The unseen light i'm referring to is the strange wave length of Infrared. Invisible to the human eye, but not to a camera with the right filter or a converted sensor with the IR prospective filter removed from the sensor. Also possible with Infrared film on a film camera. The photographic spectrum for IR is between 700nm and 900nm (nanometre).
I prefer the use of a digital SLR with an IR filter in front on the lens. This way the cameras sensor is unaffected and can still shoot 'normal' photographs with the visible light spectrum. However, there are several 'problems' to IR photograph using a non-converted digital SLR. The first is the filters density, it lets in no visible light, so it is impossible to look through the viewfinder. One must set up their composition before hand.
Another 'problem' is the length of the exposure when using a lower ISO (typically sub ISO 400 for detailed landscape work). Usually around 15-20 minutes at ISO 200 on a bright day stopped down to F/8-F/11. The last 'problem' is the focusing, IR light focuses at a different distance compared to visible light. The difference must be accounted for before taking the photograph.
Some of these 'problems' are not real Problems. The extremely long shutter speeds can yield wonderful results when working with moving clouds and artistically blurring foliage. The other two 'problems' are only annoyances.
- The easiest way to find the best exposure is to first test shoot with a very high ISO like ISO 3200 and shoot wide open, lets say F/4. keep adjusting until a perfect exposure has been found. From there it is simple Doubling and Halving to work back towards a low ISO and a an F/stop that will have enough depth of field for the scene.
Technical details: Canon 7D, Canon 10-22mm lens (@10mm), Intervalometer, Singh Ray I Ray (IR) Filter, ISO 250, F/8, 16 Minute Exposure.
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