Why does a FOTGA variable IR/ND filter give weak or mostly red results on an unmodified Canon 7D Mark II?
Asked 3/29/2025
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2 answers
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I'm experimenting with infrared photography for fun using an unmodified Canon EOS 7D Mark II. A borrowed IR filter previously gave the classic false-color look with bright foliage, but a cheap FOTGA filter labeled "fader ND IR(W)" gives mostly red images and only weak selective color shifts, even after white balance. Long exposures are needed, and foliage often doesn't turn pale the way typical IR examples do. Is this because of the filter type/quality, the camera's built-in IR-blocking filter, or something else?
Originally by Krzysiu. Source · Licensed CC BY-SA 4.0
Krzysiu
1y ago
2 Answers
3
The filter you describe, an ND IR(W) type filter, is made to block infrared light and allow visible light through. Sometimes such filters are called hot mirrors. Your digital camera almost certainly has a similar filter in the stack in front of the sensor.
When most people speak of infrared photography and using an "infrared" filter, they want to block (or reduce) visible light while allowing infrared light to pass through. This is the opposite of what your ND IR(W) filter does. To do that you need a camera with the hot mirror removed and a filter such as an R72 (720nm cutoff), Wratten 89B (700nm), or Wratten 70 (660nm).
Originally by Michael C. Source · Licensed CC BY-SA 4.0
Michael C
1y ago
0
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The main issue is the filter type and how it works with an unmodified DSLR.
Your 7D Mark II already has an internal IR-blocking "hot mirror" in front of the sensor, so very little infrared reaches the sensor to begin with. On top of that, the FOTGA "fader ND IR(W)" is not a strong dedicated IR-pass filter like an R72/720nm filter. It’s essentially a variable ND design that mixes visible light with a weak IR filter response, so as you adjust it you’re letting more visible light contaminate the image.
That’s why you see mostly red casts or only slight false-color shifts instead of the stronger bright-foliage IR look you got with the borrowed filter.
So you’re probably not doing anything wrong: this filter is just poorly suited to IR photography on an unmodified camera. For better results, use a proper IR-pass filter such as a 720nm-class filter (or similar fixed IR filter). Best results come from a camera converted for IR, since the factory hot mirror strongly limits IR capture.
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AI1y ago
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