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Infra-Red With Digital cameras - by Chris Maddock

Infrared photography is not restricted to film users, but can be possible for many digital camera users. This article is intended to explain a little about how to take digital IR photos, what equipment is needed and how to process the images. I've split it into several sections for clarity;

Cameras and Lenses
Filters
Taking the images
Conventional Monochrome IR processing
False-colour IR Processing
Dealing with “Hotspots”

Cameras and Lenses

Not all digital cameras can take IR photos. To reduce IR blooming in images the sensor assembly often includes an infrared blocking filter, which rather restricts what can be done if you do want IR. The first thing to do if you want to try it is to find out if your camera can see IR with any degree of success. The easiest is to look on the Internet to see if anyone is already successfully doing IR with the same model. If you have no luck there then you can at least test to see if your camera can at least see any IR light. All you need is a TV remote control – in a darkened room point it at the camera (and the camera at it) and take a photo whilst holding down one of the buttons on the remote. If the resultant image shows a bright spot then the camera can see IR light. Unfortunately, that is all it will tell you. To test further you need an IR filter.

Lenses can also be a source of problems. Some lenses have coatings that will cause a bright “hotspot” in the middle of the image, which will render them unusable for IR work. In some cases the hotspot is not excessive and can be dealt with in Photoshop – I’ll cover how I deal with this on my Canon G5 at the end of the article. A partial list of Canon lenses that do and do not cause a hotspot can be found here

Filters

There are quite a number of these on the market, the principle difference being at what point in the light spectrum they block light. Sometimes (but not always) the filters have names based on the cut-off point, a higher number cuts off more visible light, if the name doesn’t relate to the cut-off point then a visit to the manufacturer’s specs will give the answer. Some filters are available as screw-in types in different sizes, others are gel type filters which tend to be cheaper but harder to handle and less durable.

Many people use an R72 or Wratten 89B filter, which cuts out light wavelengths under 720nm (nanometres), but these tend to be expensive – especially when experimenting. A 49mm Hoya R72 filter is around £24, the price can increase sharply for bigger sizes.
A cheaper alternative is the IR87 from the Lee technical filters range. This cuts off at 730nm and is a gel filter. A 100mm square will cost around £13 and can be cut to mount in a Cokin P-series frame or cut to size to make several for lenses that take rear mounted filters.

An even cheaper alternative if you can find it is Ilford’s SFX filter, which is not always available since Ilford only produce small amounts of the SFX film from time to time. This is intended for use with SFX200 InfraRed film. Although Ilford don’t state the cut-off point they do list the R72 and Wratten 89B as suitable alternatives. I reckon the cut-off point is lower than those since I can just about see through it (unlike the others) which means it is passing some visible light. This is my personal preference since I like to produce false-colour IR photos and some visible light is needed for these. The SFX filter is a gel filter in Cokin A and P sizes.

A word of warning about gel filters; whilst these can be mounted in Cokin type holders I found that the length of exposures needed (anything up to 30 seconds) can allow sufficient visible light to leak behind the filter and reflect off the filter and any dust particles.
My solution is to sandwich a layer of the gel between two screwin skylight filters. This stops the light leakage and protects the filter.

Taking the Images

Because of the IR blocking filter, most digital cameras have a much lower sensitivity to IR light than they do to visible light. The result of this is that exposures are long – depending on the camera, subject, aperture and ISO setting used they can be anything from 5-30 seconds – using a tripod is strongly recommended, as is bracketing.

Correct white balance is important. The IR filter will add a cast to the images, ranging from a strong magenta to a strong red depending on the camera and filter used, so it’s strongly recommended to set the camera to a custom white balance. This is usually done by taking a shot of a sheet of white paper (with the IR filter fitted) then setting the camera to calculate and use the white balance from that – check your camera manual for the exact method. If you shoot RAW, then this can be dispensed with and the white balance chosen when processing the images.

Framing the shot can be very difficult, especially when using an R72 or Lee 87 filter, as there is hardly any visible light transmitted. If you use screw-in filters then you can set the framing up before fitting the filter. Drop-in behind-the-lens filters are a real pain but pre-framing can be done – but don’t forget to put the camera body-cap on whilst fitting the filter, to prevent dust getting into the mirror box whilst you’re doing it. The simplest solution I’ve found is not to use an SLR – my Canon G5 can rather *********** well through my preferred SFX filter, so I use that and view it on the preview screen. As a committed SLR user I knew I’d find a use for that screen eventually ;-)

Focussing is the final hurdle to overcome. IR light focuses at a different point to visible light and most modern lenses don’t have an IR depth of field scale – if they have a DOF scale at all. Older lenses that had a proper DOF scale often had an additional line for IR focussing. You can focus manually using visible light then back it off a little although how much requires practise. A reasonably small aperture (f8 or so) will help to correct any inaccuracy. If you have an autofocus camera (thinks, are there any non-AF digitals?) then your luck may be in. Provided that you don’t use too dark a filter the camera may be able to autofocus successfully, and that will take into account the required shift for IR focussing. It’s worth a try, even my Canon D30 (not the best of AF systems in the world) can focus most of the time with a Lee IR87 filter.

OK, we now have some images to work with, so let’s get processing.

Conventional Monochrome IR processing

We’ll start off processing an image for the traditional infrared black & white look. I shot this one in RAW so didn’t worry about the white balance. Here is how it looks when opened in RawShooter, apart from the amount of foliage it’s very Martian.

After selecting a grey area of the clouds to set the whitebalance, things look far better, so I convert it and open in Photoshop.

A quick Desaturate and Curves adjustment to bring up the contrast gives me the finished image, ready for framing.

False-colour IR Processing

An interesting variation on infrared is to use some colour but partially reverse the colours by swapping the red and blue channels. This can produce striking images but doesn’t work for all scenes.

This is another shot which I took in RAW, so once again we have a strong red cast when opened in RawShooter.

Again, I select the whitebalance from a grey area of cloud and convert it before opening in Photoshop.

To perform the Channel Swap go to Image->Adjustments->Channel Mixer.

With the Red Output Channel selected (default) change the Source Channels – Red to 0% and Blue to 100%. Leave Green alone.

Now select the Blue Output Channel and change the Source Channels – Red to 100% and Blue to 0%. Again, don’t make any change to the Green channel.

Once I’ve done this, the image looks like this;

A quick application of Auto Contrast and Auto Color and this one’s ready to be framed as well.

I’ve automated the Channel Swap using a Photoshop Action which you can get here

Dealing with “Hotspots”

As I said in the Cameras & Lenses section, the coatings on some lenses can cause a bright hotspot in the middle of the images. I found that my G5 does give a hotspot but that it’s not too excessive, and most of the effect can be edited out. If you look closely at the false colour image you can see a patch in the middle of the image that is slightly brighter and slightly yellowy, this is the hotspot.
It can’t be removed completely, but it can be reduced – this is the method I use for my G5 images.

Once the image has been converted and opened in Photoshop I make an oval selection somewhat larger than the hotspot, and feather it by 200 pixels (Select->Feather)

I then open Image->Adjustments->Hue/Saturation and tweak the settings. Somewhere around Saturation –40 and Lightness-10 usually does the trick

Now remove the selection (Select->Deselect) and perform the Channel Swap as above. Once again, a quick application of Auto Contrast and Auto Color finishes it off.


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All text and photos are copyright © Chris Maddock, 2007