Film scanner
I checked my Epson V 600 film holders and both the 35 mm and 120 holders look like only 1 mm. I am measuring from the base to the place where the film sits on top of it. I haven't tried the coins yet. I might try US pennies since they are fairly thin and dark. I'm attaching a Zebra picture from a 35 mm negative that I scanned using the Epson holder.
US dimes are thinner than pennies, so they might give you more precision.
Years ago, I decided to test my Epson Perfection 4990 to see if I could find its optimum film-to-glass distance. What I used was an unmounted slide and pieces of film as shims to vary its height. Doesn't have to be film for shims. You can just as easily use paper or, well, shimstock, far as that goes. Anyway, after futzing around with it for way too long, I finally determined the optimum distance -- and it was
exactly the height of my negative holder. Which, incidentally, was half the thickness of a mounted slide. I mean, think about it. The Epson slide holders don't actually hold the slides. They just position them on the glass. So for a slide's image to be in focus, the correct distance must be half the thickness of a slide mount.
Also, I've scanned some negatives that have had some rather pronounced cupping. I was concerned that the centers of the images would be soft because that was where the image distance would be farthest from the glass. But it wasn't a problem. Which tells me that Epson has built in enough depth of field with their optics such that they can handle some variation in image height. That's probably why it took me so long to figure out the optimum image-to-glass distance.
