In this tutorial we go step by step through shooting infrared with your normal, unmodified digital camera.
Infrared photography is, quite simply, stunning. So it is natural for a photographer to want to give this a go. We cover film infrared photography elsewhere, so here we will concentrate on digital infrared here.
Digital camera sensors are sensitive to infrared light. Because of this, camera manufacturers place an IR blocking filter in front of the sensor to improve color rendition by blocking infrared light. These blocking filters are not 100% effective, and so we can still shoot IR. Unfortunately the strength of the filters has been growing over time and so more recent digital cameras have blocking filters that will cause your exposures to be quite long and certainly requiring a tripod.
You will need a filter to block the visible light through the lens and only allow the infrared through. The common one and, in my view, the best one to start with, is the Hoya R-72 filter (for a more extensive discussion, see our article “Choosing a Filter for Infrared Photography”). This filter lets through a tiny amount of red light but the full range of infrared. Your R-72 filter goes on the lens and thus stops you from seeing through the camera, in the case of an SLR. In many compact cameras the Live View on the LCD with still work and display the scene. With dSLRs with Live View you would expect this to work, but in practice it does not. The only company’s camera that has Live View that works with an IR filter attached is Olympus. With their cameras you will get a darkish but quite workable monochrome view, but you must turn on Live View Boost from the menus.
If you examine the graph above which shows the sensitivity through the Bayer RGB filter that is part of every camera’s sensor you will understand why a camera responds the way it does in the IR. With this Kodak sensor (reproduced with permission from Kodak), we can see that while the red filter remains transparent to light from the red (as you would expect) right through to the IR, this is not true of the other filters. The green and blue filters drop to opaque beyond their range, again as you would expect, but become transparent to infrared above a certain wavelength. Since filters are defined by the point at which their transparency to IR rises to 50%, with this particular filter the green channel acts as if it had a 780nm cutoff filter and the blue as if it had an 800nm filter, approximately. So in a camera with this sensor the red channel will see IR from 720nm on, green from 780nm and blue from 800nm on. This explains why the resulting images from your camera will usually be very red: the camera is capturing more light in the red channel because it can make use of a wider range of IR light and covers an area where the sensor is more sensitive to IR, which the green and blue will be less so. This obviously varies from sensor to sensor because of the different dyes used in manufacturing the Bayer filter.
What this means in practice is that your camera will produce a more extreme infrared effect (darker skies, more contrast) in the green and blue channels than in the red.
The fact that the three color channels will see different parts of the infrared spectrum (depending on the filter you put on the lens) means that there is some residual information you can exploit for false color infrared, if you wish to. We will look at this in the companion article “Processing Infrared Photography Step By Step”.
The above difference between the three-color channels in how much of the infrared spectrum they see means that there is a significant difference in exposure between the three when you take a photograph. In many cameras I have tested there is approximately a 2.5 stop difference in exposure between the red and the green channel. Because most infrared scenes will, with an unconverted camera, not use the full tonal range the camera can capture, you will end up with a lump on each channel that does not cover the whole range (see the histogram below). This lack of use of the full range means that you will normally, in processing later, need to do a Levels adjustment and stretch the tones out. If you are primarily stretching the image data up to create a white you will also be magnifying the image noise. So to create a good image for later work you will wish to expose your image to the maximum possible level without white clipping occurring. That means you need to choose which channel is most important to you, the red, with a more moderate IR effect but the highest exposure, or the green, with a stronger IR effect but requiring a longer exposure, and thus more noise.
Once you have determined the above you can start shooting. Framing is an issue with dSLRs (as well as film SLRs for IR work). With the filter on you cannot see. There are three possible solutions to this issue:
- Use an external viewfinder mounted on the hotshoe for framing;
- Frame by guess and then refine once you have taken a shot;
- Frame the camera and then screw on the filter.
Personally I do not like the last one because there is too much handling of the filter and thus more risk on dropping it or marking it. I prefer the guess approach, so let me illustrate that below.
Setup the camera on your favorite tripod, with a cable release (you can use the self timer if you do not have one) and the R-72 or similar filter on the lens. Point the camera by eye and set a starting field of view if using a zoom lens. Take a shot. Look at the result and change the setup if the framing does not please. Remember to use the eyepiece blind or little plastic thing on the camera strap to cover the eyepiece. There is the risk of light leaking in and fogging part of the image.
Determining your exposure is an easy process. Depending on how recent your camera is, set a starting exposure of somewhere between ½ a second (older camera) to 15 seconds (newer camera), f4 and 100 or 200ISO. Take a shot. Adjust the display on the LCD screen so that a histogram is also displayed if your camera is capable of doing so (normally done by cycling through the modes with the display button, but check the manual). If you can display individual channel histograms adjust your exposure up or down to put the desired channel as close to the right of the histogram as you can without clipping. If your camera only displays one, combined, histogram if you place the right of the histogram up near the right of the graph you will be properly exposing the red channel. Find by experiment how many stops from this you need to shift to correctly place the green channel. If no histogram can be displayed you must do it from the image. If the result is very bright red but with no obvious clipping then you are exposing the red channel properly. Again, experimentation will show you how much you must shift from this to get the green right. This will result in an over exposed red image showing lots of white, on the LCD.
With most dSLRs I have tested you will get a decent result with autofocus. This is because, with the R-72 filter on the lens, only IR light is going into the camera and so the sensor must focus using this light. I would advise using a moderately stopped down aperture if the exposure sensitivity permits. What I mean here is that many cameras only allow you to set a shutter speed up to 30 seconds. If your camera requires a full sun exposure of 30 second, f2.8 and 100ISO, then you want to stop down further you will need to either use bulb and using a watch or stopwatch time the exposure or increase the ISO, which may not be desirable to you because of a rise in noise. Remember that (see the separate article on Diffraction Effects) blurring occurs faster as you stop down to really small apertures in infrared than it does in the visible. So I rarely go below f11. Again you need to take some shots and then look at them at 100% in Photoshop to see how well the focus works. Manual focusing can be hard with dSLRs because many of the lenses no longer show and IR red focus line. If you have a lens with this mark you can, of course, focus without the filter, then attach it and adjust focus.
Don’t be surprised if, when you examine your images either on the LCD or back home on the computer, that they feature a central glow or low contrast, fogged looking area. Many lenses do not perform well in IR, at least on unconverted cameras. Since the IR blocking filter in the camera is one that reflects away the IR light rather than absorbing it, the majority of the IR getting through your R-72 filter will be reflected back at the lens. This IR will reflect around and, because the multi-coatings may not be designed to handle IR, eventually find their way back to the sensor, causing a central spot. Some lenses will only do this at certain zoom settings or certain apertures. If you have a lens which does this badly the only real solution is to buy a different lens. See the reference lists elsewhere on this site for our listings and those on other sites.
You will also find that your camera is more sensitive to lens flare in IR than in visible light. So you need to be more careful about effective lens shades and how you point the camera, unless you like this effect.
That’s about it for the shooting. See the companion article on Processing Infrared Photography Step By Step.