Somewhat continued from this page.
Most mirrorless camera users would simply purchase accompanying lenses from the camera brand. There is no reason not to do that. However, other photographers would know they can use older SLR and rangefinder lenses, too.
In that case, aperture control of older lenses would be “manual.” If you turn the aperture ring, diaphragm closes and opens. LCD viewfinder of mirrorless cameras is very clever. Brightness automatically adjusts according to diaphragm size.
I believe it is rather welcoming, than being only able to see the viewfinder at aperture wide-open. With SLRs, yes, it is better to see it brighter. With mirrorless, you can see the bright view it would exactly photograph, plus with correct depth-of-field shown (somewhat with help of focus-peaking).
Traditionally, professional lenses used to have “manual” aperture control. Older SLR lenses can now forget about “spring-loaded” auto-aperture. Welcome to professional use of lenses. KT