[alexbock]

I enjoy monochrome infrared photography at wavelengths longer than 1000 nm using a Sigma sd Quattro (no Bayer filter and the sensor IR filter is reversibly user-removable). To reduce exposure times as much as possible, I've been testing an AR-coated near infrared achromatic doublet (AC254-050-B-ML) with a 3D-printed housing to attach it to the camera mount with a manual focusing barrel. The next step is to design a fast double gauss lens that can similarly be assembled from readily available NIR-coated stock lens elements.

[buildbot]

I've wondered about doing this a lot! How is the image quality with the doublet? I love the Quattro series of cameras but hate the software personally. I may someday pick up a sd Quattro H...

[alexbock]

The center of the image it produces is perfectly sharp, but using a single achromatic doublet as a photographic lens at f/2 necessarily produces significant eighteenth-century softness as you move out to the edges. This image [1] taken to demonstrate that red wine (left) is clear and nearly as transparent as water (right) in the near infrared provides a decent example of how the images it produces wide open soften outside the center much more quickly than modern commercial camera lenses which typically don't lose sharpness at maximum aperture until the corners.

However, the sd Quattro is sensitive enough from 1000 nm - 1100 nm that I can take handheld shots outdoors on a sunny day while stopped down to f/5.6, and the smaller aperture gives more consistent sharpness across the frame. It also only takes a few seconds exposure on tripod to capture astrophotography of red giant stars that emit significant infrared like Betelgeuse.

Incidentally, the original reason I wanted an infrared-sensitive camera and a 1000 nm long pass filter was to photograph stars in the sky during the middle of the day, taking advantage of the quartic dependence on wavelength in Rayleigh scattering to remove the overpowering brightness of the sky.

[1] https://alexbock.github.io/blog/nir-examples/near-infrared-8... (note: this image used an 850 nm long pass filter rather than 1000 nm but was taken with the same doublet described before)