Originally posted by RKKS08 1) Change the path of light before it reaches the mirror. This affects sensor and VF.
2) Change the path of light between mirror and VF. This would affect VF only.
Item 1) is possible, with the SpeedBooster, et al., but wasn't what I was referring to. Of course, any such adapter would make the viewfinder brighter, but we would need to change the optics for the viewfinder to make it bigger (which would make it dimmer).
Item 2) is what I was referring to. I'm saying that if you have a hypothetical 33mm F/0.92 lens on APS-C, you can make the viewfinder as bright and as large as a 50mm F/1.4 lens on FF. The size and shape of elements 5/6/7/8 in the picture below might have to change, and 6 + 8 might likely be multiple elements, but the function would still be the same. In this thought experiment, the size of the mirror remains unchanged.
In practice, camera makers don't optimize viewfinders for F/0.9 lenses, or even F/1.4 lenses, so bigger, brighter viewfinders are the province of full-frame cameras.
If you want to have some real fun look through a MF viewfinder. Truly awesome.