Great info from Dave. Here's my take:
Originally posted by Kenty1 1) As you can see I have a few lenses - is there any 'best' focal lengths/ranges to use?
2) Does ISO matter?
3) Is there a best F No to use?
4) I usually shoot in RAW (PEF). Are JPGs better?
1) Wider lenses are better, aesthetically. Narrow-FOV IR shots look to me like surveillance. But your wide zooms take large filters -- will Cokin work with them? I use the kit 18-55 @18mm to hang 52mm filters on. Too bad I can't hang filters on my ultrawide nor fisheyes.
2) Lower ISO is better. Higher may be necessary depending on subject. Remember that you're essentially throwing away 3/4 of your pixels, only getting data on the R channel. IR turns a 12mpx camera into a 3mpx image system.
3) Tighter f-stops are better. You'll get enough softness anyway, which doesn't look great in IR. Maximize DOF and detail. There are dramatic exceptions of course, but IR 'scapes demand detail.
4) RAW is just about mandatory. Much PP intervention is needed. And because of the effectively reduced resolution, you'll probably want to downsize the images you produce.
What you didn't ask, and what is very very important, is: Which IR-pass filter should I use? These are labeled by their cutoff wavelengths. A #25 Red filter is essentially 680nm and gives a pseudo-IR effect with lots of visible light. 720nm is common and admits a bit of visible light. I often use 780-900-930-1000nm filters which pass almost no visible light (780nm) to total blockage (the others). These deeper IR-pass filters are effective for deep ND effects, forcing LONG exposures on unmodified cameras. My 1000nm filter is something like ND50.
So the filter used determines just which part of the EMF spectrum you're slicing. Near-IR (680-720nm) looks much like visible light, just very contrasty. Deep-IR (900-1000nm) looks totally different than what we normally see. In PP you get to decide how to render those spectral slices as impossible colors.
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I also like to play at the other end of the visible spectrum. Early photo emulsions could only see ACTINIC (UV-violet-blue) light. A few decades of chemistry slowly produced the dyes that made plates and film sensitive to green-yellow-orange-red-IR spectra. If you'd like to replicate US Civil War-era photography, use a Blue or violet-CC filter to block longer wavelengths. You can use the Blue digital filter in your camera but that's rather harsh; I prefer a #47B blue-violet or a #80C light-blue filter for handheld shooting. Like an IR-pass filter only feeds the R channel, so a deep-blue filter only feeds the B channel. You'll want the same low-ISO thick-DOF settings as with IR work.
Last edited by RioRico; 08-23-2011 at 09:00 AM.