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Just bought and am learning about my new K-01, and about APS-C photography. Coming to it from an Olympus XZ-1... with a much smaller sensor.
I enjoy low-light photography. But the camera gods giveth, and then taketh away, don't they? The XZ-1 has f/1.8 28mm equiv lens, but... because the sensor is small, you get a lot of noise over ISO 400. Then, there's the K-01. I can take a perfectly good picture at ISO 1600... but my 18-55 kit lens (27 - 85 equiv) starts at f/3.5 then goes to f/4.5 at 24mm (36 mm equiv) and so on.
So this got me looking -- and learning -- about Pentax lenses. Looking for a 28mm prime (42mm equiv) at f/1.8, or f/2. Well, forget that one! VERY expensive... like the 31mm (47mm equiv) f/1.8 Pentax sells for $1300. And that's dollars, not pesos, unfortunately. Wow!
Photography. There's no free lunch, is there? The bigger the film or sensor, the larger the lens to cover it, and then much more costly glass -- even more costly to stay up with new high-res sensors.
Side notes -- thought maybe I could stick Nikon lenses on the K-01. You can, sort of... old non-AI lenses will go on but not click into place. And you can get an optical adapter (with its own lens) for AI and newer lenses... but. The adapter lens adds a stop, or 1.5 stops, and you are back where you started.
But I am buying a $50 brass adapter to mount medium-format lenses. They are so sharp on my 645N... figure it's worth a try to see if they stay sharp on the K-01. Again, a big trade-off... a 2.6X crop factor... that 75mm yields a 195mm field-of-view on the K-01.
Solution? Write your Congressman. Instead of millions for lithium-ion batteries that light up Dreamliners, how about a national project to develop a $200 K-mount 28mm f/1.4 lens? Yes!!!