Adjusting for field curvature

[Jonathan Leavitt]

When you focus on the center of a flat test pattern using a lens that has noticeable field curvature, the measured sharpness always tapers off from the center because it is going progressively out of focus towards the edges. You would need to re-focus the lens at various points from the center and somehow incorporate those results into the data. How can one do that?

For example the attached Zeiss Biotar does not improve at the edges between F2 and F8. I assume this is field curvature, because stopping down the lens ought to have more effect on the edges.

[Frans van den Bergh]

Hi Jonathan,

The MTF Mapper GUI is not really well suited to a workflow that requires combining multiple result sets from multiple images into a single output. Keep in mind that the GUI is really calling the command-line MTF Mapper in the background.

I know that other MTF Mapper users have performed analyses like you are interested in, e.g., you capture a sequence of through-focus images and then extract the best result from each part of the field to build a composite result. This requires some scripting, but I think something like ChatGPT might be able to help you write a suitable script. The way I would go about it is to run the command-line version of MTF Mapper with the "-v 2 -q" options (amongst others), and extract the results from the "edge_mtf_values.txt" file. You can then simply grid the sensor into rectangular regions to create a low-resolution "image", and keep the best MTF50 value for each of the Sagittal and Tangential directions for each of these low-resolution cells. If you process a through-focus set of images, the result should be the "what if I focused in this region of the image" output you are looking for.

I'd be happy to help answer any further questions, but I do not have any plans to add support to the MTF Mapper GUI for combining results across multiple images.