Okay, so I'll preface this by saying that I am still very much a novice. Here is the situation - a week ago, I was shooting (hand-held) a roll when all of a sudden, there was a slight click, the viewfinder was black, and the shutter would not release. I just assumed the battery had died - didn't really think about it, changed the battery, maybe fiddled with it a little, and went back to shooting. The negative for that frame was entirely black.

Today i was shooting for about 6 hours - moving very quickly, a lot of action, flipping between portrait and landscape a lot - when it happened again. Since I knew it wasn't the battery, I actually took a second to think about it and realized the mirror had locked up. Though the toggle seemed to be in the normal position. Flipped it to lock and back, and the shutter released. This happened repeatedly for several shots on that roll, but not once afterwards. I shot probably 5 rolls before and 6 after.

Is it possible that I am just hitting the mirror lock-up when switching from landscape to portrait? Normally I turn my right hand up, but I've been turning my left up recently because I strained that wrist. Or could it be something else?

Lastly, when this happens, how can I keep from losing a frame?

Any sage advice on what i am sure is a dumb error on my part would be greatly appreciated.

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