Depending on type of material focus distance changes

Hi Everyone,

Thanks for the help with loading libraries and adding lenses. I have been testing to find the sweet spot for focus on the various lenses I have. Looked at all the videos and followed the directions, but still have a problem. I used an aluminum black coated business card, 30% pwr, 150mm/sec sp, 40 khz, line interval 0.025, no scan angle. Found what I think is the focus distance by using the brightest and loudest test. Measured the distance and got one number. Repeated the test with plain aluminum ( non-coated ) and got a different focus distance measurement. Unless I am wrong, I do not think that you have to have different lengths for the various materials. What am I missing here and does anyone have an explanation or a more accurate way to get focus distance on a manually focussing machine?

Addition: Lightburn video has person testing black-coated aluminum business cards, does the coating make a difference here? If so, why did they use the coated card?

Thank you,

Jim

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