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Tree at Hatchet Pond - IR

I’ve posted briefly about infrared before, and also mentioned that the Canon 20D isn’t very suitable for it. This is true because the blocking filter (or “hot mirror”) in front of the 20D’s sensor blocks just a little too much infrared light. Normally this is a good thing – digital camera sensors are sensitive to infrared, much more so than to visible light, and so without filtering the visible image would be swamped by the infrared signal – but it seems on their DSLRs Canon have used a hot mirror which strongly cuts off any light with a wavelength longer than ~700nm. So when using an R72 (or similar) filter which cuts off almost all light with a wavelength shorter than 720nm and transmits only the longer wavelengths there is very little overlap, and so the net result is a camera that is not very sensitive at all.
For example, with Efke IR820 film and an R72 filter, on a sunny day at f/11 I can get good results from a ¼ second exposure – an effective ISO rating of about 2. With the 20D in those conditions I might need 5 minutes or more, a difference of at least 10 stops and so far beyond the bottom of the ISO scale it’s not possible to quote an equivalent.
So we’re stuck with long exposures, even wide open we’re typically going to need 15 seconds or more, and that’s trickier because of the slight focus shift when shooting in infrared, and so it forces us to take a certain kind of photograph, with blurred movement and quite a lot of noise. However, with the right subject and composition these can be beautiful, striking images, as the above example hopefully shows. One noticeable advantage over the IR film I have is that this is a colour image, but it’s best referred to as a false colour image because the colour comes from the processing of the image.
Unadjusted image taken with R72 filter
The R72 filter is so often referred to as “very dark red” – it’s almost entirely opaque to the human eye – so what you really get in-camera is an image in shades of red. But notice that the foliage in the scene is very bright, approaching white and that the sky is very dark. This is the key to the otherworldy look of IR photography – unlike most visible wavelengths, infrared light is not absorbed by the chlorophyll in plant foliage, so it is strongly reflected. Similarly whereas the sky is normally blue because blue light is scattered strongly by the atmosphere, it scatters very little infrared, and so appears very dark in infrared wavelengths.
Mono conversion
By doing a simple grey-scale conversion of an image like this we can get a “traditional” infrared shot – because it consists pretty much of just the one channel, the method of conversion (desaturate, channel mixer, etc.) doesn’t make very much difference to the output. We have a dark sky, black water, bright ghostly plants. So how do we get the blue skies of the first image in this post? There are two elements; white balance and channel flipping.

By setting the white balance of the shot to a very low colour temperature the camera is effectively boosting the gain on the blue and green pixels, so that the limited light levels that make it through the R72 filter and the hot mirror, and through the colour filters on each photo site are given a bit more amplification. Instead of shades of red we can get purple shades in the landscape with muddy yellow skies. The best way to achieve this is to fill the frame with foliage and take a shot, and use this to calibrate the camera’s white balance, or to use foliage to select the white point when setting the white balance in post-processing.
Custom WB set in DPP
A word of caution though, I’ve found a problem when processing images using Adobe Camera Raw (ACR) when I’ve set a white balance in this way. ACR has a minimum colour temperature of 2000K and it will use this to override any lower setting, even if it’s in the original RAW file. Opening files that come out of the camera with blue/brown tones, ACR will adjust the setting to its own minimum and turn the image a range of purples and orange. The solution is to use the software from the camera manufacturer. Canon provides Digital Photo Professional for free with its DSLRs and it is a very capable piece of software, although the user interface and workflow leave a lot to be desired compared to the relative richness of something like Adobe Camera Raw.
Infrared shot with Adobe Camera Raw white balance
It has a higher minimum colour temperature, but unlike ACR doesn’t override the RAW file settings. The difference can be seen here with the DPP result above-left and the output from ACR to the right. You can see that the DPP output has a much wider range of colours than the more limited pallette in the ACR processed image. But still, we’re not done, while interesting, the yellow sky is not very attractive, so to get a more “natural” looking blue sky as a background I swap the red and blue channels of the image.

The easy way to do this in Photoshop is via the channel mixer; for the Red channel simply slide the Red slider from 100% to 0% and the Blue slider from 0% to 100% and conversely, for the Blue channel slide the Blue slider from 100% to 0% and the Red slider from 0% to 100%. Similar tools exist in most photo applications, or you might have to resort to pasting from channel to channel or the equivalent.
Image after swapping Red/blue channels
The result can be seen to the left – a slightly odd blue sky and a range of orangey pinks. Much more pleasing to the eye than the unflipped version. Since this is the way I process virtually all my IR shots with the 20D I saved a preset in the channel mixer to do this in one go.
After this we can pretty much proceed as normal in post processing. Because of the long exposures we need to be conscious of noise. I tend not to shoot IR at above a setting of ISO 200 on the camera because the noise becomes too much. There are also the usual problems with sensor dust, but also the additional issue of hot pixels, or even cosmic ray detections on the sensor.

Another problem I have occasionally is light from the viewfinder leaking around the mirror. This seems to happen only when the camera is in a vertical orientation, and when the sun is behind the camera. I can only assume that when vertical, the mirror sags slightly allowing a small amount of light to leak through. Because it’s a long exposure, a small amount is all that’s needed to show up quite strongly, especially since light getting in that way is unfiltered. You can see the effect in the tree image at the top, an irregular orange band on the right of the shot. In some cases I manage to clone or smooth it out, in others I just crop.

The only thing left for this shot is to do some levels and curves adjustments to boost the contrast a little and brighten it up, and we have a final image:

Image after curves adjustment

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