Vignetting analysis

This feature requires the Analyze-License

After selecting a f-stop in the tree view a vignetting analysis view appears on the right side:

Vignettierungs-Analyse

In the upper part you can see the graph "Vignetting profile", which shows the Standard illumination fall-off behaviour in [ % ], dependent on the distance to the vignetting center.

In addition the following buttons are available:

Export View ...: Saves the displayed diagram as a Tiff-file with a resolution of 1600 * 1200 pixel.
Vignetting profile ...: Saves all vignetting profiles as ssv-file, which can be imported into any conventional spreadsheet application.
See below for further information.
Close: Closes the analysis window.

The two check boxes Fixed Range and Smooth Curve serve the same purpose as they do the distortion analysis context (see section Distortion analysis).


Display the vignetting as profile

Vignetting profile

The curve represents the standard illumination fall-off (IFOStandard) in [%], dependent on the distance to the vignetting center (on the original, distorted image).

IFOStandard considers the circumstance, that the strength of the vignetting effect also depends on the subject brightness B (beneath the captured parameters focal length and aperture). The relationship between brightness and illumination fall-off is determined by many factors, among others to what extent contrast and brightness were preadjusted by the camera.

Therefore this dependence is approximated by the following curve:

Dependence between brightness and illumination fall-off

IFO = IFOStandard * (B / 25 - B² / 2500) for B < 50%
IFO = IFOStandard for B >= 50%

According to this approximation the affected image area ought to be brightened up by the amount of IFOStandard for a brightness greater than 50%, in order to compensate the vignetting effect. For e.g. black (brightness = 0%) the area would not have to be brightened up.

Note that the illumination fall-off always refers to the white level, and not to the examined grey value. If the color depth is 8 bit for example, the correction factor, by which the grey value finally must be multiplied, can be determined as follows:
Factor = 1 + (IFO * 255) / (100% * B)

Example for a color depth of 8 bit per channel (RGB):

For a pixel (RGB value: 110, 0, 43), which has a distance of 1100 to the vignetting center (on the distorted image), we want to know the adjusted brightness value for each color channel.

The standard illumination fall-off for this distance can be read off from the above vignetting profile and amounts to about 6%.

Dependence between brightness and illumination fall-off

The 8 bit pixel brightness value is calculated to
(110 + 0 + 82) / 3 = 64

... and its relative brightness (at a maximal brightness = 255) thus amounts
64 / 255 * 100% ~ 25%

Based on the approximation expression (also see above curve) for B < 50% thus the illumination fall-off can be calculated in percent to
IFO = 6% * (25 / 25 - 25² / 2500) = 4.5%

The correction factor results in:
1 + (4.5% * 255) / (100% * 64) = 1.1793

The adjusted pixel can be determined by multiplying each color channel with the correction factor:
R = 110 * 1.1793 ~ 130
G = 0 * 1.1793 = 0
B = 43 * 1.1793 ~ 51

All precise standard illumination fall-off values for all focal lengths and all f-stops can be found in the ssv-table "Vignetting profile ...".


Save vignetting profile in ssv-table

Note: Please read in the section "Save distortion profile in ssv-table", how to import ssv-tables into OpenOffice or Excel!

Use the button "Vignetting profile ..." to save the complete vignetting data from the current Calibration-Set in a ssv-table (semicolon separated value).

The contained standard illumination fall-off values are grouped by focal length and f-stop and are indicated for 24 distances from the vignetting center.