Assuming human pupil have a radius of 0.25 cm and a comfortable viewing distance of 25 cm. The minimum separation between two objects that human eye can able to  resolve at 500 nm wavelength is      
 

minimum angle to resolve by any optical instrument is $\theta =\frac{1.22\lambda }{D}$

D = diameter of human eye

and minimum distance between two points will be $\mathrm{\theta } x$, where $x$ is the distance between the object and eye.

$\theta x=\frac{1.22\lambda }{D}x =\frac{1.22\times 5\times {10}^{-7}\times 25\times {10}^{-2}}{2\times 0.25\times {10}^{-2}}=30\times {10}^{-6}$