DxOMark image quality testing protocols for lenses and sensors
Testing protocols for distortion, LCA, and vignetting
We measure Lateral Chromatic Aberration and distortion on the DxO Labs dot chart, which is a pattern of regularly distributed black dots on a glass support. We chose glass because it provides the flatness and shape stability necessary for these measurements. The dots printed on the chart are circular and perfectly aligned, forming a grid.
Measuring vignetting (the darkening of an image near its corners) requires using the white background of the same dot chart. Before shooting, we align the camera sensor on the target plane and carefully check the uniformity of the lighting to ensure that it remains within the +/-4% range. To further enhance the accuracy of the vignetting measurement, we characterized the actual illumination uniformity of the chart using a calibrated camera-lens couple. We also set the color temperature to 5500K (corresponding to daylight).
We take a picture for each focal length and aperture. The camera remains at the same shooting distance so that the same chart area is framed, which means that the chart illumination is exactly the same for every focal length and aperture.
Finally, we take two additional pictures for each focal length at two different distances, with the lens focused at infinity, in order to calculate the Effective Focal length (EFL).