Originally posted by JeffJS I believe you. I am not arguing with you. It would be nice though if we could make up our minds when answering somebody's questions about it. One poster says it Can, the next, says it Cannot (light pass through the viewfinder to the mirror box when the mirror is up).
Well, perhaps we can agree that when answering somebody's question, it is always best to speak from experience, rather than hearsay :-)
I'm not confronting you, by the way. It's just that your post appeared to make this point equivocal (or at least tried to do so), when it is not. And you should care one way or another, actually, because now you can say with confidence which is the correct answer.
Of course, to be consistent, it is best to see for yourself. If you have a really dark ND filter (like ND400), try shooting with the sun behind you for the longest duration possible (shoot F22, ISO100). If you can stack NDs or a polarizer, the better. Take two shots, one with the cap (or hat) off and one on. The difference will be astonishingly great if the exposure is any longer than a few seconds :-)