Single lenses cannot produce a flat field image. ( see https://en.wikipedia.org/wiki/Petzval_field_curvature ) Since we're using aspheric singlets for scanning, we can cheat by re-focusing during the scanning motion to absorb this error.

This makes it possible to get a sufficiently balanced spot size across the whole scanning motion

This OSLO PSF simulation has been performed to illustrate the spot shape over lens displacement. As the object distance (LD-Lens) is around 4 mm and the image distance roughly 40mm +/-0.3mm equals a 6mm scan line length which means that with a 3.3mm diameter LD every 5mm, sub-25µm spot size is achievable. If one emitter fails, the stroke can be increased to +/-0.5mm, covering the now empty segment with the neighbouring scanlines.