Interest has been expressed in the distribution of color (luminance and chromaticity) across the sensor frame when a Color Parrot white balance measurement diffuser is in place, illuminated by a certain incident illumination.
Because of the inhomogeneity of the diffuser (its distinct "central" and "annular" stacks), we might imagine that to be affected by such matters as lens aperture, focal length, and focus distance setting. We recently ran some tests to look into that. The camera was a Canon EOS 20D, equipped with a Canon EF 24-105mm f/4 IS lens.
In order to give some insight into the contributions of the central and annular regions of the diffuser to the pattern, we ran comparable tests with the entire diffuser face active, only the central "spot" area active, and only the annular region surrounding the spot active.
In these tests, the diffuser was exposed to our favorite "daylight filtered through a white venetian blind".
In each case, the exposure was metered (in Av mode) to give an essentially consistent photometric exposure at the center of the frame (EC was +1.0 Ev unit).
The following parameters were varied among these specific values:
- Diffuser active area: full face; central spot only; annulus only
- Lens aperture: f/4; f/8
- Lens foal length: 24mm; 105mm
- Lens focus setting: infinity; closest focus
Al 24 combinations of these variations were tested.
Here, for ease in visual comparison, we see all 24 frames in a montage, labeled with the parameters in effect:
Note that because of the "uniform photometric exposure" situation, we cannot judge directly the relative contributions, at any place across the frame, of light from the central and annular regions. I am contriving another test that will allow that to be most easily visualized. (There seems to be some anomaly in the exposure of the 7th image in the first row. We will be looking into that. It's so hard to get good laboratory technical aides anymore.)
However, based on other testing, it seems likely that in most of these situations, the light on the center of the frame comes preponderantly from the central region.
Linear Mode
