Infrared (IR) Photography Overview

Background
Infrared photography (IR) relies of light that our eyes cannot normally see. This light produces a beautifully bright canvas that suggests a dream realm. The most recognizable trait of IR photography is that green leaves become nearly white while blue skies become nearly black.

However, in reality, IR photography is actually NEAR-infrared photography. True infrared photography is only obtained with thermal imaging. The IR photography that most of us are familiar with relies on reflected infrared light, rather than emitted infrared as is the case in thermal imaging. The sun produces enough IR for objects such as trees to reflect that IR light back to our cameras. However, this does mean that traditional IR photography does not "see in the dark". In fact, it takes much MORE light to process because so much of the light is blocked out.

IR photography is a relatively recent development in photography with the first IR photograph being published in 1910 and taken with then-experimental film. "Normal" photographic film is not IR sensitive but since the first 1910 photo there have been large leaps in IR film technology.

Film
The original popular IR film was black and white. Like most black and white film, it trended towards the blue end of the light spectrum. This led to beautifully cool feeling IR photographs. Even with IR sensitive B&W films, it is often necessary to use orange or red filters. Some still use IR filters on top of the IR sensitive film to increase the IR look. Without the additional filters the loss of contrast can "wash out" the IR look.

Color IR film tends to create "false colors". Reflected infrared becomes red, red becomes green, and green shows as blue. Most color IR film does not have to be loaded into the camera in complete darkness as B&W IR film does. However, due to B&W IR photography and the upsurge in digital photography, color IR film is becoming hard to find. Kodak recently stated they would stop making their 35mm color IR film. "IR" filters are available for use that can approximate the IR film feel but the results tend to lack the clarity and depth of IR sensitive film.

Digital
Digital sensors are generally not sensitive to IR light. Some actually go so far as to use IR blocking in order to increase focusing efficiency in the visible spectrum. Many people use IR specialty filters in order to attempt digital IR photography. However, some cameras will still produce false colors that must be corrected in a digital darkroom product. Sony, Fuji, and Sigma do make cameras that allow the removal of the IR blocking.

Universal Issues
Because IR filters remove a large portion of visible light, autofocus will not work properly. This means you must be able to focus manually to take an accurate image. Also, because so much light is removed, exposure times become very long. A tripod or other sturdy base must be used to prevent camera shake. Light meters do not record IR light, they measure visible light. Remember to bracket your exposures until you get a feel for what the right exposure compensation should be.

Source : http://photography.about.com