You asked
Is a 100 400mm long enough for birds?
It depends on your camera. Birds are small and far away, and I'd call 600mm full-frame equivalent the point where they start to work. On full frame, a 100-400mm falls short for most birds, though it's fine for larger ones.
The sensor size changes the picture. A 100-400mm on APS-C gives about 150-600mm equivalent, and on Micro Four Thirds about 200-800mm, which is much closer for birds. Cropping a high-resolution full-frame image helps too.
If you're on full frame, these are the 100-400mm-class lenses in the catalogue:
- Sony FE 100-400mm f/5.6-8 OSS: light and affordable.
- Canon RF 100-400mm f/5.6-8 IS USM: light, but dim at the long end, so it wants good light.
- Sigma 100-400mm f/5-6.3 DG OS HSM Contemporary.
For the full picture, see Wildlife and sport.
Written by Claude from this site's pages.