Originally posted by pxt 1. Is a change to noise in comparision to K3 II which I have?2. How much IQ is affected?3. For what it can be use in reality?
A cropped K-1 or K-1ii, compared to your K-3ii, will IMO show a small improvement in noise while sacrificing a little less resolution. The numeric difference going from 24mp K-3 to 15mp crop K-1 is not as significant as it appears; a 300dpi K-3 print becomes around 240dpi on the K-1. You won't notice it at most viewing sizes.
My uses for crop mode (in software because I use raw DNG rather than JPG) are with 2 particular lenses, when saving weight and bag space justifies compromises in image quality.
1) DA 55-300. Full frame telephotos are heavy so I only bring them when I plan to photograph wildlife or other specific subjects. The 55-300 is compact enough to stick into a small bag when I want to travel lightly.
2) DA 15 Limited. This lens works well in 4:3 and also 1:1 crop mode. Like the DA 55-300, I'll take the DA 15 if I don't expect to need a lens like the heavy DFA 15-30, but want flexibility in case a wide angle opportunity unexpectedly pops up.