The perfect lens does not exist. Chromatic and geometric aberrations distort images. Software corrects them during shooting or later in post-production.
Chromatic and Geometric Aberrations
Which lens is flawless? No one. When we look at how it works, two types of problems appear: chromatic aberrations and geometric aberrations. Chromatic aberrations cause colored fringes, especially visible in the corners of the image. They are caused by the dispersion of the refracted white light and its decomposition into several colors. Geometric aberrations create a distortion of the image, most often a type of barrel (convex appearance) or pincushion (concave appearance).
Sometimes the distortion takes the form of a mustache, with a barrel distortion in the center and a pincushion distortion on the edges. Geometric aberrations are more frequent on zooms than on fixed focal lengths: barrel with the wide angle to pincushion toward the telephoto position. To these defects, add the vignetting that darkens the edges of the image, especially visible at a wide aperture.
The ideal is a lens without aberration or vignetting. But its price would be stratospheric. The manufacture of a lens is a compromise between the ideal and what can be effectively corrected through software image processing. The aberrations are corrected at the time of shooting or later in post-production through the modeling of optical aberrations. The programs apply specific corrections for each fixed focal length (or different focal lengths if it is a zoom), for each aperture, according to the focusing distance. They automatically remove unwanted color fringes and distortions.
Over the last twenty years, lens and camera manufacturers have gradually included corrections in the JPEG and Raw files. When you shoot in JPEG format, the image is automatically corrected. In Raw, it retains its aberrations, but the integrated profile makes it possible to display the corrected image. Olympus and Panasonic, in 4:3 format, include a lens correction profile with the Raw file.
In post-production, most image processing software offer correction profiles for each combination of the camera and lens. DXO has been integrating corrections adapted to camera-lens pairs since 2004. Updates are regular and performance is optimized. Competitors have followed, including Adobe which integrated in 2010 an automatic optical correction in Camera Raw 6.1 and in Lightroom 3. Capture One 7 added it in 2012.
Correct With Subtlety
Automatic corrections eliminate chromatic aberrations, distortions and vignetting. The first should be systematically applied, because fringes are most often annoying, but distortions and vignetting do not necessarily degrade the image. A subject without a straight line (a portrait, for example) can remain as is. And vignetting can darken the corners of the image with success. It’s a matter of personal judgment.
Built-in Profiles and Post-production
When the cameras’ built-in profiles for the Raw format (Olympus and Panasonic in 4:3) are recognized by Lightroom and Camera Raw, Adobe does not provide a custom profile. In the lens correction panel, the “i” information warns that the built-in profile is applied. It is the same with the Fujifilm X or Leica Q Raw. This does not prevent you from exploring the possibilities of adjustment in manual mode if the built-in profile shows a little distortion. If Lightroom warns “Unable to find a matching profile automatically” (this message is sometimes displayed), the profile is in fact applied.
DXO does not take into account the built-in profiles of the cameras. It applies its own profiles, which are better than the built-in profiles. DXO can show the uncorrected image. We then see the extreme distortions of short focal lengths in 4:3 and the inevitable cropping due to geometric rectification.
Discover the photography courses at Spéos
Spéos offers various training courses ranging from simple one-week photography courses (initiation and advanced level) to 3-year courses. The long courses to become professional photographers allow you not only to master all the photographic techniques and its vocabulary (blurs, hyperfocus, sharpness zone, depth of field, backlighting, focal length, shutter release, autofocus, wide-angle, rule of thirds, etc.), but also all the stages of shooting and image processing.
Visiting the school allows you to discover the premises, the studios and the equipment, and is undoubtedly the best way to familiarize yourself with your future way of working. This is why, in addition to the open days, Spéos offers throughout the year personalized visits by appointment to come and discover the school with a member of the team.
Without a correction profile, the image can show a very pronounced distortion, especially with wide-angle lenses. With a proper profile, the lines are square. DXO profile from DXO PhotoLab 6. Olympus EM5, 12 mm f/2 lens.
Text and photos: Philippe Bachelier, teacher of Printing techniques at Spéos