• Members 208 posts
    March 27, 2023, 11:23 a.m.

    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.
    PK 28mm vari ND.jpg

    Image uploaded directly

    PK 28mm vari ND.jpg

    JPG, 164.3 KB, uploaded by petrochemist on March 27, 2023.

  • Members 509 posts
    March 27, 2023, 12:40 p.m.

    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

  • Members 208 posts
    March 27, 2023, 1:49 p.m.

    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 :
    live.staticflickr.com/2930/13972162589_79b7c77bc8_c.jpgBTTC grid in IR by Mike Kanssen, on Flickr
    (sorry this shot's not adapted glass)

  • Members 40 posts
    Jan. 1, 2024, 5:45 a.m.

    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.