Please explain diffraction in lenses

[jp]

Great discussion!

[Struan Gray]

One way to understand diffraction is to ask yourself why light *shouldn't* spread out after passing through a hole.

Water waves do. Sound waves do. All waves do.

The reason most light rays don't spread out very much is that they have such tiny wavelengths compared to the holes they are usually passing through. Also, light from conventional lamps and discharge tubes is mostly incoherent, so the final effects of any diffraction they do experience are less dramatic. Really coherent light, from a high class laser, diffracts by itself, as the beam width acts as an effective aperture.

Subsurface waves on the underwater thermal/saline interface in the Straits of Gibraltar, diffracting off the Pillars of Hurcules.

[Old-N-Feeble]

That is a very nicely demonstrative image, Struan.

[Struan Gray]

Thanks OnF. Strictly speaking, those are non-linear waves, or solitons. Like a tidal bore, but underwater. Diffraction is the same, even for them.

[rbultman]

Is this why the disks used in soft focus lenses, like the 150mm for the RB, are perforated? You essentially have more sources of diffraction with the disk having the smallest holes providing the fuzziest (softest) image?

[Struan Gray]

That will provide a contribution, although the softening given by diffraction is less than many people imagine, and it tends to be fairly hard-edged.

The soup-strainer discs are a way of reducing the total amount of light, while allowing some rays from the edge of the lens to form the image. You are mixing in aberrations which would normally be eliminated by stopping down.

[N Dhananjay]

Is this why the disks used in soft focus lenses, like the 150mm for the RB, are perforated? You essentially have more sources of diffraction with the disk having the smallest holes providing the fuzziest (softest) image?

No, that is different. Soft focus lenses (at least of those designs) rely on spherical aberrations. Spherical aberrations are reduced as you stop down. So the perforations are ways to allow spherical aberrations at smaller stops. That is, at smaller stops, light is not made up of only light from the center of the lens but also from the periphery (which gives you the spherical aberration).
Cheers, DJ

[rbultman]

Thanks for the explanations.

[N Dhananjay]

Really coherent light, from a high class laser, diffracts by itself, as the beam width acts as an effective aperture.

Thanks for mentioning this. Again, it underscores the fact that diffraction is a fundamental aspect of wave phenomena. If you have waves, you always have the potential for interference effects. It is probably the biggest reason we have to live with wave-particle duality - there is no simple way to explain diffraction with a particle nature (at least, not without recourse to quantum weirdness...:-).

Cheers, DJ

[Jim Andrada]

When I was in college we didn't have to worry about the self diffractive property (or any other property) of lasers because - there weren't any lasers! Damn - makes me feel OLD!!!