I recently bought a Zenit B and have just been playing around with it. My first roll came out fine, but the most recent negatives came out with these marks on it (see attachments). Not every photo came out like this and only one photo came out with the crazy what-appears-to-be-light-leak spread across the majority of the photo.
What do you think? Light leaks? Dust?
I don't particularly hate this new 'effect' but I don't want these marks to be on every future photos I take with the camera.

Thanks heaps!