It's not so much the filters not being "true infrared" as the sensor not having much if any ir sensitivity.
Yup. Every camera has different sensitivities to IR transmission. Camera that have a strong on sensor filter will only record the tiny bit of visible light that passes through the filter. This makes some cameras less-suitable for IR photography in their unmodified form.
The
Leica M8, for example, is well known to have "issues" with the IR spectrum. It was to the point that it was causing color issues on the final images (synthetic blacks went purple and skin color was on the pinkish side). The solution was to use IR cut filters on the front of the lenses. This issue can be used to the advantage for those shooting IR. WIth the M8 in bright sunlight, I can obtain hand holdable exposures. This is with the 720nm filter
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This is with the Red and Blue channels swapped
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Leica pretty much corrected the situation with the M9 with a stronger IR cut filter on sensor. So it pretty much makes the M9 near-useless in IR photography. I tried... I get almost a black frame with deep-red silhouettes. I would have to resort to long exposures in order to get a less-than-stellar image.
If you really like infrared, the best solution is to convert a camera. I have an Olympus E-PL1 that I removed the on sensor filter stack resulting in a full-spectrum camera. It allows me to choose which filter to place in front of the lens. Shooting infrared with the full spectrum E-PL1 is pretty much like shooting with a regular camera.... you get normal exposures as you would expect in any given situation. This is with the 720nm.
The E-PL1 converted (DIY) cost me about $80. Instructions on Petapixel. Next step for me is to get my hands on a 820nm filter to further cut out visible and process in B&W.
As for processing, I found that it makes it a whole lot easier if you set your camera's custom white balance with the selected filter on grass (or whatever is reflecting IR) prior to shooting. This makes a huge difference in processing afterwards. Also remember that bayer filter based sensors have twice as much green sensor sites than red or blue.... you are blocking a lot of green. At least for my E-PL1, I like to over expose just a tiny bit.
