By Peter Cox
INFRARED photography is a way of capturing scenes in a completely different light – literally. Objects react differently to infrared light than visible light, resulting in an otherworldy view on the world. This technique has been around for almost as long as photography itself, but has become far more accessible with the advent of digital cameras.
What is Infrared Light?
Light is just a form of electromagnetic radiation. All this radiation exists in the electromagnetic spectrum. Our eyes are sensitive to wavelengths from around 400nm (which we see as the colour violet) to 700nm (which we see as a deep red). Between those two extremes are all the colours of the rainbow. Outside that band are other types of radiation, including x-rays, microwaves and radio waves.
Infrared light exists just beyond the visible red part of the spectrum, and eventually manifests as heat. Thermal imaging systems use that deep part of the band, but the area we’re interested in is called ‘near infrared’ and exists from 700nm to about 1100nm.
I’ll explain how it’s captured later in the article, but for now I want to explore the creative possibilities.
The Wood Effect
The Wood Effect is named after physicist Robert W. Wood who, in 1910, was the first to make infrared photographs and note the effect this light has, particularly on foliage.
You can see the effect in the image above. The leaves and grass seem to glow. This is because chlorophyll (the pigment that gives most plants their green colour) is transparent to infrared light, allowing it to be reflected back much more strongly than visible light. It’s a common myth that the bright glow is a result of some internal fluorescence within the plant – this is not the case. While there is a tiny amount of fluorescence from the chlorophyll itself, it’s not enough to be visible.
This reflectance is why infrared photographs are often mistaken to be of snowy scenes – the landscape appears as though it’s had a fresh dusting of snow. This is quite understandable. The way the plant cells reflect infrared is actually very similar to the way newly-fallen snow reflects visible light.
Other than the way foliage is recorded, there are a number of other ways in which infrared photographs differ. Blue skies become very dark, but clouds appear very bright. The skies are dark because the scattering of light caused by the Earth’s atmosphere (which results in the skies being blue) is reduced in the infrared. Clouds are bright because they reflect infrared light very efficiently.
You can see this effect in the image above. Note also the difference between the lit part of the valley and the shaded part – the difference is far greater than it would be with visible light.
Another effect is atmospheric haze. This is caused by small particles in the atmosphere which scatter light and reduce visibility. Infrared light is less prone to this scattering and so ‘cuts through’ haze quite dramatically.
While not a particularly creative effect, infrared images tend to have a much narrower dynamic range than usual. In virtually all images, it’s possible to capture good, clean shadow detail and not overexpose the highlights. This allows you to photograph a scene that would require strong filtration in visible light, or multiple exposures blended in HDR.
As a result of this characteristic, all but the highest contrast infrared images need a substantial ‘levels’ adjustment in postprocessing to show their full potential.
So How is it Captured?
All digital cameras are capable of recording both near ultraviolet and infrared light. In order to protect your normal, visible light images from these wavelengths, the camera manufacturers place what’s called a ‘hot mirror’ in front of the sensor. This reflects everything but visible light – resulting in photographs that look as you expect them to. The key to infrared photography is to block visible light from reaching the sensor, permitting only infrared light to pass.
In the days of film, you’d place an infrared filter in front of your lens and use film that’s sensitive to infrared light (the filter is needed as well because the film is sensitive to both visible and infrared light). With digital, you can still place your filter in front of the lens, but the hot mirror will block most of the infrared light from reaching the sensor.
It won’t block all of it, however, so it’s still possible to make an infrared image this way – it’ll just involve a very long exposure time. Typically shutter speeds will be in the one- to two-minute range in broad daylight. Also, hot mirrors are getting more efficient at blocking infrared light as technology improves. More recent digital cameras need longer exposure times than the ones released a few years ago.
This method is clearly inferior to using infrared film, which permits normal shutter speeds. It also has the disadvantage of not permitting you to compose through the filter – because only infrared light gets through, you won’t be able to see through it. Nor will the camera be able to autofocus, and autoexposure won’t work either. You must compose and focus with the filter off, then put it in place and make the exposure.
Converting the Camera
So is there a better way? There is – it involves replacing the hot mirror in front of the sensor with a so-called ‘cold mirror’, which permits only infrared light to pass. This allows you to use normal shutter speeds, eliminates the need for a filter in front of the lens, and permits autofocus and autoexposure.
The downside to this process is that it is basically a permanent conversion of the camera. The procedure is not trivial and is best performed by trained personnel in a clean room environment. There are several companies who provide the service, and the cost will be in the range of a few hundred euro. Most of this is labour – the cold mirror itself is relatively inexpensive.
