If F is measured in mm, the plate scale in radians per mm is given by p = θ / s = 1 / F
since s = F θ
Plate scale is usually expressed in arc-seconds per mm
Plate scale on JWST FGS/NIRISS
The plate scale of the James Webb Space Telescope component Fine Guidance Sensor and Near Infrared Imager and Slitless Spectrograph is about 0.065 arcsec/pixel. It uses a 2048 x 2048 pixel array with a pixel size of 18 microns a side with a field of view of 2.2' x 2.2'