Originally posted by anthony mazzeri I agree. Just get a K-M adapter and use all your fine Pentax SLR lenses to begin with before going to the expense of a whole new set of duplicate rangefinder ones.
Well, there is quite a difference in size, weight and comfort. Even the smaller DA Limiteds, when clamped on the K to M adapter are big, clumsy, uncomfortable and tedious to use. The 40mm pancake might be the nicest of those. 42mm screwmount lenses aren't quite as bad with the m42 adapter but still, it's much clumsier with those, too.
For example, I have tried a 135mm Leica R lens with the LR to LM adapter, but the experience was not that pleasant, nor usable. The lens is long and heavy, heavier than the GXR and the module together, and handheld shooting felt a bit daft. Not much smarter with a tripod, either. A reasonably fast 50 or 58mm m42 or LR lens wasn't much better, either. It''s still an awkward chunk of glass and metal uncomfortably far from the body itself, although a bit more usable.
I ended up selling the R lens and buying a native M lens to replace it, and the difference was like night and day, even though I lost half a stop of speed, and a little bit of focal length. Shooting with the short SLR telephoto was tedious and hard to do handheld, but quite enjoyable with the native one. Same with wideangles and standard lenses. I still have and sometimes use the m42 to M adapter and a couple of m42 lenses but, I am in the process of gradually replacing all of them with proper native M lenses. The m42 lenses are much better and nicer with typical (D)SLR bodies.
The GXR really is a fully digital rangefinder. Sort of. Not just a small mirrorless DSLR substitute. Putting a huge DSLR lens (along with a chunky adapter) into the GXR defeats the whole purpose of the system. IMO, anyway.
Besides, the GXR M module has been designed with the typical (symmetric) rangefinder lenses in mind and optimised for them, unlike the other mirrorless systems. That's why the M module has a real M bayonet to begin with, the sensor has no AA filter, and apparently the IR filter is also thinner than usual, much like in the digital Leica M bodies, and unlike in other mirrorless bodies.
Therefore it makes more sense (again, IMO) to use the GXR M-module as a digital M-mount rangefinder with the classic M-mount rangefinder lenses, rather than as an awkward mirrorless body with chunky adapters and all sorts of lenses designed for entirely different systems. Vice versa, it sort of makes more sense to use the classic M mount lenses in either a real Leica or the GXR, rather than in other mirrorless systems with an adapter. A non-native lens with an adapter is always a compromise, and in the case of the GXR, perhaps more so than with some other mirrorless systems.
In other words, whilst it's possible (and maybe cheaper) to use the GXR mount (+ K-M adapter) as a K-01 substitute, I for one don't recommend it. At least not as a permanent solution. It's not particularly fun or comfy, either. Want to use the old K lenses with a small mirrorless camera, buy a K-01. It's cheaper, too, these days.
-just my 2c, and I'll come up with my lens recommendations in another, hopefully shorter post.