Ok, through some research and what not I have a question for all you bird shooters out there.
Does this math work out pretty well as far as focal length is concerned on birds:
*For every 10-20ft 100mm of focal length is needed depending on size to fill the frame*
Size for 10ft being small birds such as sparrows and redpolls and 20ft being for larger birds such as blue herons
So here goes:
I'm seeing a redpoll at about 40ft away, so I'm gonna need roughly 400mm to fill the frame with it?
I understand one can "crop" at 100% but, I'm wanting QUALITY so I'm wondering if my math is correct?