How Infrared Photography works

Summary

Calculator for infrared photography: applies the filter factor for IR-blocking filters (R72, 89B, 87) and reminds you about the focus shift IR introduces relative to visible light.

How it works

Infrared (IR) film is sensitive to wavelengths that are just past visible red. To use it you put a deep red or near-opaque IR filter on the lens to block the visible light, leaving only IR to reach the film. This modal handles the exposure compensation that filter requires and the focus shift IR introduces.

Filter factor

Each IR filter blocks light by a known amount. A Hoya R72 blocks roughly 3 to 4 stops. A deep red R25 blocks about 3 stops. The modal lets you pick the filter and applies the right factor.

Scene bias

How much of your scene actually emits IR. Foliage and clouds reflect lots of IR (the famous Wood effect with white leaves). Skin and water reflect very little. The bias slider biases the recommendation toward typical IR scenes.

Focus shift

IR comes to focus at a slightly different distance than visible light. Most older lenses have a small red dot or line on the focus scale labelled R or IR. After focusing visually, shift the focus ring so your distance lines up with the red mark instead of the normal index.

Caveats

Some modern lens coatings and some camera bodies (especially those with strong IR-blocking sensor filters) make IR difficult or impossible. Test before you commit a roll.