Generally, the longest focal length lenses available are 800mm f/5.6 options, aside from rare or discontinued examples. And even as a prime, such a lens is massive. So, you can imagine just how big things get if you want to make a zoom that can reach 800mm with an f/5.6 aperture, but that is just what this lens known as the "Sigmonster" can do.
[ Read More ](Originally posted by Alex Cooke)