Originally posted by RKKS08
So you either get a picture as bright as with FF, but half the size, or you optically enlarge it by 2x in the viewfinder path (ocular) and get half the brightness.
My K-3 has 100% coverage with .95x magnification.
The K-1 has 100% coverage with .70x magnification.
Mathematics says the K-3 has .25x greater magnification.
Luminance flux (perceived as brightness) is measured in terms of light energy per unit of angular measurement, so frame area (crop) is coincidental in regards to measurable brightness.
That leaves magnification and apparent distance relative size of the 3:2 frame (angular view). Edit: The above line is clumsy and not quite accurate. The apparent viewfinder frame size for the two cameras is very similar with the K-1 appearing a little larger. The K-3 screen may appear somewhat dimmer as a factor of the greater magnification. This subject is deeper than it first appears. It is helpful to consider the focus screen as an array of emitters of set density viewed through a simple lens and go from there.
Steve
(Total light to the screen or sensor is not related to brightness/exposure to either.)