Originally posted by cdd29 I do landscape and night photography primarily and starting to dabble in astrophotograohy. What would be some (if any) advantages moving from full frame to medium format?
Originally posted by MarkJerling I think, for studio work, product photography and the like, the 645D or 645Z would be a good choice. For landscapes and night photography, I think the K-1 would be your better choice. No doubt, the 645D or 645Z would do well in those situations too, but it's more suited, I think, to studio work.
The main advantage of medium format over FF includes a 4:3 frame (more square) vs. 3:2 frame; beautiful shallow depth of field or bokeh, and perception wise from clients, it separates you from "small format" photographers. I've used my medium format exclusively outdoors and even handheld, although it is more commonly seen in studios. The 645D is weather-sealed for a reason.
Pentax does make a 35mm 645 prime (28mm FF), a 33-55mm zoom (26-44mm FF), and a 28-45mm (22-36mm FF) wide angle options. The last one is AW rated, although having non-WR or AW rated primes has rarely stopped me from shooting outdoors (with caution).
With that said, I agree with Mark especially in terms of cost. The K1 is by far more affordable when you look at the overall system costs, and is more than capable to do landscapes and astrophotography. If you hit the jackpot, and don't mind the bulk, then I'd go for a 645Z, but given your budget, the K1 makes more sense and you'd have less regrets than getting an older MF 645D.