Originally posted by acoufap My "strategy" depends on the FF specs and budget.
FF should ...
... give me 1-2 Stops better ISO performance than my K5
... cost limit 2.000,- €
On same sensor technology this is 1.15 stop more or less in theory... And that match the DxO score the sensors get.
But don't forget that to get it you'll have to live with the shallower deph of field too. Thus the better high iso performance work really when you are at the widest apperture of the lense, would want/accept shallower deph of field to lower the iso but can't because the lense doesn't support it. If you happen to have the equivalent lense with the same apperture... of course.
So yes a 35mm f/1.4 on APSC can be replaced by 50mm f/1.4 rougly. 50mm f/1.4 get replaced by 85mm f/1.4...
But many time a 200mm f/2.8 will get replaced by 300mm f/4... Because 300mm f/2.8 is simply too big, too expensive. And thus you'll not leverage at all the better high iso performance from the FF.
Even if you do no better than 108mm f/2.5 on FF, you aren't going to see any improvement for noise compared to 77mm f/1.8