Optimal aperture position?

Started 2 months ago | Discussions thread
ProfHankD
ProfHankD Veteran Member • Posts: 6,820
Film-based apodizer

E Dinkla wrote:

Using analogue thin emulsion B&W lith film with a developer that delivers a softer image may be the best choice, the PET film there has more optical transparency and the silver halide image will be quite neutral. It may still be possible to print a negative radial gradient with an inkjet printer and use it in contact to expose the B&W lith film. Some trial and error steps to get the gradient right in the lith film.

Using silver hallide film (or that type of emulsion on a glass plate) is the best method I know for repeatable production of high-quality apodization masks. Actually, I've also done this for making flash projection masks for structured light 3D capture. Mask quality this way can be excellent, although it does mean bringing out an old film camera and remembering how to process film. Medium or large format are needed for larger apertures, but 135 film can handle masks for most fast 50s and shorter focal lengths because they tend to vignette a lot with a larger aperture.

The sneaky trick is that we're trying to get a gradient, not a sharp image, so the way I create the image is to basically photograph a computer-generated display with a long exposure and enough (deliberate) motion to blur any raster pattern.

PS: Reciprocity failure can be your friend -- that's how you can get arbitrarily higher contrast on film than the image you photographed had. When photographing the negative image, keep the exposure balanced so the stuff you want clear on the film is below the reciprocity failure threshold and the bright parts are above.

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