I learned from a reader recently that the Sigma SD1 Merrill is trivially converted to infrared by removing the internal filter inside the mirror box—reversible too. The sensor in the SD1 Merrill is the same sensor as found in the Sigma DP Merrill compacts. See the recent blog piece: Pixel for Pixel, *Nothing* Beats a Sigma DP Merrill.
Sigma has graciously agreed to loan me an SD1 Merrill along with a lens—probably the excellent Sigma 35mm f/1.4 DG HSM. I look forward to seeing how the camera performs in infrared with its true color sensor using various filtration.
The bad news: the SD1 Merrill has no Live View feature (it is an older design than the DP Merrill compact cameras). Which means that infrared backfocus needs to be dealt with. But perhaps the AF system will focus properly in infrared with the filter over the lens, which would go a long way towards mitigating the issue. Still that’s a huge hassle with deeper infrared (800nm and deeper), as it means constant on/off of the filter (to focus vs to compose vs to shoot).
Removal of the blocking filter means that the camera can see in “full spectrum” also: visible light as well as infrared, and possibly ultraviolet as well when using the UV-transmitting Coastal Optics 60mm f/4 UV-VIS-IR APO Macro (modern lens coatings strongly block UV with most lenses). Of course, it also means that the lens requires a filter to achieve the desired spectral transmission (e.g., infrared pass or infrared block or UV pass).
At about $2299 the SD1 Merrill is not a modest purchase, but considering that a DSLR infrared conversion costs $350 to $500 to do right and is not easily reversible, the ability to convert and deconvert the SD1 Merrill has some merit. The one disappointment is that I won’t be able to use my Zeiss 25mm f/2.8 IR-Distagon, since the SD1M has a Sigma mount (IMO, Sigma ought to offer the SD1M in Nikon mount, which would make it far more versatile).