IR

[compilerbitch]

I just got an IR filter (square, Cokin P-series, 89B). To the naked eye,
it looks completely black -- if you look through it, you can /just/ see
bright objects as a dim red image. I wasn't stupid enough to look at the
sun through it, of course (it's pretty much transparent at IR frequencies).

Anyway, putting it in the filter slot in the compendium shade on the
Cambo worked a treat. I focussed the image with no filter in place on
the ground glass, locked everything down, then inserted the BetterLight,
stopped down to somewhere between f16 and f22 and put the filter in
place. Doing a prescan, I found I'd way underestimated the sensitivity
at IR, so I ended up upping the line time to about 1/250th and dropping
the ASA rating to the minimum (just under 400). The focus looked OK, so
I made an exposure and checked it in Photoshop. It wasn't bad, but the
focus was slightly off -- this is not too unexpected for IR. I then used
the BetterLight's focus meter to get it dead on -- the next scan was
fine. This is a crop from that scan, actually a fairly small area in the
top right of the original.



The shot was taken in reasonably bright sunlight, about an hour before
sunset. I took the image data from the red sensor only, but
(interestingly) I found that all three sensors (R, G and B) seemed to be
about equally sensitive to IR.

Comments

[cuthalion.livejournal.com]

From the rainbowing on the moving objects in the big color jpeg, I'm guessing the back has a filter with three large strips of red, green, and blue instead of a mosaic or something? Anyway, CCD responses are similar to CMOS ones even in IR, the 89B is pretty similar to an R72, and it seems probable your R,G, and B filter response curves extend past visible similar to canon's, so you might be able to pull some neat false color out of the full color picture!

Here is something I wrote about about false color IR, although the response spectra graphs honestly aren't all that accurate.