In-camera lens corrections

Digital technology provides the ability to adjust images in-camera or on a computer after capture. One area in which this can prove useful is in correcting for shortcomings and quirks in optical performance.

When lenses are designed, optical physics dictates what is possible, along with the price point of the lens and the complexities of manufacturing. In any case, it is fundamentally impossible to make a perfect lens – so, based on its design, every lens will exhibit a greater or lesser degree of optical irregularities. Typically, these manifest themselves as vignetting, where the corners of an image are slightly darker than the centre as a result of light fall-off, and chromatic aberration or colour fringing along high-contrast edges, where the lens has been unable to focus different colours or wavelengths of light to precisely the same point.

However, every design of lens has its own characteristic quirks and flaws. By mapping the performance of each specific camera and lens at a variety of focal lengths, focus distances and apertures, it's possible to correct for these irregularities and bring the lens performance closer to perfection.

These corrections were first made available in Canon's Digital Photo Professional (DPP) software, but the increased processing power of cameras has made it possible to carry out corrections in-camera as the images are captured if you're shooting JPEGs or during RAW processing in-camera if you're shooting RAW. You simply switch each correction on in the camera's Lens aberration correction menu.

In 2008 the EOS 5D Mark II and EOS 50D introduced Peripheral Illumination Correction. This adjusts images as they are shot on the camera to correct for vignetting or corner shading, making the brightness more even across the scene.
This correction is designed to work with Canon lenses. If you're using a non-Canon lens, it can result in unwanted artefacts, in which case it's best to turn off Peripheral Illumination Correction.

In 2012 the EOS-1D X and EOS 5D Mark III introduced Chromatic Aberration Correction to remove colour fringing and haloing around high-contrast edges. It improves the overall image quality and maximises the performance of Canon lenses.

Diffraction is the bending of the light path as it passes through the lens. It's an issue at small apertures because a greater proportion of the light is bent (in comparison to shooting at wide apertures), resulting in reduced image sharpness. This is frustrating, because typically you'll select a small aperture in order to try to extend the depth-of-field and achieve maximum sharpness across the image, and yet perversely diffraction will be reducing sharpness. Landscape photographers often need to find a compromise between these two conflicting optical phenomena.

In 2016, the EOS-1D X Mark II introduced Diffraction Correction, which compensates for the loss of sharpness at small apertures. It also compensates for the slight reduction in resolution that can occur as a result of the presence of a low-pass filter in the sensor. The low-pass filter is important for avoiding moiré interference in images with fine repeating patterns.

Initially, distortion correction data was stored on the camera for some lenses, and if you wanted to use other lenses the optical correction data for those lenses had to be downloaded via the EOS Utility software and registered to the camera. However, since the introduction of the EOS 5DS and the EF 11-24mm f/4L USM lens in 2015, this data has been stored in the lens itself. This means that the camera can access the lens data and apply it when processing JPEGs in-camera. If you're shooting RAW, however, you will still need to download the appropriate lens profile on your computer.

Canon first introduced its Digital Lens Optimizer (DLO) as a function in Digital Photo Professional (DPP). It is able to correct a wide variety of aberrations including coma, astigmatism, sagittal halo, field curvature and spherical curvature, which could not normally be addressed.

The EOS-1D X Mark II featured the Digital Lens Optimizer as an in-camera post-shot editing function. However, in 2016, DLO was introduced as a shooting feature with the launch of the EOS 5D Mark IV.

In-camera, DLO applies its corrections to JPEG files. When it's activated, Chromatic Aberration and Diffraction Correction are not selectable within the main menu, but are part of the DLO corrections. It is also possible to turn off Peripheral Illumination Correction if, for example, you want the framing effect of vignetting in a portrait.

All EOS R System cameras use lens correction (and other) data that's stored in the lens. Thanks to the super-fast communication speed of the RF mount, particularly when you're using RF lenses, shooting with DLO turned on does not reduce the maximum continuous shooting rate, as it did in earlier cameras such as the EOS 5D Mark IV.