Vignetting or light shading consists of the attenuation of light
far from the image center. There is a natural cause of
vignetting: An object that is off-axis “sees” the aperture with
a smaller solid angle (the apparent surface of the aperture stop
as seen from the object’s position). For large-aperture lenses,
another phenomenon also impacts the illumination of the sensor
in the image. In these cases, not all rays from an off-axis
element that go through the stop of the lens will reach the
sensor. Some of them will be blocked by mechanical elements
inside the lens. DxOMark measures light vignetting by imaging an
evenly-lit white target. Pixel values are retrieved from all
parts of the image. The ratio of these values over the maximum
value (usually the value at the image center) found in the image
gives the vignetting measurement for each pixel. Vignetting is
expressed in exposure value (EV): a variation of 1EV is an
attenuation of a factor of 2. We create a field map of the
vignetting as well as an average