My ASA N12 has fairly bad vignetting in the corners , about 45- 50% illumination. Under vary transparent skies the images appear well corrected by flat field images. However, when the transparency falls the images often show a radial effect. This includes some really good night not bad clouds.
Essentially the corners brighten or darken differentially compared to the center of the images as transparency changes. Anyhow, This is my interpreption of the effect.
Is there any mathematic explaination as to why flat field correction might be effected by transparency?
If so, are there any image processing approachs that could help?
The effect is often quit small and the contrast is still excellent. I would use these images but they often produce a uneven background if enough are add to a integration.
Max
