One of the biggest benefit of mirrorless cameras is the WYSIWYG. WYSIWYG stands for: what you see is what you get. Changing the exposure make the image in the viewfinder brighter or darker at the correct aperture and depth of field…
The problem with “what you see is what you get” cannot anticipate extra lights like flash or or lights that will be turned on…
Most of the time with flash, the EVF or the LCD screen will look dim, very dim. The problem is that the camera cannot anticipate what the flash will do because in only decides during the pre-flash of the TTL system.
OVF of the X-PRO1 / X-PRO2
Using the optical view finder is quite good in the dark. Almost all the light comes through and I see very well in the dark.
EVF of the X serie
You will need to disable the “what you see is what you get” of the Electronic Viewfinder.
SCREEN SETTING > PREVIEW EXP./WB IN MANUAL MODE > Select OFF
If you need to do this on a regular basis, you should set a shortcut to the menu.