Pixiq

Digital sensors capture images that reflect what our eyes see but infrared photography is about recording invisible light. The spectral range where red wavelengths are no longer visible starts at about 700 nanometers, give or take a few nm. In order to capture infrared images,  visible light wavelengths below 70 nm must be blocked so that they don’t reach the sensor. Which brings us to filters. If you shoot with a digital camera that has not been converted to IR-only, you will need to use a filter to capture infrared images.

How Filters Work?

It is a common misconception that a colored filter adds its color to an image. When colored filters, such as a 25A Red, are used in traditional photography they transmit wavelengths that are a similar color to the filter and block ones that are not. For example, in traditional color photography, an 85A blue filter used to color-correct photographs made under incandescent light does not add blue to the image. It subtracts the red and yellow tones, rendering the image more neutral in appearance. Depending on the density of the blue filter, it blocks more or less of these other colors. In much the same way, the various deep red (becoming almost black) filters used for infrared photography totally or partially absorb or subtract visible light while allowing the transmission of infrared light.

The information in this chapter is about using filters to produce monochrome or black-and-white IR images. (Black-and-white images are monochrome, but not all monochrome images, as you will see throughout this book, are black and white.) Images captured by all of the methods suggested in this book start as RGB (color) digital files and have to be processed to make them appear monochromatic. (Even when you use a camera’s black-and-white mode, the sensor is still recording RGB information—the camera then has to process that image to make it black and white.) Creating IR color images is a “faux” process that requires creative postproduction techniques, covered in more detail in subsequent chapters of this book.

So what kind of filter do you need for your digital camera?

Filters for IR Photography

Filters have a long history of use in photography for both practical and creative purposes. In this guide, however, we are concerned with filters that specifically absorb or reflect visible light, allowing you to record the infrared spectrum. The classic IR filter is a round piece of glass within a metal or plastic mount that allows it to be threaded on the front of a lens. These are sold in various sizes to mount directly onto different sized lenses. A quality filter is made of fine optically flat material mounted within a frame that’s strong, durable, and not prone to binding or cross threading. This takes precision manufacturing, which is why a good filter can be expensive (but will last a lifetime if well cared for).

Filters come in many different shapes, sizes, and materials, including delicate gelatin filters that drop into holders or lens hoods, round glass filters, and square or rectangular glass or optical-plastic ones that fit into modular holders.

The modular concept was popularized by Cokin, whose system includes a holder that attaches to the front of the lens with an adapter ring. The system’s square filters slip into the holder. Though you may need different sized attachment rings to fit your various lenses, you won’t have to buy a different IR filter for each lens because the same holder and filter can be used with different sized adapter rings. On the downside, since the filter is not physically threaded into the front of the lens like a screw-in type, visible light can also enter from the sides, polluting the finished image. Is this a good thing or a bad thing? Depending on your desired effect, it’s a little of both.

Semi-Famous Filter Facts

Many filters have a number designation based on a system of tolerances first developed by Frederick Wratten, who pioneered the use of colored filters at the beginning of the 20th Century. His company, Wratten and Wainwright, made photo materials for the commercial printing industry. He had established his filter numbering system, the “Wratten system,” by 1909 when he published The Photography Of Coloured Objects. In 1912, George Eastman acquired the company and Wratten’s system became a standard for filters used in photography, astronomy, and other applications. Eastman Kodak Company still manufactures and markets KODAK Wratten Gelatin® filters. Other filter manufacturers have their own designations, but they also often refer to the Wratten equivalent.

There is not a great deal of intuitive logic in the organizing system for Wratten numbers. Number 25 in the Wratten system is a red filter that passes visible red as well as infrared wavelengths to the film or sensor (thus blocking wavelengths shorter than 580 nm—remember that the wavelengths of visible light range from approximately 380 to 700 nm). You can easily see through a red #25.

Other filters that block all (or nearly all) visible light, from 700 nm and below, will be much darker than the Wratten #25. Depending on their specifications, these opaque filters are extremely difficult or impossible to see through.

If you happen to be shooting black-and-white IR with a red filter, you won’t need to compensate for focus (to use the infrared focusing mark if it exists on your lens, or hyperfocal distance) because most of the light hitting the sensor is from the visible spectrum. This means the focusing distance is the same as it would be for an unfiltered photograph. All this changes when an opaque filter is used, because only infrared light reaches the imaging sensor, and a slight focusing adjustment needs to be made. If the lens doesn’t have an infrared mark, Eastman Kodak Company recommends that you first focus through a KODAK Wratten Gelatin ® #25 filter using a small aperture, such as f/16 or f/22. Then switch to a darker filter because the depth-of-field at these small f/stops usually offsets the slight difference between the visual and infrared focus. (For more on focusing, see pages 30-35)

Cleaning Filters

When cleaning glass screw-mount filters, you should take as much care as when you clean your lenses. It’s a good idea to use a microfiber cleaning cloth, but never chemical lens cleaning fluid, which might damage the coating. Filters manufactured with optical plastics require even more care because they scratch easier than glass.