I am trying to put together a system of lenses that has a probe with a 2 mm diameter. The system has a probe with an objective and a relay and a set of lenses with a larger diameter that function as a microscope, magnifying the image generated by the objective. The attached Figure 1 gives a rough description of the system. The problem is that the generated useful image is too small, as seen in Figure 2. I thought that this problem would be solved by selecting appropriate field lenses that would image the exit pupil of the previous lenses set onto the entrance pupil of the next, but it seems it is not to be the cause of the issue. I calculated (paraxially) the focal length of the field lenses considering the positions of the entrance and exit pupils and tried a few field lenses based on that result with no success. What can be done to increase the size of the useful image? Ideally, the size of the useful image should be as marked with a red circle in Figure 2.