Infrared Photography with a DSLR 

The following educational material is extracted from the video course “The Infrared Photography Masterclass”.

Radiation in the near infrared spectrum is the nemesis of the digital color camera. This is because near infrared radiation still has photon energy that would be detected by the digital camera if it were not filtered out. The additional strength of the radiation would increase the number of photons that strike the photosites of the camera's sensor and overload many of the photosites. In addition, it would be impossible to properly encode blue, green and red color into an image to simulate the colors of the scene that the camera operator expects.

To solve this problem, nearly all digital camera manufactures install an infrared cut filter just in front of the camera's sensor. This is typically called a "hot mirror". It is called a hot mirror because it reflects, or rejects, all of the near infrared radiation, while passing most of the radiation of the visible spectrum. Conversely, a "cold mirror" reflects, or rejects, all radiation in the visible spectrum while passing most of the infrared radiation. Digital cameras use a hot mirror to cut near infrared radiation.

As discussed in my course “The Infrared Photography Masterclass”, in infrared photography we are trying to map the energy of radiation to tones that are within human recognition. But the hot mirror that is installed deep within the camera body blocks infrared radiation and allows only the visible portion of the spectrum to pass to the sensor. We need to get that hot mirror out of the optical path and replace it with a filter that passes infrared radiation while blocking visible radiation. This type of camera conversion is called "dedicated" because the hot mirror is permanently replaced with a cold mirror.

We sometimes want a camera without a hot mirror installed at all so that we can decide filtering depending our interest or need. A camera with no internal filter installed is called a "full spectrum" camera. A full spectrum camera allows us to determine the filtering to be used in a given application, typically with lens mounted filters. But, of course, a full spectrum camera requires the investment of lens mounted filters, which can get pricey. Most infrared photographers will choose the less expensive route of the dedicated conversion.

Dedicated and full spectrum conversions can be performed for a DSLR camera body. However, a camera's operation is affected by a conversion. It's important to understand how the functions of the camera are affected as an aid to practicing infrared photography.

This topic is covered in more detail in my course “The Infrared Photography Masterclass”. But let’s review how the DSLR operates to help understand how a converted DSLR for infrared photography.

The DSLR Light Path

Referring to the illustration, the blue lines illustrate the light path within a DSLR camera body. Radiation energy is focused onto the internal semi-transparent view finder mirror which splits the light path to the optical view finder system, the main sensor and the auto focus system. Many DSLR camera's have a preview feature, which is an image derived from the primary sensor, processed by the camera's computer and displayed on the camera's LCD as a JPG image. Take note of the hot mirror mounted directly in front of the primary sensor.

Metering

As infrared photographers we need to consider the fact that the metering system of the typical DSLR is calibrated for color, shown in the illustration as the “Transparent Matte LCD”. The reason this impacts infrared photographers is because the color-calibrated metering system of a DSLR can't be fully trusted when shooting for infrared. When a camera has been converted as "dedicated", where the hot mirror in front of the sensor is replaced, the metering system still functions assuming the visible spectrum. This is because some of the radiation is split to a sensor intended for metering. When a DSLR has been converted as "full-spectrum", where the infrared filter is mounted on the lens, the meter system still functions assuming the visible spectrum, but is now receiving filtered radiation. In either scenario, the metering system won't be accurate with respect to infrared radiation, although will perform slightly better with a full spectrum conversion because it's metering on radiation filtered to the infrared spectrum.

Auto Mode

The color-based metering system of a DSLR will typically misjudge the photographic settings when the camera is used in auto mode where. The camera's computer determines such settings as aperture, shutter, ISO and exposure based on the metering system. I personally have never had much success with infrared photography using the auto mode of a camera, with the color-based metering system being one reason.

Focus

There's a couple factors that we need to consider pertaining to focus. When a DSLR is set to auto focus, the camera's computer will typically use a process called phase detection that attempts to detect contrast within the scene in the camera's field of view. This is called "passive auto focus". Passive auto focus relies on a separate sensor within the body of the DSLR labeled in the illustration as "Auto Focus Detection". And, as you can see in the illustration, radiation is split to the auto focus detection system. The camera's computer looks for edges as revealed by differences in contrast of objects in the scene and then adjusts the lens focus motor until the edges are clearly defined. The idea is to achieve focus by obtaining sharp edges.

The issue for infrared photographers is that the passive auto focus system is calibrated for optimum results in the visible spectrum. With that said, I have found that the auto focus systems of the better DSLR camera's and lenses will still work well when shooting for infrared, especially in brightly lit situations. This is because the passive auto focus system is using contrast detection, which is dependent on tones, not color. And high contrast is one of the unique characteristics of infrared photography, for instance plants against a dark water body. See “How Infrared Radiation Behaves” in my course “The Infrared Photography Masterclass”. I have found that the passive auto focus system of the DSLR shouldn't be entirely trusted.

