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What's the easiest and most affordable way for obtaining UV reflectance images?

Started Apr 24, 2021 | Questions thread
ProfHankD
ProfHankD Veteran Member • Posts: 9,147
Shifting the focus plane

petrochemist wrote:

Not surprisingly focus afterwards was way off.

This is actually my #1 issue with full-spectrum conversions. Generally, if you remove a 1mm thick piece of glass typically with index around 1.5, you've basically pushed the sensor about 0.5mm closer to the flange. MFT uses a 4mm cover glass, so we're talking about a potential shift of 2mm!

For DSLRs, that's well out of the adjustment range for aligning the focus sensor (or screen). Fortunately, most lenses just loose a bit of close focus with a shift like that. However, I own some lenses (ultrawide/fisheye) where the entire focus range is a fraction of a mm. So, the take-away message is that you might want to make a custom adapter that compensates for the missing cover glass.  That's easy if you 3D-print adapters, it can be a bit harder to change a commercial adapter's length that much, but it is do-able.

I'll also note that focus plane for NIR or NUV is often pretty far off from visible, so that alone might make you want a customized adapter if you're shooting only with one of those bands.

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