Diffraction lenses, any thoughts?

[jamaeolus]

I read this site regularly and this popped up. Some on here might be interested.


https://theconversation.com/a-new-thin-lensed-telescope-design-could-far-surpass-james-webb-goodbye-mirrors-hello-diffractive-lenses-206055

[RokkorDoctor]

I think someone got confused as to the difference between Fresnel Diffraction and Fresnel Lenses when describing the original Fresnel lenses as diffraction lenses.

Fresnel lenses use conventional refraction as a way to form an image. Diffraction is a different optics/physics phenomenon.

If you were to make the spacing of the Fresnel rings in a Fresnel lens small enough (approaching the wavelength of light), then you would see diffraction effects appearing.

This sounds simply like a more intelligently constructed Fresnel refraction lens with and additional etched diffraction grating/pattern on the refractive surfaces. Thus it looks like they combine the effects of refraction and diffraction to improve the imaging quality of a Fresnel refractive lens.

[Lightshow]

I don't know how stable a Fresnel lens would be in space.
Also I very much doubt it would outperform JWST in mid-IR, the lens would be glowing at those wavelengths, which is why the mirrors are coated in gold.

[kathala]

This has been done commercially by Canon for twenty-odd years, sometimes quite decently (4oo 4 DO II), sometimes not (all others).

I am staggered that "an astronomer who studies astrobiology" should be "co-leading a team that is developing a new kind of space telescope". No wonder there were so many science sceptics during covid.
I am a professional photographer - maybe I should build my own little nuclear power plant to recharge my cameras, or at least co-lead a team of astrobiologists to build it.

[papasito]

A famous photografer in Spain said that he uses Canon 70-200 IF lens because at f/11 or 16 it has not Diffraction problems at all.

Can be possible?

[visualopsins]

[kathala]

[calvin83]

[stevemark]

[RokkorDoctor]

Re. the above discussion;

The theoretical discussions re. diffraction-limited resolution usually only consider the the Airy-disk diffraction pattern of a round opening in relation to the circle-of-confusion size. However, in digital photography diffraction-limited sharpness is affected by a number of factors, not just the aperture size:

- aperture size
- aperture shape (e.g. hexagonal, round(ed), star-shaped etc.), which affects the diffraction pattern observed (Airy disk pattern, converging diffraction spikes, diverging diffraction flares etc.)
- The convolution between the particular diffraction pattern and the mathematical complex modulation transfer functions of the anti-aliasing filter & Bayer filter (which themselves are also related to pixel size of the sensor).

E.g. on FF, at f/64 everyone will notice diffraction softening regardless of lens or camera sensor used, but in the "grey" area (say, f/8 to f/16) it depends on the particular lens and filter/sensor combination used. For some combinations of aperture shape and filter/sensor it may become noticeable at f/8, for others not until maybe f/16. And that is assuming an ideal lens without any other lens flaws that may mask the onset of diffraction softening to some extent.

EDIT, incidentally, at a relatively simple level, CD players use a diffraction grating to create a very sharply defined pattern of laser light peaks; depending on the configuration of the photodiodes that read the reflected light different diffraction gratings are used; some players just create a central spike for both data readout and tracking (Phillips CDM transports), others create a pattern of three spikes (one central for reading data, two adjacent for tracking).
The reason I mention this is that this neatly illustrates how diffraction patterns and sensor patterns can interact in terms of readout resolution.