Frequencies and Filters
time for some cool graphs
A wave is described essentially by its frequency (the number of wavelengths that fit into a second) and its amplitude (the hight of its crests); in our case, frequency translates into color and amplitude into intensity.
Our eyes are incapable of capturing waves longer than 700nm, and it is not a matter of intensity (of how much IR light there is): it all has to do with frequency.
Digital cameras can catch up frequencies up to 1200nm, in the so-called Near Infrared (NIR), and that's where we want to look into.
Once you have converted your camera to full spectrum
(i.e. removed the hotmirror), you will have to cut out the visible light by placing an IR filter in front of the lens, and you will have a few options to choose from.
The most common and versatile filter is 720nm, but you can also get 760, 850 and 950 nm. These values represent the shorter wavelengths that will pass through the filter, so the higher the number, the deeper you dive into NIR, the harsher the contrast will be in your image.
You can also find filters that cut above the invisibility line,
at 590nm, 665nm and 680nm.
Physically this is not infrared, as it will still let part of the visible red spectrum through: it is, in fact, a BVR filter (Barely Visible Red).
720nm filters, though technically is IR, will still possibly leak some visible light, also due to the fact that 700nm is an average value for human eye sensitivity.
If you shoot 760nm, you definitely are in the invisible zone.
For my personal experience, 680 and 720 are the best choices if you are working with false color, as they still let some visible light in and thus leave you more freedom in postproduction.
When shooting bnw, I definitely go for 720 or 760, depending on atmospheric condition.
These sample images (copyright: KolariVision) give you a good idea of how different filters affect false color and
I have recently experimented more with the 850nm - which renders pretty harsh contrasts, and despite my initial concerns about serious limitations in postproduction due to the way 850nm renders a harsh BnW, I was very happy with the results from my shooting session in Kazakhstan in 2019.
Check out the Kazakhstan 2019 gallery!
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