I started to read CTEIN book and at first page , there was an idea. He wrote square wave light is sharper.
Is there a square wave light source or a lens transforms the curvey psf to square.
This is possible with Radar.
Wrong.
1) It's on page 3.
2) ctein actually wrote:
I’ll spare you the math, which is beyond the
scope of this book (it involves Fourier transforms,
if you want to work it out for yourself ). The difference
between a sharp edge and a fuzzy one (a
square wave and a sine wave) corresponds to a
difference in spatial detail three times finer than
the line spacing.
3) There is no mention of square wave light.
4) What is discussed is square wave edges in images. That means where the image (negative) presents an abrupt sudden change from light to dark or the reverse.
5) Since light is simply electromagnetic radiation, square waveform electromagnetic radiation would be possible, but it would not look like light, since only the fundamental frequency would be visible. The rest of the harmonics needed to actually produce a square wave would be up in the ultraviolet and x-ray radiation regions.
6) Light is a narrow band of wavelengths, basically from 400 nanometers (nm) to 700 nm. You need to comprehend this fundamental fact of great importance to understanding photography.
7) A study of the waveform harmonic characteristics of square waveforms shows that they are composed of a harmonic series of the odd integer harmonics. That means frequencies 3, 5, 7, ... times the fundamental frequency. When this is combined with point (6) above you have the reason for point (5).