Friday, 7 June 2013

Infrared Photography

Acacia - Canon 40D, Canon 17-55, 61sec, f/5.6, ISO 1600, IR filter


Infrared photography has been around for a long time, and is always good for unusual and sometimes disturbing images. Being something of a geek, I have been drawn to it for quite a while, and now I've finally gotten around to writing about it.


Technical Stuff


Infrared light is merely ordinary light whose wavelength is a bit longer (i.e. redder) than the human eye can see. Light visible to the average human eye ranges in wavelength from 390nm to 700nm, whereas the pictures on this post are made from 760nm light. Don't confuse this with thermal imaging, which uses much longer wavelengths.

Certain films and digital camera sensors are sensitive to infrared light. You can try this by pointing a compact camera at a typical TV remote and pushing a button on the remote. On your camera's screen you should see an infrared LED light up at the front of the remote, but your naked eye won't see it.

760 nm infrared filter

To take pictures exclusively in the infrared, you need to mount an infrared filter (like the one pictured) onto your lens. This is where the challenges start. For one, your viewfinder will be pitch black; after all, the point of this filter is to block out all light except for the infrared. This means that you should mount your camera on a tripod and compose your image before screwing on the filter. Be sure not to change the zoom on your lens while you're mounting the filter (and the lens hood, which is a good idea here).

Notre Dame Cathedral 1 - Canon 40D, Canon 17-55, 30sec, f/5.6, ISO 250, IR filter


Once you've composed the image and mounted the IR filter, the next challenge is focus. The infrared image typically focuses at a different distance than the visible light image, so you will need to refocus once the IR filter is in place. Some lenses have a little red mark on the focus scale indicating where to set the focus manually for an infrared image, but most modern autofocus lenses don't seem to have these any more. The good news is that many SLRs (such as my old Canon 40D) can autofocus on an infrared image - at least as long as the camera hasn't warmed up too much in the sun. That's what I use, and it's very accurate when it gets a lock. Sometimes you need to play with the AF point a little.

Notre Dame Cathedral 2 - Canon 40D, Canon 17-55, 120sec, f/4, ISO 200, IR filter


The reason for this is that different wavelengths of light are refracted differently by the lens elements. This causes the problem of chromatic aberrations (which led Newton to discover the colour spectrum - he was wondering why he got funny colour fringes with his telescope!), and one of the reasons modern lenses are so complicated is that one must combine several lens elements made of different types of glass to correct for this effect. But the correction is typically only done for visible light and the image produced by infrared light is usually quite a bit off the visible light image. For this reason, infrared images focus at a different distance setting.

Notre Dame Cathedral 3 - Canon 40D, Canon 17-55, 74sec, f/4, ISO 100, IR filter


For this reason, also, most cameras have a filter placed in front of the sensor that cuts out infrared light (perversely, this is also called an infrared filter, even though it has the exact opposite effect as the IR filter you screw onto your lens). This is to prevent the uncorrected (and thus out-of-focus and differently sized) infrared image from interfering with the corrected visible light image on the sensor. A defect in this filter embarrassed Leica with their M8 digital rangefinder camera, and the rich people using this thing had to shoot with a special infrared-blocking filter mounted on the lens. The problem was corrected in the Leica M8.2.

Chicago Bean with Ghosts - Canon 40D, Canon 17-55, 30sec, f/2.8, ISO 100, IR filter


So now you see another challenge: the filter on the camera's digital sensor is deliberately blocking out infrared light, so how can we take infrared photographs? One solution is to modify the camera by removing this filter. Some crazy people do this, but the camera then becomes quasi-useless for visible light photography, so you're now the proud owner of an infrared-only camera.

Baha'i Temple - Canon 40D, Tokina 11-16, 108sec, f/4, ISO 100, IR filter


The other solution (and whether this is feasible depends on the camera) is to just make very long exposures. Some infrared light does make it through the blocking filter, and as long as this is substantially more than the visible light that manages to get through the IR filter on the lens, a long enough exposure will produce a decent infrared image.With my Canon 40D this works quite well, though the exposure times are typically very long: e.g 30secs at f/4 and ISO 400 on a sunny day.

The Eye of Sauron - Canon 40D, Tokina 11-16, 78sec, f/5.6, ISO 100, IR filter


The resulting image looks pink in the camera, since the red pixels on the sensor are more sensitive to infrared light than are the green or blue pixels. This is due to a combination of the transmissivity of the different color filters in front of the red, green and blue pixels, as well as the relative gain ("boosting") of the three color channels.
 


Quiver Trees and Clouds - Canon 40D, Canon 17-55, 10sec, f/4, ISO 800, IR filter

But is it art?


Okay, so now we have a pink image of an infrared scene. Next, one will want to convert this to black and white, and perhaps add a little toning to taste. But what makes infrared images different from normal black and white photography? Well, things look a little different in the infrared. The sky, for one, is typically much darker in the infrared than in visible light, thus bringing out the clouds better, just like using a red filter in normal BW photography. Green foliage, on the other hand, is highly reflective in the infrared, so that trees and grass look almost white and ghostlike - such as in the picture of the acacia tree in the Namib at top of this post.

Young & Tall - Canon 40D, Canon 17-55, 10sec, f/4, ISO 800, IR filter


Another effect is simply the length of the exposure blurring out motion. This can give the clouds a vague look, blur people into ghosts, but also messes up trees swaying in the wind.

The overall aesthetic of an infrared image is different, and if you know what you want, you can use this to good effect. Add a little alien split toning and the image can become rather disconcerting. As with all photographic techniques, it's fun to try out different stuff, but it requires a lot of thought, practise and artistic vision to turn a gimmick into art.

Alien Planet - Canon 40D, Canon 17-55, 10sec, f/4, ISO 800, IR filter