Here's one of my adapted lenses PK mount 28mm, fitted with a variable ND filter on my full spectrum A7ii - the filter set to it's darkest effectively blocks visual light but transmits IR longer than 850nm quite well.
Image uploaded directly
March 27, 2023
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I have a 720nm converted camera. Would a variable ND be able to take the cut off out to 850nm in conjunction with the existing 720nm? If so, can you point to a suitable filter?
Ta
Dave
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Your converted camera will have had the internal IR cut filter replaced by one that transmits above 720nm, any filter that blocks in the 720-1100nm region should make some difference to what your sensor sees. So it should do, most polarizers transmit IR & variable ND filters are basically two polarisers that can be rotated relative to each other. Polarisers that polarise in the infra red region are specials that cost more - so would be unlikely to be used for a variable ND filter - but you can't be sure without trying.
If you don't already have a variable ND it would probably be better to just buy a 850nm filter. I normally use IR filters but didn't have one with me on this occasion.
Both sorts of filters should be readily available on e-bay, or amazon (sorry currently a dirty word) I've brought cheap Chinese ones for IR by this route.
Generally there's not a vast difference between 720nm+ & 850nm+ monochrome images. The 850 ones MIGHT be a bit sharper & will have longer exposures. There may be special cases like shooting through inks where the difference becomes significant but I've yet to see anything like this - 720nm sees through quite a few inks as shown by the grid girls cat suits here :
BTTC grid in IR by Mike Kanssen, on Flickr
(sorry this shot's not adapted glass) -
I wanted to comment and say I purchased a Konica ar 28mm f3.5 and it's a great lens for IR for me (on my Nikon Z7). I had first tried the Nikkor non-ai 28mm f3.5 and I could not get anything sharp with that lens. It was difficult to achieve clear focus and even when I did the sharpness was horrible. The Konica and like night and day. The edges are perfect but the lens in general is great for me.