Canon’s Dual Pixel technology is fairly common today, found on many of the company’s DSLRs. It splits every pixel into two halves which enables phase detection autofocus (PDAF) on the sensor itself. In general, on-chip autofocus relies on the slower contrast detection method, which limits performance in live view (including when shooting video).
As it turns out, fast AF performance is not the only thing that can be accomplished by splitting the pixels into two. Imaging-Resource published an in-depth look at how the new Dual Pixel RAW on the 5D Mark IV enables a tiny amount of post-production magic. Because the pixels “know” when something is out of focus, that data can be manipulated later to slightly refocus an image. You can think of it like Lytro’s computational photography, only on a much, much more limited level.
In fact, Canon was careful not to use the word “focus” in its description of the new technology at all. In the press release, it describes the feature rather vaguely, calling it an “in-camera Digital Lens Optimizer during JPEG shooting and Diffraction Correction technologies.” It is followed up by a four-sentence footnote warning about the system’s limitations.
There’s a good reason for Canon’s caution here. Dual Pixel RAW can only make micro-adjustments, which will mostly only be visible in macro photographs or at very wide apertures. It won’t be helpful in every situation, but may occasionally be able to turn a rejected photo into a keeper by fine-tuning focus on a subject’s eyes or other important details that were off by just a small margin in the original exposure. Dual Pixel RAW can also make small bokeh-shift adjustments and reduce ghosting.
The feature is currently only compatible with Canon’s Digital Photo Professional (DPP) software, although presumably Adobe could build in support for it in future versions of Lightroom. It’s also not turned on in the camera by default, and doing so will result in larger file sizes, about 72 megabytes per image. For those critical shots, however, that is probably a small price to pay.