I follow you here and yes, it does make sense... less width blur between light and dark areas - sharper lens, simple!rjlittlefield wrote: ↑Wed Mar 29, 2023 3:24 pm...That original thing will have one constant brightness value on one side of the edge, and a different constant brightness value on the other side of the edge, with a sudden transition between those two brightnesses.
... Note that the blur width is a constant value, despite that the first edge has high contrast and the second edge has low contrast.
Very roughly speaking, what MTF Mapper does is to look around and find the values of locally constant dark and light on opposite sides of edges, and then it measures the width of the blurred edge that transitions between those values. Sharp lenses will give narrow blurs, soft lenses will give wide blurs. The MTF50 value that is reported by MTF Mapper is an indication of the width of the blur.
So, the answer to your question is that all the information comes from the image. MTF50 is a measure of how quickly the pixel values change from A to B, not how much different the values of A and B are.
Make sense?
--Rik
After about an hour mediation on this "width blur" I somehow am not convinced that width blur between the dark and light areas is the whole story...
In your example blur width is the same, but if I shoot backlit target and one lens is capable of image which is close to black and white (high contrast) as it should be with proper exposure without clipping (top figure in your example) and the other record some gray much (bottom one, low contrast) then I do not see how these lenses are the same.
If only the blur width matter there will be no need for black and white target or proper exposure, or evenly lit background...
I will think more about that and run some tests...