Ideally, a lens will render straight lines as straight. Reality is that many lenses are not perfect and straight lines are rendered with a curve. The amount of curve can be vastly different from lens to lens and from focal length to focal length within the same lens.
Lens distortion is most noticable when straight lines run parallel to and near the edge of the frame. The custom checkerboard distortion test chart provides those lines for a worst-case result.
Pictures are worth thousands of words. Following are four distortion examples - use the links below the image to see the designated distortion type.
Barrel distortion appears as a bulge in the center of the image. Pincushion distortion is the opposite effect. Mustache or wave distortion combines barrel distortion in the center with pincushion distortion in the corners - creating an old-fashion mustache shape in the lines that follow the edge of the top of the frame.
Some types of photography can tolerate distortion better than others. Landscape photographers shooting an ocean scene will not be happy with a non-straight horizon. Architect photographers will not accept curved buildings in their photos - nor will their clients. Even framing a scene level is difficult with distortion in the viewfinder.
Other types of photography are far more forgiving of a distorted view. If there are no straight lines in the frame, it may be difficult to see any effect of the distortion in your images. People have no straight lines - and may even appreciate a little thinning effect from pincushion distortion.
Distortion Target Alignment
Because the checkerboard target is so unforgiving, the slightest framing error becomes visible. Rotational alignment is really hard to get perfect - especially when looking through a distorted lens - so you might see some slight variation in this regard. Target alignment is laser-controlled.
The primary checkerboard distortion target measures 47.25" x 31.5" (1200mm x 800mm). Focal lengths over 460mm may be tested on a smaller-sized target.
Lens Distortion Can Be Corrected Using Software
Software can be used to straighten your lines - removing the distortion from the image (though wave distortion is harder to correct). But, this correction is a destructive process. You lose image quality during the process of remapping the pixels in the image. A distortion free lens delivers a better final image.
RAW-captured images are processed to black and white (removes CA effects) with a very high contrast setting. An f/8 aperture is used for all samples.
What Is The Small Gray Frame?
The small gray frame seen in full frame lens results represents the APS-C/1.6x Field of View Crop Factor (FOVCF) found in many of Canon's Digital SLR Cameras. Nikon's APS-C sensors have a 1.5x (or 1.55x?) FOVCF, so the APS-C frame is slightly tight for the full frame Nikon lenses. If you are using one of the the APS-C DSLR camera bodies (and don't plan to move to a full frame body in the near future), you can disregard the distortion results outside of the gray frame.
Back to the Lens Distortion Comparisons