if outside temperature is 20F degrees below inside, you need to let the lens cool for 30 min or so before it is usable.
mounting the lens on a 36mm extension tube further improves close focus - without sacrificing infinity focus! (normally, it can focus way past infinity)
sharper, better built and with less vignetting than the Samyang 500mm f/8.0 mirror lens - although plastic base plate is dodgy