So image circle is the lens coverage before light falloff occurs, therefore I will not need a center filter for my Rodenstock Grandagon-N 90 mm f 6.8 lens (image circle 221 mm) on a 5 x 7 camera. Thankk you for the information. I guess someone just wanted to sell me a center filter.
NONONONO!
The image circle is the lens coverage before the image
sharpness decreases to below some level decided upon by the lens manufacturer, NOT the point at where light fall-off becomes noticeable!
I've posted (repeatedly) an image shot with a 90mm f:8 Super Angulon on 13x18cm film, where the light falloff is "camouflaged" in the dark valley sides. And (also repeatedly) a link to a page where I use a 90mm f:6.8
Angulon on 13x18cm film, showing both the extent of the image circle, the circle of illumination, and the degree of light falloff. The plain old Angulon shows no vignetting, but severe falloff. The newer 90/8 SA shows far less falloff, but vignettes
sharply at the end of the image circle.
With some scenes you can easily get away with not using a center filter on a 90mm SA on 5x7" film. With other scenes it will be a huge improvement.