hi guys, decided not to go with mirror lens but have bought a sigma 500mm tele f/7.2, i know its not the best lens in the world but i got it at a good price so could not turn it down. we have a lot of woods around my area and i thought it might make a cheap entry into wildlife photos, i am also considering a 2x converter or would this downgrade the quality too much?, any info is appreciated, thanks. Neil