What should I do if the eyepiece of SLR viewfinder is not bright?

What is the concept that the eyepiece is not bright is puzzling.

Seen from the eyepiece, the brightness is very dark.

Guess 1: the lens aperture is small, the aperture is not large enough, and the brightness is insufficient when the light is dark. There is no other way but to use a large-caliber lens.

Conjecture 2: The low-end SLR brightness of pentaprism is lower than that of pentaprism, and the brightness is worse with the first factor. There's nothing I can do.

Conjecture 3: For the old manual aperture lens, the full aperture lever does not match, or there is no lever (such as folding lens). When the aperture is narrowed, it will be dark. In this case, you can only open the aperture to check the focus, and then narrow the aperture to shoot.

Guess 4: Old-fashioned film SLR, using old-fashioned pure frosted glass focusing screen, the brightness is very low. There is no way.

All black:

Guess 1: the high-end SLR has an eyepiece curtain (to prevent light from entering the eyepiece to interfere with imaging when taking a selfie). Did you close the curtains?

Guess 2: if the lens cover is not removed, it will definitely not light up, and it is estimated that it will not.

Conjecture 3: When the reflector is pre-raised, the eyepiece is not bright.

Conjecture 4: In the real state, the eyepiece is not bright either.