How can I prevent focus shift between slides when projecting them?

Question

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)

Answer 1

A few things can cause this issue:

1. Reversed slides: the image is on the emulsion side, and due to the thickness of the backing, will not be in the focal plane of the lens.

2. Different slide mounts: some processors may use mounts of different thickness from others, so you may need to remount all in a uniform thickness, e.g. B&H slide mounts.

3. Thermal expansion: the heat of the projection lamp can cause a slide to pop or "oil can". To resolve this, you may need to remount the slides as above and/or add more infrared filtration in the lens chain.

Answer 2

If you haven't yet made the move to digital projectors, another possibility is movement of the projector, as the carousel turns, or if people are sitting near it.

You can try moving the projector farther away from the screen to increase depth of field. Or maybe add an aperture to the lens.