No lens sees forever. Two stars, two cells, two anything — bring them close enough and every optical system blurs them into a single blob it can never split, no matter how you sharpen. The wall is set by the wavelength and the aperture (Rayleigh’s criterion). Slide the two points together and watch the twin peaks merge into one at the limit.
A point source images not to a point but to an Airy disk of angular radius ≈1.22λ/D (wavelength λ, aperture D). Two sources are ‘resolved’ only if their separation exceeds this — Rayleigh’s criterion: the peak of one Airy pattern falls on the first null of the other, leaving a ~26% central DIP. The instrument sums two point-spread functions and checks for that dip; a fail-loud self-check throws unless a wide pair shows a dip (two peaks) and a sub-limit pair shows none (merged to one). Below the wall the information that there were TWO is simply gone from the image. ◆ real optics, node-verified.
A Gaussian stands in for the Airy PSF (same resolvability behaviour, simpler tails); the exact 1.22λ/D constant is the circular-aperture Rayleigh limit. Super-resolution methods exist but need extra priors or data — they don’t un-blur a single frame for free.