I wanted to see how F-Stop and the zoom of the lens effected light
falloff. I did the following test below.
Using my Nikon CoolPix 995 digital camera
I took a piece of velum and placed it in front of the lens.
Evenly light the velum by bouncing light off the ceiling onto it.
Took two pictures, one rotated 90° from the other.
Averaged the two pictures.
Made a duplicate layer, flipped horizontal, averaged and merged with original.
Made a duplicate layer, flipped vertical, averaged and merged with original.
cropped a rectangle from the center to the corner. 100 pixels wide by 1222 pixels long. (smaller re-sampled version below)
Resized image to 1 pixel by 1222.
Made an image of the Histogram.
To read the values for the image I ran a gaussian blur to average out the values. The image with the light falloff correction did not have any blurring.
Note: I also took some images with my FC-e8 fisheye adapter but I do not think I evenly light the velum. It is much harder to get the velum to cover the entire lens and have it evenly light. I will try again, maybe with some sort of lamp shade.
center to shortest
center to corner.
Center - corner
|Made levels adjustment to both images, to emphasize the differences
Std Dev: 7.17
Center to Corner: 25
Std Dev: 0.72
Center to Corner 3
|Film camera. 28-90 mm lens @ 28mm with UV filter on front. Film scanned|
Adjusting levels elaborate the light falloff
PanoTools | Correct | Radial Luminance
R, G, & B = 35
Trying to eliminate the vignetting has caused the center to go too dark.
|PanoTools | Correct | Radial Luminance
R, G, & B = 12
The Light Falloff eliminated but the vignetting remains.
Page last modified April 18, 2006