Originally Posted by
Johnny Bravo
The poster was looking for advice on a 'first lens' aka his 'primary' lens for bird photography. In those cases, I'm firmly of the opinion that the 100-400 is a very poor choice--aka 'the Black Plague'. Dropping 1600 on that lens, and then suffering through the disappointments of trying to use it as a primary birding lens isn't a great way to start this hobby. For several hundred less he can pick up a prime and get superb results--hitting the delete key a lot less for sharpness issues.
The MTF charts tell the tale, as do user ratings on lenses--primes are the way to go for long telephotos. Zooms are a compromise with flexibility as the key delervable, at the cost of sharpness across apertures.
For a long zoom the 100-400 is a fine lens. It just doesn't deliver like a prime. (Hence the long lines of long primes one sees at sporting events.)