What's the difference between z_i and z_s? Looking at the diagram, I would have thought that z_i is the diagram focus because that's where the lines converge.

jshergill426

Z_I is the distance where the Blur Circle of Diameter C is in the focused object plane, while Z_S is the distance where the same Blur Circle is an unfocused virtual image.
https://en.wikipedia.org/wiki/Circle_of_confusion

What's the difference between z_i and z_s? Looking at the diagram, I would have thought that z_i is the diagram focus because that's where the lines converge.

Z_I is the distance where the Blur Circle of Diameter C is in the focused object plane, while Z_S is the distance where the same Blur Circle is an unfocused virtual image. https://en.wikipedia.org/wiki/Circle_of_confusion