The main issues are pinhole geometry and diffraction, not lens-like aberrations.

For a short pinhole-to-film distance, you generally need a very small hole to keep the image sharp. But if the hole is too small, diffraction softens the image, so there is an optimum size rather than “smaller is always better.”

Another important factor is the thickness of the material at the pinhole. With a short focal length, rays to the edges hit the pinhole at steeper angles. If the pinhole is made in thick material, the sides of the hole can block those rays, causing mechanical vignetting. That’s why pinholes are usually made in very thin metal or foil.

The increased path length to the edges mainly causes light falloff, not optical aberrations. A pinhole camera has no lens elements, so you don’t get normal lens aberrations from glass. The softness you see is more likely from the pinhole being too large, too small, or too thick-walled.

So the key challenges are:

• making a very small, clean pinhole
• using very thin, opaque material
• balancing sharpness against diffraction
• managing edge falloff/vignetting