DPP4 (and 3!) diffraction correction (Or you don't have to afraid of f/16)

[KeithB]

I was perusing the manual for DPP 4 and came across the "Digital Lens Optimizer" function which

This function enables the resolution of images to be increased by
removing any remaining aberration related to image-forming capability
or any deterioration of resolution resulting from diffraction phenomena.

You actually have to download the correction data to do this. I assume it does a convolution of the data with the theoretical MTF data.
Has anyone tried it? Bob, where do you think it might actually help?

[KeithB]

Also to bring out the fact that this correction is now built into the 1DX Mk2.

[Bob Atkins]

I've not tried it but I will take a look at it if I can find the download.

I'm not sure it can actually correct for diffraction though. You really can't correct for diffraction. Maybe they mean something else, or maybe they just mean they add some sharpening.

Unlike distortion or CA, which distort the information in an image, diffraction reduces the information. You can correct for distortion and CA by "undistorting" the information, but you can't put back information that isn't there in the fist place. Diffraction reduces resolution and you can't get it back. You can sharpen the image to make it LOOK sharper and you can even try to guess at what the missing information might be based on some sort of estimation function, but it's just a guess. The fact that they say you need to set sharpening back to "0" before using digital lens optimization suggests that sharpening is involved.

[Bob Atkins]

I just tried it with some images I recently shot with a 50/1.8. I'd shot these test images of a resolution chart at various apertures for testing out some software, so I was ready to go!

My conclusion is that, as I suspected, it can't and doesn't remove diffraction effects. It just ads a bit of sharpening. Shots at f22 with "digital image optimization" showed lower resolution than uncorrected shots at f16, which in turn showed lower resolution that f11 etc.  f4 is the sharpest aperture of this lens, with or without digital image optimization applied. At f4 without DIO it's sharper than at any other aperture with DIO and much, much sharper than at f16 with DIO.

You can't get around diffraction. There's just no way. If there was, astronomers wouldn't have to build bigger and bigger telescopes to see more detail on bright objects. A tiny backyard 4" reflecting telescope would resolve as much detail on a bright object, like the moon or mars, as the largest telescope in existence. It doesn't work that way.

Digital image optimization is just a fancy term for a process that corrects as many of the image aberrations as possible and ads a bit of sharpening to try to mitigate the effects of diffraction softening, but the effect on resolution is minimal.

Why do Canon talk about diffraction? Probably a mixture of marketing hype and Japanese-English translation!