When you receive your 28-20-46-000 20X HR Lens from Excelitas Technologies, you may find that the images lose sharpness towards the corners of the lens.
As confirmed by Excelitas, the 28-20-46-000 20X HR lens is designed to have diffraction limited performance (sharp focus) only near the center of the field of view. If the user start moving towards the corner of the field of view the lens will start to be less sharp. This is a physical limit of the design of this lens.
28-21-11-001 can be considered as an alternative. It has a smaller numerical aperture, but is flat field corrected to ensure the user gets good performance to the corners of its field of view.
