I'd like to extend my capability for bird photography and reckon my budget could buy either a secondhand Nikon 500mm F4P manual focus or new Sigma 500mm F4.5 AF. I would appreciate hearing from anyone who's been in the same position, particularly with practical experience of the Sigma. I've ruled out secondhand Nikon AFI or AFS lenses as I've never seen one for sale in the UK. TIA, Martin
