OT - Flash Photography

[dennis]

No its because you asked for an answer that no one else can give. If you can't work out why then its pointless me trying.

[Fredxxx]

That just shows you have no idea what determines the diffraction limit of a lens.

[dennis]

Well the airy disk refers to the size when focused at infinity, it varies with focal distance not focal length. So I know more than you.

its about 7 microm at infinity and somewhat larger at general photographic distances as you don't appear to know.

I will ignore the fact that it varies with wavelength as well.

[Fredxxx]

You haven't a clue. It is dependent on f-number and the wavelength of light of interest.

Please feel fee to ignore other facts as well and introduce some erroneous ones as well.

[Martin Brown]

Actually the lenses have improved a lot, but the thing that has set them free is that it is now possible to use additional degrees of freedom that were never allowed in a film camera. Namely that the red, green and blue images no longer need to have exactly the same linear magnification and the resulting image can be resampled in software during the demosaic stage. This has allowed much wider zoom ranges to be achieved than was possible with conventional visual/film lenses.

Aspheric optics and high dispersion ED glass has also helped. Fluorite gets used in some very high end telescopes but it is hell to work with.

Any half decent lens will ultimately be diffraction limited if you stop it down far enough - typically f8 is the sweet spot. What is impressive is to have diffraction limited optics at apertures of f5 or faster.

It is you that is the utterly clueless f****it. No surprises there.

The size of the Airy disk in um produced by any diffraction limited imaging optics is *entirely* determined by the working focal ratio. It is usual to oversample the resulting image by a factor of at least 1.5x or better 2x if possible to avoid Moire effects.

The Airy disk is defined for a star like point source at infinity.

FWHM ~ lambda*f

So for 550nm and f=5 that is a 2.8um psf.

The Airy disk out to the first zero is somewhat larger ~6.7um

[Fredxxx]

I'm surprised, I thought all lenses were designed achromatic, and most de-Bayering filters rely on the relative position of the R and B pixels wrt green. Perhaps achromatic isn't quite the right word in this context.

I guess this is only considered where lens costs exceed all others?

I recall some old TV camera lenses, with possibly very small pixels using a lens that had a neutral density filter to stop down without going to ridiculously high f numbers.

Unfortunately Harry has a habit of reinventing the laws of physics.