5.6.

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