Infrared Photography Quiz

Infrared light occupies wavelengths beyond red in the visible spectrum, starting around 700 nanometers. Camera sensors are sensitive to this range but standard hot-mirror filters block it. Removing that filter or using an IR-pass filter over the lens allows only these longer wavelengths to expose the sensor.

Healthy leaves reflect a large share of near infrared light, so foliage records very bright, often almost white, in an infrared photo. This is called the Wood Effect, after physicist Robert W. Wood, who made early infrared photographs. Leaves do not absorb infrared, so they never turn black.

Camera sensors are naturally sensitive from ultraviolet through near infrared, and a filter in front of the sensor normally blocks infrared. A full spectrum conversion removes that filter and replaces it with clear glass. Filters placed over the lens then choose which wavelengths reach the sensor.

The R72 is an infrared pass filter that blocks visible light below about 720 nanometers and lets longer infrared wavelengths through. It looks nearly black to the eye. 72nm and 7200nm are far outside the range cameras can record.

Blue sky scatters and reflects very little infrared light compared to what it reflects in visible wavelengths, so it records as dark, often near-black or brownish in raw infrared captures. The familiar pale sky of visible-light photography inverts to a dramatic dark tone.

In a standard false-color infrared file, red and blue channels are swapped in post-processing. This maps the sky, which recorded in the red channel, to blue, while foliage that recorded bright is shifted to yellow or white. The result is an image where sky and leaves read in more familiar colors.

In infrared, some lenses show a bright, often discolored circle in the middle of the frame. It comes from infrared reflecting inside the lens barrel or off coatings that were designed only for visible light. It gets worse at smaller apertures, so lens choice matters for infrared work.

Lenses are designed to focus visible light, and longer infrared wavelengths come to focus at a slightly different distance. Focusing by eye or with visible light can leave an infrared image soft unless the focus is corrected. Some lenses have a red infrared mark for this, and stopping down helps cover the error.

Foliage is the brightest thing in most infrared scenes, so a custom white balance measured from sunlit grass or leaves gives a neutral starting point. Without it the image comes out strongly red or magenta. Auto white balance usually cannot make sense of infrared light.

False color infrared takes the infrared tones the camera recorded and assigns them to visible colors, often with a channel swap in editing. The colors are an interpretation, not what the scene looked like. It is a processing choice, not a sign of a broken sensor.