Originally posted by thibs
A mirror lens would be a very bad idea.
For the bokeh club, yes. For us BoB shooters (Bird on Branch) we often find uniform blu sky behind our subjects.
Originally posted by timautin
+1. Lightweight is important, but less than IQ imho... For the 500-600 range, I'd prefer to carry a 3-5 kg lens, with great IQ, than a 600g one which produce bad pictures.
You already have lots of nice options that should fit. I don't. Carrying a 3+ kg lens is not an option. Buying one is not an option eater. Yes, I must pay more attention to the background, but I'm willing to pay that price for saving a pile of money, back pains and a hard argument with my wife. I will go through enough just to buy the K-1.
All mirror lenses I have seen and read about to date is low quality usually designs from three decades ago without a hood. A large hood and internal baffles are important to increase contrast. The blackness and rib shapes on baffles and hood are important too. I know of only one mirror design with AF and that is only in Sony alpha mount. I have seen a couple of mirror lenses with a built in ND filter wheel to control the amount of light as an alternative to having a variable aperture, but none with hood or AF. I have seen custom made mirror lens designs with an asymmetric design (no doughnut), but never one with a camera mount. I'm sure that If Pentax wanted to make a modern mirror lens according to the digital age and high demands, they could make a mirror lens that could change the principle view on them and virtually put a low quality tag on all the rest. A mirror lens is not a bad idea, its just historically poorly conducted.
Originally posted by D1N0
But those aren't very light

They are extremely light .. like in photonic light, for their focal lengths.