Hi AlexC.,
Yes that IOR. To calibrate the "Lens" I would set a camera straight to the image and far away, with a 500 or 1000mm focal length. Then dial the IOR until the image is close to the original (meaning it is not stretched and does not contain parts of the other image).
(Image below shows too much magnification)
Screenshot 2025-07-25 at 10.44.44 AM.jpg
Yes, three is possible, with three stripes and a higher IOR. Three stripes means slimmer stripes and more magnification. Here, the feeling of these Lenticular Postcards becomes more apparent, and its imperfections deliver that feeling even more.
If these artifacts are too prominent, then the lens needs to be smaller, or perhaps shaped differently
Here is a snapshot from a postcard I bought in the spring while visiting Germany and the Filmmuseum in Frankfurt. If I'm not mistake it has five images in use. This also demonstrates the analog artifacts of this technique very nicely. In my opinion, perfection here would ruin this.
Filmmuseum_Frankfurt_IMG_0620.jpg
I assume that the aesthetic of this effect should be evident, if it is just a blend of images, which could be done faster, as mentioned above.
Example with three: (Images for demo use here only)
https://projectfiles.maxon.net/Cineversity_Forum_Support/2025_PROJECTS_DRS/20250725_CV4_2025_drs_25_ANll_03.zip
My best wishes for your project