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Optics / Lenses / Filters Questions

Question Asked:

How do we perform a lens calculation for spacers? Is there an excel spreadsheet

1 Answers

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Perry West - President
Automated Vision Systems, Inc.
perry [at] autovis [dot] com
(408) 267-1746

I'm assuming you need to focus closer than the MOD (minimum object distance) for the lens in question. The total lens extension (LEtotal), focusing plus extensions, is simply the magnification (PMAG if you wish) times the focal length.. The lens focusing range (DImax) maxes out at the focal length squared divided by the MOD minus the focal length. So, the minimum extension required is LEtotal minus DImax. There are some extension amounts which cannot be achieved in practice. For example, C-mount extensions can be up to 2mm with washers or 5mm or more with extension tubes. The extension range between 2 and 5mm is not practical. This is often a problem for short focal length lenses. Also, most machine vision lenses are infinite conjugate designs. This means the lens designer optimized performance for a focus at infinity. As the lens is focused closer, its performance degrades, Down to the MOD, the degradation is small. Focusing closer further degrades performance.

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