Distances with diffusers, cameras and LEDs?

Caveat - I know it’s going to vary between set ups, but I’m trying to get my head around some general rules of thumb, and haven’t found a lot searching old topics surprisingly.

Generally with a direct LED camera top down digitising setup (no mirrors/bounces/etc), are there general rules or ratios of how far the diffusion should be from the light source? And then how far the film negative should be from the diffusion?

Obviously LEDs with tighter arrays need less diffusion, etc, but i’m going in circles fine tuning the variables in my beta set up, so it would be handy to have some general starting points to launch from!

A few rules of thumb:

1) Your film should not be touching the led (or diffusor) but ideally it should be close to touching. You don’t want it touching because it can cause newton rings (or other distortions). But you don’t want it too far because it introduces opportunities for reflections and other issues which create uneven light.

2) If you are using something to mask out the light source around the film, you want it to be very thin and as close as possible to the film itself. If you have a mask that is too far from your film, it will basically cast a shadow from the light source on the edges, which will result in orange fringes once inverted.

Hope that helps!