Infrared photography


In infrared photography special filters are used which block visible light (and ultraviolet light), and only infrared light is recorded. It's really intriguing to be able to capture stuff that we can't see with our own eyes. Especially foliage and grass shine really bright, giving an almost wintery look to the pictures, even when taken in midsummer. Unfortunately my infrared camera broke down after a couple of years, but maybe I'll make a new one some day.


Below are some examples which show the difference between infrared photos and regular photos. I've also included the channel swapped versions, because I think it looks even better like that (the process of channel swapping is explained at the bottom of this page).


Type of light


Whereas harsh midday sunlight is often said to be a bad thing for convential color landscape photography, but this is far from true for infrared photography. Overcast light sometimes gives quite dull results where harsh sunny light gives a nice contrast to the photo. The difference can be seen in these two examples below. In both cases there was either shadow or sunlight on the land parts.




The sensors of digital cameras are sensitive to both normal light and infrared light, but we usually want to take pictures of what we see, and we don't see any infrared light. Therefore, cameras are equipped with so-called hot mirrors, which is a piece of glass in front of the sensor that blocks infrared light (as well as ultraviolet).


Cameras without a hot mirror are called full spectrum cameras and, because the infrared light is not blocked, pictures get a strange pinkish hue, like in the example on the left. This is not an actual picture taken with a full spectrum camera, but an approximation that I made in Photoshop by combining a normal picture with an infrared picture turned to pink.


If you want to take infrared pictures with a digital camera, there are two options:

• You use your camera as such and put an external infrared filter on your lens, which blocks visible light but is transparent for infrared light. The advantage of this system is that you can use a regular camera, but the disadvantage is that you will need very long shutter speeds (tens of seconds, or maybe even more) because of the hot mirror in your camera. Having an external filter on your lens which blocks visible light, combined with a hot mirror on your sensor which blocks infrared light, means that very little light reaches the sensor, which is the reason why you need really long shutter speeds.

• You convert your camera to a dedicated infrared camera by replacing the hot mirror with a so-called cold mirror, which blocks visible light, but lets infrared light through. The disadvantage is that the camera can only take infrared pictures, but it has several advantages. The camera will take pictures with normal exposure times, you don't need any external filter, and you can freely exchange lenses. A bit of care should be taken with replacing the hot mirror though, because it might be that it also acts as the anti-aliasing filter (read more about anti-aliasing filters here).


In the first case, you first need to see if your camera is suitable for infrared by performing a very simple test. Just take a remote control, press any button, and take a picture while pressing the butting (and aiming the remote control at your camera of course....). Remote controls work by sending infrared light, so this is a perfect way to test your camera!

To the right are some pictures that I took while testing my cameras. The first photo was taken with my normal camera and there is hardly any light seen coming from the remote control, which means that this is not a very suitable camera for infrared photography. The second picture is from my converted infrared camera, and shows a totally different picture!


So if your camera is sufficiently sensitive to infrared, you can put an external infrared filter on your lens, and take pictures with really long exposure times. But it's not very convenient, both because of the long exposure times, and because it is really difficult to compose with a black filter on your system. So you have to take off the filter, compose and focus, put the filter back on, and take the picture. I did this a couple of times, but it made me feel really frustrated and I quickly stopped taking infrared pictures.

So I have converted my old camera to a dedicated infrared camera. You can either send your camera to specialized companies, or buy the cold mirror and install it yourself (doing it yourself is cheaper and a lot more fun, but since it is hard to work dust-free, you're bound to get some dust stuck on your sensor). And that's when the fun starts! No more long exposures and no more filters on and off!


Camera - internal filter vs external filter


A thing to keep in mind is that pictures with a dedicated infrared camera differ quite a bit from pictures from a combination of a normal camera and an external infrared filter. This is because an infrared camera will only capture infrared light, whereas a normal camera with an external infrared filter will capture both visible light and infrared light, which will give a picture which is somewhere in between a dedicated infrared camera and a normal camera, probably not too far from a full spectrum camera. (As explained above, the external filter will block most of the visible light and the internal filter will block most of the infrared light. As a result, small amounts of both will reach the sensor, making it more or less a full spectrum camera with a very low sensitivity.)

I have tried to show this principle with the photos on the left where all photos were taken with the exact same type of camera (I have both a normal version and a converted one of this camera), so only the filters differ and there is no difference in the sensors. I converted the photos to black and white to make the differences more apparent.

The first one is a normal photo, the second one a photo with a normal DSLR and an external infrared filter, and the third one is from a dedicated infrared DSLR. It is clear that using an external filter results in a picture somewhere between a normal picture and a real infrared picture.