Lens Considerations

When using lenses designed for color photography, which is almost always the case, there needs to be a slight compensation for the longer wavelength of the infrared spectrum, which lens manufactures don't usually take into account. Since the passive auto focus system has been calibrated to the lens in anticipation of the lens working in the visible spectrum, the auto focus will often misadjust the lens focus, resulting in a slightly out of focus situation. The auto focus system has no way of knowing that you are shooting for infrared. And, the slight out of focus situation is very difficult to detect in the field and correct during image processing.

Focus fine tuning is actually an important field procedure for infrared photography because the auto focus system can't be fully trusted when shooting for infrared. Using the camera's preview zoom feature, assuming the zoom feature is available, it's easy enough to perform a check of the focus and fine tune the lens focus manually, although this can become tedious in the field.

What I normally do in the field is use the camera's auto focus system to get the focus close, then switch off the auto focus and fine tune the lens focus manually. I use the camera's electronic zoom feature to zoom in on the LCD preview, and then manually adjust the lens focus if needed.

What the better camera conversion services do is calibrate your camera body to a specific lens that you send along with the body to be converted. The idea is to use that body and lens combo when you shoot for infrared. You can also perform the calibration yourself for one or more of your lens as this Canon 7D menu selection shows. But, of course, do this micro calibration after your DSLR body has been converted. Check if your DSLR supports lens calibration and follow the procedure for your camera model.

The optical coatings of lenses are typically tuned for color. As a result, some lenses are prone to flares and hotspots when used for infrared photography. Flares occur when the infrared radiation bounces around inside a lens barrel, or the camera body, because the lens coating was intended for the visible spectrum. Flares are often handled with lens hoods or repositioning yourself to another location in the scene. Hot spots occur because some lens coatings, having been intended for the visible spectrum, are not fully transparent in the near infrared spectrum and tend to block a percentage of the radiation energy. This image illustrates lens flare.

The LCD Preview

Next, let's consider the LCD preview feature of a typical DSLR camera. This is the simulated image preview and information display from a Canon 7D. The preview image is a JPG simulation that is produced by the camera's computer based on what the computer interprets considering metering, which is calibrated for color as mentioned. The preview is not exactly what the sensor is detecting, rather is an approximation of what the camera will output, which is why it's called a preview. Why this matters to infrared photographers is because you are only able to view an approximation of the image the camera will output. The preview is the camera's best guess...in anticipation of a color image. With a camera converted for infrared, if you depend entirely on the preview in the field, you will be mislead for purposes of setting the optimum exposure given that you are working in the near infrared spectrum.

The LCD preview will also reveal the inability of the camera to properly set the white balance for the scene being shot. You'll see a pink, brown or purple tint in the preview itself as shown here. The tint depends on the infrared filter being used. This often obscures your ability to determine if the exposure is optimal or if the focus is sharp. However, many DSLR camera's have an electronic zoom feature that aids tuning for sharp focus, which I mentioned earlier.

The preview is not by any means useless for infrared photography. It's still useful for composing a scene, fine tuning the focus, reading information about the camera settings and to get a general idea of the image that will be taken. But, it shouldn't be depended on entirely.

White Balance

White balance is needed to remove a color cast that is common to nearly all types of light. Tungsten light bulbs, for instance, emit a purple color cast. Florescent bulbs emit a greenish cast. Because these color casts are recorded in our images, which effectively ruin our photographs, a method is needed to remove the cast. White balance adds the opposite color to cancel out the color cast.

Summary

I wanted to point out a few issues that we must consider when using a DSLR for infrared photography. The coating of lenses are intended for the visible spectrum, leading to a higher probability for lens flares and hot spots in the infrared spectrum. The color-calibrated metering system may not provide the best aperture, shutter and ISO settings. The LCD preview is still useful in the field for composing a scene, but doesn't represent the final image that the camera will output in an infrared application. The auto focus system, which is calibrated for color, could result in a slight focus error if we entirely trust the camera. And, the color tint issue that relating to the inability for the camera to white balance in infrared radiation.

These DSLR issues could be considered equipment problems that infrared photographers face. But I don't consider them problems, just characteristics of a DSLR camera designed for color photography that need to be kept in mind when shooting for infrared. We have several ways of dealing with these issues that become clear with a good understanding of the camera and experience with infrared photography.

Please refer to my infrared photography course “The Infrared Photography Masterclass” for more information about the digital camera for infrared photography.