This is the largest (and my favorite) lens I own.....the Pentax SMCP-A* 1200/8 ED-IF. It is a beast of a lens but produced nicely coloured and incredibly sharp pictures.
This is not an easy lens to use, with the slightest movement or vibration being amplified by the focal length. The depth of field is reduced to 1 cm, (half an inch) or less when used at f8, which means that a birds eye can be in focus but it's beak with be blurred. Good luck getting a sharp shot if the bird moves, which of course they always do.
Pentax1200mmA120012008EDIFPentax 1200Pentax A1200Pentax 1200mm
From Pentax A Star Series Telephotos