Chromatic aberration is the effect produced by the refraction of different wavelengths of electromagnetic radiation through slightly different angles, resulting in a failure to focus. It causes colored fringes in the images produced by uncorrected lenses.
You may be surprised (or not) to learn that when passing through a lens light waves of different color and wavelengths don’t focus on the same plane. Much like light through a prism, the glass elements in a lens produce a rainbow of color called chromatic aberration as light passes through them. When light passes through the Extra Low Dispersion optics found in some lenses, these specially formulated glass elements minimize the amount of refraction that occurs when forming an image to produce a straighter path for the light.
Most lenses, however, can’t focus infrared wavelengths on the same plane as visible light. That’s why when you shoot infrared images it may be necessary to slightly shift the lens’ focus. Typically, the smaller the lens’ aperture and the longer its focal length the greater this shift will be. That’s why lens manufacturers used to place an infrared mark on their lenses to help you make this shift. With autofocus lenses, I set the camera in Aperture Preferred mode, set the lens aperture at f/16 (and sometimes f/11) and just shoot excpecting that the image will be in focus. And this technique usually It usually works for landscape images.
For manual focusing wide-angle lenses, like Voigtlander’s 15mm, the Hyperfocal distance is my favorite method for focusing when shooting infrared IR.
Hyperfocal distance is the point of focus where any object that is between the distance to this point and infinity is in focus. Here’s how it works: You pick an aperture, such as f/16 or 22, then rotate the focusing ring setting that aperture opposite the infinity mark. On the 15mm lens this produces a depth-of-field from about five inches to infinity—more than enough to take care of any focus shift cause by invisible infrared light waves.
If you’re interested in infrared photography, I’ve found that LifePixel does a good job of IR conversions who converted the Lumix G5 used to make the above images. If you use coupon code: Farace-IR, you can get $50 OFF the Priority Processing Upgrade
You can see another image from this shoot on the back cover of my book The Complete Guide to Digital Infrared Photography. The book is currently out of print but you can get an affordable used copy or not-so-affordable new copies of the book from Amazon.com.