Tuesday, 4 August 2015
For black-and-white infrared photography I wanted to get darker blue skies and whiter foliage than I could using my converted Nikons with 590 and 720 nm filters over the sensor. So I bought a cheap 850 nm filter to fit over the lens. The grand total of £19.99 paid to an eBay seller soon had a made-in-China Zomei 77mm 850 nm filter arriving by post.
The only disadvantage of this route to ‘deep’ infrared photographs (actually ‘near’ infrared to physicists) is the opacity of the filter. You have to work with a tripod, frame the shot and then screw the filter onto the lens before pressing the shutter. Since I will use it for non-moving subjects the addition of the filter is only a slight inconvenience.
First of all I checked what exposure I would need compared with the exposure metered by the camera without the filter. I soon found that with the filter in place my 590 nm-converted camera on aperture priority over-exposed by about 6 stops, as you would expect since the meter is working somewhere in the visible light range. Kolarivision suggest a light loss of about 1 stop with their 850 nm filter. Sure enough, when I made a series of test shots at 1/3rd stop intervals in sunlight, 1 stop over the metered exposure was the best. However, 1 stop is the starting point for adjustment up or down depending on the amount of infrared radiation relative to the amount of visible light coming from the scene.
I keep my 590 nm-converted camera at minus 2/3rd stop (-0.7) exposure compensation (slightly less than suggested by Kolarivision for their 590 nm conversions). For convenience when using the 850 nm filter I now turn compensation to 0, take the metered reading on aperture priority, switch to manual exposure and add 1 stop. I also focus manually before adding the filter.
The two photographs, from my usual test site of out of the bedroom window, below show black-and-white conversions in Lightroom from my 590 nm converted Nikon without and with the 850 filter.