[Rondec]

Actually, if you look at Canon's 70-200/4, that's nearly the same weight as the Pentax 50-135/2.8. Size is also not far off, although the Canon is longer. And when the first is used on a fullframe camera, and the second on an APS-C camera, they are more or less "equivalent" to each other, at least in terms of AoV and DoF ranges.

Of course. I don't argue that fact. But the sensor sizes and mount sizes and registration distances are pretty similar between those lenses. A K3 is really going to end up being pretty close in size to whatever the full frame Pentax is called. But if you take a lot smaller sensor and a shorter registration distance, you can create very small camera systems.

That is my point.

If you need a system that is equivalent to your full frame system, than you had best go with full frame. If, on the other hand, you value other features (portability, small size, relatively inexpensive telephoto), then a different sensor size may be better or, at least as good.

[starbase218]

Of course. I don't argue that fact. But the sensor sizes and mount sizes and registration distances are pretty similar between those lenses. A K3 is really going to end up being pretty close in size to whatever the full frame Pentax is called. But if you take a lot smaller sensor and a shorter registration distance, you can create very small camera systems.

That is my point.

Taken.

If you need a system that is equivalent to your full frame system, than you had best go with full frame.

Yeah, they don't come more equivalent than that.

If, on the other hand, you value other features (portability, small size, relatively inexpensive telephoto), then a different sensor size may be better or, at least as good.

Yup. That's why I'm sometimes still thinking of getting a M4/3 camera, when I'm not thinking of getting a used Nikon DSLR that is .