Canon ef 400mm f 5 6l usm vs sigma 500mm f 4 dg os hsm sports?

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From Sigma 500mm f/4 DG OS HSM Sports: Sigma 500mm f/4 DG OS HSM Sports: A prime lens by Sigma for Full frame cameras, 500mm f/4. Specs - Maker: Sigma - Focal length: 500mm (prime) - Maximum aperture: f/4 - Made for: Full frame sensors - Mounts: Canon EF, Nikon F - Stabilised: yes - Autofocus: yes What it frames like - On full frame (Canon EF, Nikon F): as marked, 500mm. - On APS-C (Nikon F): like 750mm on full frame; f/4 gives about ...

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 ...

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