If focal length F is measured in mm, the plate scale in radians per mm is given by p=θ / s = 1 / F, since s=Fθ.
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' 
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