I was thinking why canon 5d mark iv still need anti allies filter even though its near to the Nikon 810 resolution. But recently I found there is a feature in Canon 5D mark iv call Diffraction Correction. I'm not so sure what it does. In some places it mentions, this feature correct the degraded resolution due to the low-pass filter. Any one has idea?


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Canon's Diffraction Correction is a camera + lens specific correction that uses a profile of the lens used to ameliorate the effects of diffraction caused by apertures narrower than the Diffractive Limited Aperture for a particular camera/sensor. It was first introduced as part of Canon's Digital Photo Professional raw conversion and image processing application. A couple of Canon's most recent upper tier EOS bodies include Diffraction Correction as an in-camera option.

The correction uses a profile of both the camera and the lens used and as such does take into account the effect of the specific low pass filter in each camera model for which Diffraction Correction is offered. But it also takes the characteristics of the lens into consideration. The shape and effect of the lens' aperture blades are particularly influential in how diffraction is manifested by a lens. Using a very detailed profile of the lens in question allows the effects of diffraction to be corrected to a remarkable extent. Some of the lens profiles used by the Digital Lens Optimization module of DPP are over 100MB in size!

Using it in camera does introduce at least a couple of caveats: the in-camera processing will take longer and use more power which will both reduce the camera's burst rate and sustained continuous frame rate capacity as well as reduce the number of images that can be captured on a fully charged battery. The in-camera version may also not be as detailed as the Diffraction Correction as applied to raw files using the DLO module of DPP. The number of lenses for which profiles are available is also fairly limited at this time to mostly higher end Canon lenses. Even many of the "L" series of lenses do not currently have available the highly detailed profiles used by these tools. (There are much less detailed profiles available for nearly every Canon EF lens for use with in-camera or DPP correction of CA, distortion, and peripheral illumination.)


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