Exposures with IR filters

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Exposures with IR filters

Postby rogerb » Wed Jun 12, 2019 9:48 am

Last night, at the monochrome group, Malcolm and I had a discussion about how different infrared filters affected exposure, and I said it would be easy to test using a camera that has had the built-in IR filter removed. Pointing the camera out of the window this morning, ISO 100, f5.6, gives the following:

IR/UV cut filter (ie visible light, or what a normal camera would do) 1/100 sec
No filter ("full spectrum") 1/250
850 nm filter 1/60
720 nm filter 1/100
760 nm filter 1/60

Not carefully controlled as I did not put the camera on a tripod, so the image may not be precisely the same in all the pictures, but near enough.

The 850 and 760 block two stops compared with no filter at all, but unless you want the full spectrum effect a better comparison is with a normal camera, compared with which they lose only one stop of light. The 720nm filter is the same as a normal camera.

The results will obviously depend on the lighting conditions. It was cloudy when I took the shots and if the ratio of IR to visible light was higher on a bright sunny day the difference would be less. (I don't know whether that is true, but it's possible.) There could be other factors, such as the way the camera's meter responds to IR and visible light (it is common to have to overexpose by 2 or 2 2/3 stops when shooting IR.) But I think this shows that, although Malcolm was right that there is a difference between 720 and 850 nm filters, the difference is probably not big enough to worry about. Unless I compare images side by side, I cannot usually tell the difference between 720 and 850 nm images when they are converted to monochrome, so there is a small (1 stop) reason to favour the 720 filter.

Roger
rogerb
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