You know, digital camera image sensors are really very interesting – unlike film, digital sensors actually capture a wide spectrum of light.
This means that your digital camera actually captures light well beyond the spectrum of human vision, including light on both extremes of the spectrum, namely infrared and ultraviolet as well.
However, being able to capture ultraviolet and infrared isn't actually desirable when you're shooting a regular photo – images tend to look either hazy or have weird colour casts if the camera sensor is allowed to capture these extremes in the spectrum.
As such, every camera manufacturer applies a two-stage process to reduce ultraviolet and infrared – ultraviolet is cut out by applying a coating on the elements inside the lens, while infrared is cut out by using a special filter that's attached right in front of the image sensor itself.
This infrared filter is usually integrated with the low-pass filter that is used to slightly blur the image to help false colour banding (moire) in your images.
Just in case anybody asks me, cameras which advertise "no low-pass filter" still have an infrared filter in front – it's just that instead of a two-fold effect of also having the blur filter and infrared, the filter in front of the sensor only has the infrared cancelling effect.
Anyway, the point of this whole discussion on physics is that despite the infrared filter on the front, a modern digital camera can actually still capture a tiny amount of infrared light, and this is where it gets quite interesting.
Infrared photos produce really otherworldy-looking images – trees have white foliage, while water comes out very dark. It really looks very interesting and good fun to try out.
SONY IR CAM: This shot was taken with an old Sony digital camera that had a special infrared mode. Turning on the IR mode on the camera would get the camera to move its infrared filter out of the way and allow for fast handholdable shutter speeds when shooting infrared shots.
How to shoot IR shotsThere are actually a couple of ways that you can shoot infrared images with a digital camera, but the easiest way is to buy an infrared filter that blocks out all light except for infrared light.
You've probably seen some of these shots online before and believe it or not, it's actually quite easy to do.
All you need is an infrared filter and there are quite a number of manufacturers that make such a filter – Japanese filter maker Hoya makes one called the R72 filter while French company Cokin makes one called the 007 filter.
Now, before you run out and buy one, remember I mentioned that camera manufacturers apply an infrared blocking filter on the front of the image sensor?
Because of this, with the Hoya R72 or Cokin 007 attached to the front of your lens, there's VERY little light coming in to the camera so your shutter speeds are going to be very, very slow – so slow, in fact, that you probably don't want to be shooting handheld, so a tripod is essential.
There are a few tips I can suggest when shooting infrared – while you can (and should) be doing your tweaking in Photoshop, there are a couple of things you need to be aware of when shooting with an infrared filter.
IR MARK: Some older lenses have a red marking to indicate focus for infrared. This 50mm however only has a small white dot to indicate the infrared focal point.
For one, infrared light doesn't focus at the same point as visible light – some lenses actually have a small (sometimes coloured red) marking on the lens scale window to give you an indication where to adjust focus to get the sharpest result.
Another tip is that you can set a custom white balance on the camera – many cameras allow you to shoot a white or grey card to get a custom white balance setting, but with an infrared filter on, you should instead point it at a patch of green grass to get your custom white balance setting.
Post-processingDon't expect your shots to look fantastic straight out of the camera – in fact, they're more likely to look like dull, reddish-toned photos, or if you've taken a custom white balance from a patch of grass, a little brownish.
If you want to keep your photos in colour, then once you have your shots, you should do some tweaking in Photoshop and the most common tweak is to do swap colour channels.
You do this by opening the photo in Photoshop, then choosing Image>Adjustments>Channel Mixer.
Choose the Red channel and drag red slider to 0% and the Blue slider all the way to 100%.
Next, switch to the Blue channel and drag the red slider to 100% and the Blue slider to 0%.
The main purpose of doing this is to get a slightly more natural look by turning the reddish skies blue.
However, if you're going to convert your infrared shots into black-and-white images, you can skip the channel swap adjustments and simply choose to desaturate your images in Photoshop.
Either way, you will also need to do some basic tweaking in Photoshop with a Levels adjustment to make the photos pop a bit more by adding some contrast in your shot.
COLOUR IR: This is a full-colour infrared shot of a road in Putrajaya was taken with a Hoya R72 filter with a custom white balance which I measured from a patch of green grass.
ConversionIf you are really interested in taking up infrared photography seriously and have an older DSLR lying around that you don't use, you might want to consider converting your DSLR into a dedicated infrared camera.
This involves actually removing the infrared filter on the front of your image sensor and replacing it with a simple clear filter.
Obviously, this isn't something you want to do yourself, but there are a number of companies online that will do the conversion for you if you send them your camera.
Unfortunately, it's not cheap and once converted, you can't use the camera to shoot regular photos.
Nevertheless, if you're interested, check out www.lifepixel.com which has a conversion service as well as a number of tutorials on how to shoot infrared.
(Photo Kit is a fortnightly online column in which Tan Kit Hoong (tankh@thestar.com.my) shares his thoughts on the art and technology behind cameras at thestar.com.my/tech.)
Source:
Photo Kit: Fun with infrared filters
An image of Saturn taken by the Cassini spacecraft with an infrared filter. 
