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Infrared Photos
Technical Information
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Infrared Photos
Nikon D70; R72 filter; f/8 1/250th sec.
Canon PowerShot Pro1; R72 filter; f/4 1/125th sec.
 With IR83 deep infrared filter
 
 
 
EOS Digital Rebel; R72 filter; f/5.6 1/200th sec ISO 100
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Technical Information
Digital cameras are designed to record light in ways that mimic human vision.
For this reason, manufacturers install low-pass filters inside the camera to block out any wavelengths
that we cannot see with the naked eye. Without these filters, the digital sensors (both CCD and CMOS) would
record both UV (ultraviolet) and NIR (near-infrared) light, which distorts natural colors.
NOTE: all graphs represent approximate values, and are intended for illustrative purposes only.
The area shaded in blue shows the light as captured by an off-the-shelf Canon DSLR. This area corresponds
fairly closely with what human vision can see, yielding very accurate colors. The green bracket above
the graph shows the range of wavelengths that most digital sensors are capable of recording.
The diagram above represents the way an unmodified camera works with the camera pointed straight up. Light from the lens first hits the low-pass filter, which blocks the IR light (black arrows), then goes
through the Bayer filter array, which is a part of the CCD or CMOS sensor. The entire Bayer filter array is transparent to infrared light, so the low-pass filter is installed to prevent that IR light from
skewing the colors in your final image.
When we modify a camera for infrared photography, we remove the original low-pass filter from inside the camera, and replace it with the opaque infrared filter.
The area shaded in red represents the light recorded with the R72 filter installed. Most of the Infrared Photos
shown above are taken with cameras modified just like this. The purple bracket below the graph shows
the range of wavelengths that are visible to human eyes. A camera modified with the R72 filter blocks
nearly everything visible to us, and records the remaining IR. However, a very small amount of visible
light does get through, allowing for some interesting color effects (see esp. the sample photo taken with
the PowerShot Pro1).
With the infrared filter installed in place of the low-pass filter, the diagram above shows how the
IR light typically activates the red, green, and blue sensors in equal proportion. If the IR activates more
of one color than another, then that produces color tints.
Of course, you can influence these colors somewhat
by using the camera's white balance settings. There is no right answer for how to set white balance on an
IR-modified camera: it is matter of taste. Remember, we are translating something invisible (IR light) into
something that we can see, so it is always going to be a false-color image to some extent. We encourage
photographers to experiment, both in camera and with post-processing, to find a look that suits them.
Infrared photography without modification
Many people read about the abilities of digital cameras to record infrared light, and attempt to take
digital infrared photos by simply placing an external R72 filter on the front of the lens - without
removing the factory-installed low-pass filter. While this
is possible, and with some cameras works moderately well, there are two significant disadvantages. First, the R72 filter is opaque, so once it has been attached to the front of your lens, you can no longer see
through the optical viewfinder. Secondly, you now have a low-pass filter inside the camera, which lets only visible light through. And you also have an IR filter in front of the camera, which lets only
IR light through. So the only thing that gets through BOTH of those filters is the stray light that both filters fail to block properly.
Note that the right slope of the blue line (the low-pass filter) doesn't quite drop to zero - it only
blocks about 98.5% of the infrared light. So using both filters at once, you get only the deep red
to near-infrared colors (roughly 700 - 750nm), and in that range, only about 1.5% of the light
is recorded. That's a loss of about 6 stops. This means a tripod is vital, even for daylight scenes.
Of course, you'll be needing a tripod anyway, since you can't see through the viewfinder!
See how both filters, when used together, block just about everything.
Compare this with the loss of light using the installed R72 filter - there you loose about 9%, which is
less than 0.2 stops.
If you don't want the colors and tints that the R72 filter produces, there are two options. One is to
set the camera to black & white mode, and the other is to install a different filter - the IR83.
Note the gap between where human vision ends and the IR83 filter begins. This means no colors at all,
just a monochrome image. It's monochrome because these longer wavelengths of IR light pass right
through all the colors in the Bayer filter equally. Equal values for the three colors means a neutral
or grayscale image.
Other applications, such as forensics, astronomy or scientific photography, require the ability to record
a much wider range of wavelengths. With a clear filter installed in place of either the original low-pass
or an IR filter, the camera is unhindered, and captures everything it can. In this configuration, the
lower limit for color is usually what the lens can transmit - most lenses absorb anything below 270nm.
The upper limit is about 1100 or 1200nm - any longer than that, and the silicon is transparent. Since
CCDs and CMOS sensors are made of silicon, they don't record much beyond that. A camera modified with
a clear filter would record just about everything inside the green bracket on the charts above.
Photographs taken with a camera modified with a clear filter would have full color, but the colors may
be distorted, because the IR light can activate either red, or green, or blue sensors. Cameras modified
in this way are frequently used with other filters. For example, astrophotographers often use filters in the telescope. The broad range of a full-spectrum camera allows them to change to a filter that
highlights the specific object they are photographing.
Focus issues
If you imagine the way a prism separates white light into various colors, you will remember that this
occurs because various wavelengths are deflected by varying amounts when passing through the glass. To
some extent, the same thing happens when light passes through the glass in your lenses. Although modern lenses are designed to focus red, green and blue light as tightly together as possible, they are not calibrated to put infrared in the same place. That means that IR images will actually come into foucus
at a slightly different point than visible light will.
After modification here at LeZot, we check focus with a 50mm lens. Images are generally in acceptable focus at f/4 or higher. Even greater sharpness can be obtained by stopping the lens down or using Photoshop.
All the sample images above use one of those two methods.
Both the autofocus system and manual focus employ visible light. The focus shift between IR and visible light is different for each lens. For some bodies, we offer custom focus calibration, so that your individual lens is calibrated to your individual body. For example, we can adjust your EOS 30D to give superb AF operation with any of your Canon lenses, without affecting how your other lenses work.
As of this writing, custom focus calibration is available on the EOS 20D,
30D, 40D, Digital Rebel,
and Digital Rebel XSi. More models are coming, so if you don't see yours, please ask us!
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