When I pop up the built-in flash on my Sony Alpha 57, the LCD and electronic viewfinder brighten up a lot, so I can focus properly in low light situations. However, when I put on my Yongnuo 560 mkII, this doesn't happen. Is there any way to change this behavior or is it standard behavior when an external flash is attached? I looked at the camera settings, but didn't see anything that would appear to affect this and without this effect, I can't see anything on the screen or viewfinder.
From what I understand, that Yongnuo is a manual flash. While I am not experienced with Sony Alpha cameras, I am assuming that it is unable to simulate any brightening the flash unit may provide through the EVF or LCD because the flash is not communicating any information to the camera. Manual flashes must be manually configured. My best suggestion would be to go into the camera's settings and increase the brightness of the EVF and the LCD to see if that helps.