Not every lens is suitable for infrared photography, and there are lists on the internet with lenses listed as good or bad for infrared photography. Most lenses are simply optimized for visible light, and that gives rise to two phenomona which can occur with infrared photography.


The problem with some lenses is that they create so-called hotspots, like in the example on the right where you can see a bright spot in the middle. It changes while changing the focal length, but it is unusable at every setting. Hotspots are most often caused by coatings on both the lenses itself, as well as the coatings on the inside of the lens.

Fortunately, many lenses don't show hotspots, or only in a very small amount. I only found one lens in my collection which did not work well with infrared, and it is actually one of the more expensive ones, which shows that normal lenses are not optimized for infrared, no matter how expensive.


And as if hotspots aren't ugly enough, lenses are also a lot more prone to the weirdest flares with infrared photography, once again because lenses are optimized for visible light. So it's even more important to shield your lens from light sources than with conventional photography. Some examples are below, where the flares can be seen as orange artifacts.


Focusing and exposure measurement


When it comes to focusing and exposure measurement, there are a couple of things to remember with infrared photography:

• Infrared light has a longer wavelength than visible light, and will therefore focus differently than visible light. But the focus sensors in your camera are calibrated for visible light, which means that the infrared light will not focus exactly on the sensor when you use autofocus! In these cases it is possible to recalibrate your camera for infrared (or, if you have an older lens, there might even be a focus mark specifically for infrared on the lens itself). How much difference it is depends on the lens itself, and in the case of recalibration, it is done on the camera/lens system, and the new calibration will only work perfect for that combination.

Some lenses have such a small difference that, when shooting with small apertures (as in the case of most landscape pictures) the depth of field will be sufficiently large to compensate for it. But others have a bigger difference, as the one in the example on the left (this is at 100 %). Autofocus gives a soft picture, and a much sharper picture is obtained with manual focus. Unfortunately, if you don't have LiveView on your camera, you have no other option but to shoot several pictures with the lens at different focus distances, and determine on the LCD screen which one is the sharpest.

• The same goes more or less for exposure measurements, since the exposure sensors are also calibrated for visible light. The result from the exposure meter in your camera is a good estimate, but always check the exposure using the histograms on your LCD screen.


White balance


If you take an infrared photo, then it will probably look very red like the example on the right. That's no surprise, since we are shooting infraRED, but it doesn't make it very easy to judge your picture. So it is best to make a white balance preset in your camera, by calibrating it on something that is rendered white in infrared, like grass or foliage. If you shoot RAW, it won't have any effect on the end product, but, as said, it's a lot easier to judge the picture with a manual white balance.














Filters can be used in infrared photography, but, unless you have special filters designed for infrared photography, they are optimized for visible light, which means that they might give different results than you would expect.


Filters - ND filter


ND filters work the same way as with conventional photography, but their intensity might change. The example below on the left shows how an ND6 filter is not a 6 stops filter on an infrared camera. A normal exposure is taken with an exposure time of 1 s, which means that it should give an exposure time of around 60 s with the 6 stop ND filter attached. But in stead, only 20 s are needed, meaning a difference of only 4.3 stops. And the same goes for my ND10 filter, which becomes about 7 stops in stead of 10.


Filters - gradual ND filter


The same goes of course for gradual ND filters, and in the case of my ND gradual filters, the intensity changes so much that there is no visible effect in infrared! In the picture to the right I am holding an ND gradual filter, and as you can see, there is no dark part to be seen on the filter. Both the dark and transparent part of the filter are completely transparent for infrared light! That's pretty cool actually, but it also makes this filter useless for infrared photography.











Filters - polarizer


Polarizers can be used, but in my case it gives a very ugly color gradient in the sky, see the two examples below. This might not be a big problem if the image will be converted to black and white, but otherwise it's pretty useless.

But there is hardly any need for using a polarizer if you want more contrast in the sky when shooting infrared. Infrared light is a lot worse at scattering than visible light, which means it will give a dark sky by itself, without the need of a polarizer. The same goes for water surfaces, which are already dark by itself in infrared. In the case of other surface reflections it might be beneficial to use a polarizer, but most of it is not really noteworthy.


Channel swapping


Channel swapping is a technique that is used specifically in infrared photography. It exchanges the information from the red channel with the information from the blue channel, giving blue skies. In the example to the right, it can be seen from the histogram that the blue and red channels are exchanged, while nothing happens with the green channel.


Channel swapping is done in Photoshop by opening the channel mixer, selecting the red channel, and adjusting the reds to 0 % and the blues to 100 %. Then, select the blue channel, and adjust the reds to 100 % and the blues to 0 %. That's it! It's possible to take different values of course, if that gives better results.