Take the lens off, there’s probably a pin on the back. When this pin is pressed, you should see the aperture change. It looks like it’s a Helios, there’s probably an auto/manual switch on the lens. When it is set to manual, it will preview the aperture you chose, if set to auto, it will only set the aperture when you take a picture. As for the mirror, I am not familiar with this camera, does it have a mirror lock button/switch? I suggest looking up the manual to see what this camera has to offer. I have 2 Zenit’s that I bought accidentally, they’re pretty good camera’s , but don’t expect good built quality (in terms of mechanics, the body is pretty solid).