[gpm]

And it's also ok if the pixels-per-degree (PPD) towards the edges is lower.

Which makes PPD analysis based on counting pixels suspect. Good optics can cheat by moving pixels from the peripheral field of view to the central field of view.

Simulavr claims to go from 24.48 PPD (pixels / field of view) to 35.5 PPD (actual pixels per degree near the centre) by doing this.

They also have a great blogpost on it: https://simulavr.com/blog/ppd-optics/