When originally conceived, the FSQ-85EDX was meant for use with APS-C or smaller CCD or CMOS chips. At image circle radii beyond about 14 mm, field curvature was evident in larger chips. This new flattener will greatly reduce this field curvature, resulting in a practically flat field out to a 22 mm image circle radios. The two spot diagrams below demonstrate the impact of the flattener on field curvature.
Spot Diagram of FSQ-85EDX Without Flattener
Spot Diagram of FSQ-85EDX with Flattener