Thursday, April 12, 2007

IR filtration

The two-layer color negative film sandwich is too strong of a filter for the IR modified Canon S300. It produces a virtually monochrome image, as seen below.

What I like and want is a duo-tone response, with red coming from a little visible red and the nearest of the near-IR, and blue coming from deeper in the IR--at least 800 to 900nm. Vegetation renders pastel blue and the sky and deep red. Most neutral items are a touch reddish, but not too much. Most black fabrics are pastel blue.

I can get it by removing one layer of the sandwich.

I believe the difference is greater when the sun is out and/or when tungsten illumination is being used.

P.S. For kicks here is the latter image with a boosted green channel, extra saturation and contrast: Cloudy Bartlett Quad, IR


Ben Y said...

That's neat looking. Why is the viewable area circular?

Dean W. Armstrong said...

The camera is extremely near-sighted without a highly concave lens in front of it, because I took out the IR-block filter which lies in the converging light path of the lens. This changes the focus dramatically. The normal focus for this camera is now just an inch away from the lens. You can replace the filter with a piece of glass with the same thickness, but I don't have any pieces small enough.

My particular highly concave lens is a 2x teleconverter from a film camera, and it doesn't completely cover the field of view of the digital camera, giving you the vignetting.