That being said, it’s by far the best way to make infrared photographs. The lack of a filter in front of the lens gives considerable freedom and allows spontaneity in photographs which is just not possible otherwise.
Many people choose to convert their old cameras when upgrading to a newer model. The resale value of a digital camera after a few years of use is a fraction of the purchase price, and getting it converted opens up a whole new world to the photographer.
Unfortunately, there’s no such thing as a free lunch. There are a few problems and pitfalls to be aware of, whether you convert the camera or not.
The major problem is focus. All wavelengths of light focus in different positions, and lens manufacturers have to compensate for that in their lens design. Typically, this is done for visible light only, meaning that infrared light will not be in focus when the scene is properly focused visually.
Most older lenses (and some newer ones) have a red dot on the focus ring which tells you where to turn it for infrared photography. If using an unconverted camera with a filter in front of the lens, you must make this manual focus adjustment yourself after putting the filter on. With a little practice you will be able to get decently sharp images this way.
If you get the camera converted, most companies will tweak the autofocus system to focus infrared correctly. Generally this is benchmarked to a specific focal length (usually 50mm, unless you specify otherwise) and may vary in accuracy as you use longer or shorter lenses. This makes life a little easier, but care still must be taken if you want critically sharp images.
Thankfully, the live view function of modern cameras makes this point almost moot. With live view enabled, you are seeing the scene as recorded by the sensor, so you in fact are ‘seeing’ infrared. This means you can magnify the view and focus manually with a high degree of accuracy.
Another problem is a centralized ‘hot spot’ in the lens. This is a circular spot that is visibly brighter than the surrounding area. Most lenses are designed with only visible light in mind, and their coatings often don’t work for infrared. The hot spot is one result of this. Almost all lenses will show hot spotting to one degree or another, but some are worse than others. There are many resources on the Internet that provide guidance in this matter, but be aware that in actual use the problem is often not noticeable.
One thing that exacerbates hot spotting is using a smaller lens aperture (higher f-number). Using a wider aperture will therefore reduce the effect.
Diffraction is the effect present in all lenses that reduces resolution at very small apertures. Due to the longer wavelengths of infrared light, this problem is worse in infrared photography, meaning that in order to preserve resolution you will need to use a wider aperture than normal for optimal sharpness if depth-of-field isn’t a concern. Obviously, if it’s needed, you’ll use whatever aperture is necessary to get front-to-back sharpness.
As stated in the ‘hot spot’ section above, lens coatings are generally optimized for visible light only, and that includes the anti-flare coatings. As a result, infrared photographs are very susceptible to flare. It’s always advisable to use a lens hood in photography, but it’s vital in infrared. Also, be aware that lens hoods rarely offer full protection and you should be vigilant any time the Sun is shining on the lens and shade it with your hand, or a hat.
This is less of a problem in digital than it is in film (as we can check the result on the spot with digital), but the camera’s light meter will still be working off visible light, which may be stronger or weaker than the available infrared light. This will result in inaccurate exposures in the automatic exposure modes. Often exposure compensation is not enough to counter the problem, so it’s best to shoot in manual more often than not.
Types of Filter
So far, I’ve referred to a generic ‘infrared filter’, but there is more than one type available. The most popular has it’s maximum effect at around 715-720nm. This permits a small amount of visible light to leak through, thus exposing the three colour channels in your camera differently, giving a ‘false colour’ image. The photograph at the top of this article is such an image.
The next most common type is a so-called ‘deep infrared’ filter, which cuts off at around 830nm. This one lets virtually no visible light through and will expose the colour channels evenly, giving a pure monochrome image. It also gives a stronger ‘Wood Effect’ than the other filters. An additional benefit is a boost to your camera’s effective resolution due to the uniform exposure of the colour channels.
There are other variations on the market as well, but those two would be the most popular. My personal preference is for the deep filter, as I find the false colour effect doesn’t work for most images, and can appear ‘gimmicky’ if overused. Most images work much better in black & white. That being said, you can always convert a false colour image to monochrome, but you can’t do the reverse.
Infrared is an exciting area of photography. It makes you see the world in a different way, and opens up many possibilities that wouldn’t work in visible light. The infrared effect is generally strongest in the middle of the day, a time which landscape photographers typically avoid. This means your shooting hours are extended – visible light in the mornings and evenings, and infrared during the day.
Want to learn more? Why not contact Peter about one of his workshops at peter[at]petercox.ie?