Nervous nellie...go with Pentax K-1 or move to Sony FF?

> DFA28-105/3.5-5.6 lens is just cheap walk-around lens for hiking, it IS NOT lens for any serious work that demands FF camera.

Unnecessary condescension. Stopped down as you have to for landscape use, there is no difference to the 24-70 in resolution. And in adverse weather, the longer range offers additional benefits.

> And it IS NOT a lens unavailable on APS-C. Just get sigma 17-70/2.8-4 and you'll get similar output with smaller and even faster and a bit wider lens (26-105/2.8-4 in terms of light and 4 - 6.3 in terms of depth of field).

The only useful measure is aperture diameter or equivalent f-number f/4-6.3, as this tells you how much light hits the sensor.

But yes, the 17-70 is a pretty equivalent lens when it comes to range and aperture diameters. But it will not give the 36MP resolution of the 28-105 and it's not WR.

> I can easily turn this backward.. show me lenses that are FF K1 equivallent of lets say:
> Sigma 8-16 (FF12-24??)

Yes, exists, since before the 8-16, actually, with larger aperture diameter. Sadly, Sigma has not done the new 12-24 for Pentax, because market size is too small.

> DA18-135WR (FF 28-200WR?)

Sadly not with Pentax, otherwise I would get it. You know, as a hiking lens to take photos I don't need a K1 for....
Anyway, if we are talking sensor sizes and not specific brands, Canon (very expensive, admittedly) and Nikon each have a weather sealed 28-300 and Sony a weather sealed 24-240. All of these offer better resolution and larger aperture diameters on FF than the 18-135 on APS-C.

> Wide angle fast fullframe lenses and even some short telephoto fast fullframe lenses simply have no counterpart on APS-C systems. And these options are only reason why one would spend more money for larger camera and more expensive lenses.

That's only half of the story. If two lenses offer the same angle of view and aperture diameter, then the lens on the larger format probably has better resolution.

> I can see the reaction already: Oh yes you can use them in crop mode...

That's not my reaction, but I still disagree with you on a number of points.