I want to project some slides with a projector.
I set the focus according to the first image. A few images appear to be out of focus. As it turns out, the images are actually in focus, but the projector is not. I readjust the focus of the projector. From time to time, an image is blurry and the focus has to be readjusted.
Why does that happen and how can I prevent it? I obviously do not want to readjust the focus for every image.
I think I have the right kind of slides and this even happens when the projector stands flat (i.e. is not angled up)