One of the most interesting new features of the two new EOS digital SLRs is the Peripheral Illumination Correction function. It harnesses the incredible power of the new DIGIC 4 image processor to improve image quality by maintaining evenness of illumination from corner to corner in JPEG images, even when shooting wide open with a zoom lens. This is a feat that previously required skilled post-processing techniques in a personal computer, but now it can be done automatically at speeds up to 6.3 frames per second with the EOS 50D (3.9 frames per second with the EOS 5D Mark II).
This new feature essentially eliminates one of the limitations of previous full-frame digital SLRs, and will be especially appreciated by EOS 5D Mark II shooters.
What is Peripheral Illumination Correction all about?
Virtually all camera lenses are designed so that the volume of light transmitted to the image sensor tends to decrease somewhat from the center of the image to the corners. Many different terms are used to describe this phenomenon, such as “vignetting,” “light fall-off,” “unevenness of illumination,” etc. Most experienced photographers are well aware of this common lens performance characteristic, and some take advantage of it for creative effects. Vignetting, for example, has been a popular artistic technique for centuries. It draws attention to a well-lit main subject by darkening the areas surrounding it. Intentional vignetting can be effective in a wide variety of photographic applications, including landscapes, portraiture, and advertising photography to name a few.
However, there are many other shooting situations where uneven peripheral illumination can be very distracting and undesirable. Examples include aerial photography, sports photography, seascapes, and any other kind of composition where consistent, even illumination across the frame is preferred. For these situations, the less falloff there is in an image, the better. Generally speaking, uneven peripheral illumination or light falloff is at its worst at the maximum aperture of the lens, whatever that happens to be. It could be f/1.2 with a fast prime lens, f/2.8 with a professional zoom lens, or even f/5.6 with a consumer-grade zoom lens. In most cases, peripheral illumination is also affected by the distance setting – it gets worse at infinity because the entire coverage of the lens is being used, but it gets better at closer distances because the lens is projecting a larger image towards the image sensor and as a result the sensor is effectively seeing a cropped view. In almost all cases, uneven peripheral illumination quickly diminishes as the aperture of the lens is stopped down. For most high-quality lenses today, uneven peripheral illumination is no longer a concern once the lens is stopped down by a couple of f/stops or more.
Why is Peripheral Illumination Correction desirable?
As previously mentioned, there are many shooting situations where consistent, even illumination from corner to corner is preferred. This is mainly because it’s the way our eyes see, so unevenness often looks unnatural. In other cases, it may be desirable to use the largest aperture on the lens in order to shoot at the fastest possible shutter speed in low light conditions, or perhaps to blur out distracting background details by minimizing depth of field and thereby draw more attention to the main subject. In these cases, stopping down the lens isn’t a satisfactory way to minimize light falloff in the corners. Whatever the reasons for shooting wide open may be, the options for correcting falloff in the resulting images have previously required expensive editing software and the expert knowledge of a professional photographer or retoucher. Canon’s original solution for this problem first appeared in the 2nd Half of 2007 in Version 3.2 of Digital Photo Professional software, or DPP 3.2 for short. With this software, RAW images captured by EOS Digital SLRs with certain EF and EF-S lenses could be corrected for peripheral illumination at the click of a mouse. As welcome as this new feature was at the time, it was still somewhat limiting in the respect that only 29 lenses were originally supported, and it didn’t work on JPEG images.
Fast forward to the 2nd Half of 2008, and there are some major improvements: DPP has evolved to Version 3.5, and 82 lenses are now supported for Peripheral Illumination Correction, including many older models that are no longer being sold. But more importantly, Canon’s newest EOS digital SLRs starting with the 50D, and now the 5D Mark II, have become capable of storing Peripheral Illumination Correction data for up to 40 EF and EF-S lenses, and then applying that data to in-camera JPEGs automatically at framing rates up to 6.3 fps!
How Does Canon’s Peripheral Illumination Correction Work?
At its most basic level, Canon’s Peripheral Illumination Correction function uses a mathematical algorithm to edit image data.To be more specific, the algorithm contains information about the peripheral illumination characteristics of individual Canon lenses.
Variables such as aperture value, focal length and distance settings are taken into account. When using a supported lens, these variables are transmitted in real time from the lens to the camera through the electronic contacts in the lens mount. The new DIGIC 4 image processor has enough power to read the data instantaneously as it changes, and then dynamically apply the Peripheral Illumination Correction algorithm to brighten individual pixels in the resulting JPEG images. The end result is that uneven illumination caused by the optical characteristics of the lens is cancelled out. Peripheral Illumination Correction data (P.I.C. data for short) for 26 of Canon’s most popular lenses is preloaded to the cameras at the factory, and the camera contains enough storage capacity to handle P.I.C. data for up to 40 individual lens models. P.I.C. data for lenses other than the 26 preloaded models can be added or exchanged using the EOS Utility software program supplied with the EOS 50D and 5D Mark II (note that EF-S lenses cannot be used with full frame cameras). The current version of EOS Utility (v2.5) contains P.I.C. data for 82 lenses, and more will be added in future versions of EU software as new lenses are introduced.
Peripheral Illumination Correction is automatically applied to JPEG images when the cameras are set to Full Auto or Creative Auto, or to any of the EOS 50D's Basic Zone exposure modes such as Portrait, Landscape, Close-up, Sports, Night Portrait or Flash-OFF (the EOS 5D Mark II does not have Basic Zones). Both cameras also have a submenu accessed through its 3-inch LCD screen that allows users to activate the Peripheral Illumination Correction function as an optional feature in the camera’s Creative Zone exposure modes, including P, Tv, Av, M, and A-DEP. The default setting is OFF, so it’s essential to activate it in the menu if you want to use it.
The Peripheral Illumination Correction function has no effect on image data when the cameras are set for RAW, sRAW1 or sRAW2 recording modes, but the P.I.C. algorithm can be applied to these images during post-processing in compatible versions of DPP, together with additional image improvements including chromatic aberration correction and distortion control.
Canon’s Peripheral Illumination Correction function gives photographers considerably more creative control over their images than ever before, yet it is virtually effortless to use. If you’re thinking about upgrading to a new digital SLR, be sure to give it a try in either of our new models!
The CDLC contributors are compensated spokespersons and actual users of the Canon products that they promote.
All images are copyright Erika Silverstein
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