Dynamic Range from Dual Pixels

I probably don't give the Canon DSLR users enough love on this site yet. So let me try to make that up a bit. 

My friend Iliah and his team have put together a new utility called DPRSplit. If you shoot the Canon 5DM4 in "dual pixel" mode, the resulting image contains two images. One of those is the usual composite frame that makes up traditional CR2 raw files. The other is made from a set of the sub pixels (what Canon calls Dual Pixel). Because those sub-pixels collect less light than the combination of the dual pixel, you essentially are shooting a second image at -1EV. 

That means that you should be able to recover about a stop more highlight room and increase the overall DR by something close to 1EV. If you can process the two sets of data.

Canon's own software allows you to do refocus using the dual pixels. DPRSplit (beta software at the moment) allows you to extract the two images and create two new DNG files from them that can be processed by any converter or software that allows HDR processing (or image stacking with blending options).

Regular raw capture on right, sub pixel capture on the left. Note the exposure differences here. In the regular raw the bank is exposed in the mid-range and should be easily manipulatable but the highlights are lost. In the sub pixel raw the bank and trees are getting down in the shadows where it's tougher to pull up detail but the highlights are no longer blown out. Image courtesy fastrawviewer.com. 

Now, purists will note that this isn't a perfect bracket set. The regular raw image is made of summing two sub pixels together. If we call the sub pixels X and Y, then raw files contain X+Y values, where DPRSplit will create an additional raw file containing just X. Obviously, there are several issues at play here that make this not exactly the same as just shooting a 0EV and -1EV bracket pair, including low level alignment (the sub pixels get their data from opposite sides of the lens, so there's an implied depth map in the X+Y versus X data). 

Still, DPRSplit makes it convenient and produces a highlight-saving potential for Canon shooters that they shouldn't ignore. It means they can push their ETTR exposure a bit further than they usually do and recover more information.

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