This is an unashamedly technical post. For those of you turned off by nanometres and transmission filters, other blogs are available.
A couple of years ago I took the plunge and invested in an infra red converted camera. Since then I’ve learnt to understand the best conditions and subject to apply infra red to, and I’ve experimented with post processing. I had my Nikon D300 converted to record infra red images in 2013. I love the effect, particularly when post processed into black and white images. This post is about the basics and is based on a presentation I recently gave to my local camera club.
The nanometre bit
Infra red light is invisible to the naked eye and has wavelengths starting at around 590nm and stretching on to 1000nm and beyond.
Most digital camera sensors are so sensitive to ultra violet and infra red light that a special filter is placed in front of them to cut this light out. Converting a camera to take infra red photographs is simply a case of replacing this filter with one that blocks visible light and transmits infra red. That’s what I had done to my D300. It gets a little more complicated because there are different filters available to allow different wavelengths of light to pass through (in the same way that coloured filters allow different wavelengths of visible light through). My camera has a 720nm filter, (which blocks light of wavelength less than 720nm). Sensors to pick up heat energy are a completely different beast and are not dealt with here.
As a converted DSLR camera doesn’t need a transmission filer on the lens, you can compose and focus as normal. The image in the optical viewfinder remains bright and in visible light. To see the effect of the internal filter you will need to use live view. If you are using an unconverted camera with a transmission filter, you will need to compose and focus with the filter removed as by it’s very definition, the filter will block out visible light.
My D300 was calibrated for focusing and exposure by the company that converted it (Protech repairs). I still find that when faced with different subjects, I need to adjust the exposure from the indicated values and a degree of trial and error is sometimes required. You’ll always find me reviewing the image immediately after taking it.
The sun emits as much infra red light as it does visible light and so it is possible, with a converted camera, to use exposure times similar to normal. The classic infra red effect – white vegetation and dark skies – happens because green leaves reflect a lot of infra red light but blue skies do not. Scientists use infra red photography to spot growth and dead vegetation in the landscape. Contrast can be high in these photographs and you have to keep this in mind when taking the shot. Water also absorbs infra red.
Infra red light penetrates skin slightly and this results in a a soft, blemish free appearance in portraits. Eyes tend to appear black. The longer wavelength of infra red light is less affected by haze and pollution and so landscape photographs appear clearer and crisper.
Flare can be more of a problem as most lenses are designed to be used with visible light. The lens coatings and internal coatings that reduce reflections aren’t as effective with the longer wavelengths. Some lenses suffer from ‘hotspots’, a bright central portion which varies (and may disappear altogether) with a change in aperture. Of the collection of lenses I’ve gathered over the years, about half exhibit a hotspot with the D300.
Lenses that work with 720nm Infra red and a D300 camera:
- Nikkor 60mm macro
- Sigma 10-20mm D f/4-5.6
- Nikkor 50mm f/1.8
- Nikkor 85mm f/1.8 (manual focus)
- Nikkor 24mm f/2.8D
- Nikkor 70-300mm AFS f/4.5-5.6
- Tamron 90mm macro
- Tamron 18-270mm
- Vivitar 19mm (manual focus)
- Sigma 170-500mm
below are a set of photos I took this morning. I’ve been experimenting with additional filters progressively the shorter wavelengths. This is very much a work in progress.