OP MAY also have one of the old old old screens that is dark, fingerpinted, dirty, and damages.
The OLD OLD screens are very dark, and the prism makes the situation worse.
Hunt down a newer screen or just buy an accu-matt as they are worth it. Especially if you are using an f4 lens.

Oh... is you lens stopped down? Just asking, as it sounds like the whole rig is new to you.