I guess affordable is very relative, though I'll need to keep under $250 for this one. I already have a Sigma Ultra-Wide II 24mm 2.8 MF (& 28mm 2.8), and while it is in otherwise 100% good condition, it does have quite a smallish and faint smear (looks like a faint fingerprint, though it is not) inside one side of the front element. Happily I picked it up for under $20 + Postage, so I'm not bothered, and will probably try to find somewhere in Sydney to get it cleaned. I have also found a SMC Pentax 24mm f3.5 that looks great, though I would be very interested in hearing anyone's opinions on this lens, or any others that are worth searching for. I will be using it mainly on an A7r. As one could probably guess, I'd like the best possible corner performance that I can get. I appreciate any direction here.