Infrared light is by definition “invisible” light that resides above the threshold of human vision. It is not Kirilian, or aura photography, though it sometimes creates an ethereal, ghost-like representation that can be quit seductive. Like most things photographic that appear visionary, there is a bit of science behind the magic. This involves disabling the “normal” operation of the camera to allow infrared level light to record, something that is usually blocked by an IR cutout filter placed in the light path from lens to sensor. It turns out that without this internal filter the digital image would be “polluted” with IR, and would, for most people, create less than desirable image quality. In truth, digtal sensors often have considerably more infrared sensitivity than you might think.
In most cameras the filter is “hard wired” into the construction, which means that if you remove it you now have a fulltime IR capable camera that cannot be returned to normal use. This must be done by a service company that knows its stuff; it’s something you cannot do on your own. Some cameras come IR dedicated, mostly those used by law enforcement agencies for gathering forensic evidence. A very few can be converted in do-it-yourself fashion, such as the Sigma model DSLRs, where you can remove the lens and literally pick out the IR filter (which doubles as a dust filter), albeit very carefully, and reinsert it later. Those who are true IR fans would do well to investigate the cameras used by police departments; as of this writing Fuji makes a few such models.
Another step that needs to be taken to capture true IR images is placing a filter over the camera lens, something that is uncommon for most digital photography (as most filter effects can be added later in software.) If you shoot IR without any filters with an altered camera you will get a sort of reverse pollution of visible light, something that certainly diminishes the IR effect.
There are three types of basic filters that can be used, with one being quite expensive and, in my experience, unnecessary to gain the effect. The filters used include a red filter, something black and white film photographers might still have in their closet, (a Wratten 25A), plus two filters that block more and more visible light and do not allow for image recording below the infrared threshold. The two other filters, which go under various names and codes according to the filter maker’s markings, block light under 700 nanometers and 830 nanometers (and some higher) on the spectrum; in other words, progressively more IR light and much less visible light. The highest blocking filter (830nm and above) gets very expensive and is only for well-heeled purists and aficionados.
The effects achieved with the two blocking filters can be amazing but you cannot see what’s going on in the viewfinder, which means that you have to frame and focus prior to placing the filter over the lens and making the exposure. Some photographers view over the top of the camera for an approximate framing. The red filter, while not as “pure” lets you at least see what’s going on in the finder. If you are an IR fan then the blockers will be your choice—those who dabble in it, as I do, will find the red filter is fine.
Exposure is unusual as well. It has nothing to do with making readings and using metering patterns, since you are not dealing entirely or at all with visible light. It’s a strange concept, but that’s also part of the IR mystique—being out of normal bounds of having to read exposures and balance highlight and shadow. The best way to work IR exposures is to start at somewhere around f/11 at 1/125 second and then review the image after exposure and adjust accordingly. The view in the finder is quite different than you’d expect, so you will have to gain experience with what a processed IR image looks like according to a certain exposure level. This is the only way you will be able to make predictions about what the correct exposure might be, or at least what it should look like upon playback.
There is something in IR known as a “focus offset”, which means IR light bends a bit differently and may arrive at the sensor plane in different ways than visible focusing. This will become critical when doing close-ups, but does not have much effect, in my experience, for photos made beyond six feet. I often shoot at f/11 or narrower with a fairly wide angle lens just to take up that slack, changing the shutter speed accordingly.
The procedure some use for shooting is to purchase a filter that is a bit larger than the largest diameter lens they own (thus it can be used on all the lenses owned). They first frame and focus the tripod-mounted camera, then hold the filter over the lens when the exposure is made. This procedure takes care as it can result in some light leak from the corners, but that is usually eliminated with some practice in proper holding of the filter. Exposure is set manually. Some bold photographers shoot handheld in the same fashion, viewing over the top of the camera to yield an approximate framing and shooting a bit wider than they normally would for a “fudge” factor. They figure they can apply a more specific cropping later.
IR black and white has always been near and dear to landscape photographers, and now that high-speed IR black and white film has been discontinued by major film makers digital seems the only way to go.
