How to see diffraction in photos

Started 5 months ago | Discussions thread
cba_melbourne Veteran Member • Posts: 5,702
Re: How to see diffraction in photos

Pete Berry wrote:

cba_melbourne wrote:

BobT3218 wrote:

Out of curiosity. while on the subject of diffraction, would one of the optics gurus care to advise if theoretically, FL is a factor in diffraction?

Optically, diffraction only depends on the physical size of the aperture (not the f/number), the distance of the aperture to the focal plane, and the wavelength of light. Nothing else.

FL plays no role, because it is taken into consideration (or offset) by the way the f/number is defined. The f/number of a lens is the ratio of it's focal length to the diameter of the entrance pupil (for example f/8 means the FL is eight times the diameter).

It is often said that in m4/3, diffraction becomes notable higher than f8, but FL is never mentioned. I'd like to know because I regularly use FLs from 12mm to 600mm.

The effects of diffraction very much depend on sensor pixel size (and on the presence or not of an anti aliasing filter in the sensor stack). The smaller the pixel size, the sooner (at a smaller F/number) will diffraction begin to limit the image resolution. In m43 with our current 20MP sensors, that threshold is now somewhere between f/5.6 and f/8. With 16MP sensors is was more like f/8, and with future higher resolution sensors it may well shift towards f/5.6.

Back in the film days, the onset of diffraction depended on film grain size. And film grain size depended on ISO sensitivity, because the higher the film sensitivity, the bigger its grain was. So swapping from an ISO100 to an ISO1000 film, was very much like if today we swapped to a much larger pixel size (lower resolution) sensor.

An optically perfect lens of any FL or aperture will halve it's resolution with every two stops increase in F-number, which halves the physical aperture diameter - and decreases light transmission by a factor of (1/2 squared) = 1/4th.

But our imperfect lenses fall more slowly in resolution stopping down, but much the same in light transmission.

An optically perfect lens is said to be "diffraction limited". Meaning the lens is so good, it's performance is only limited by diffraction.

Diffraction-limited lenses are lenses with aberrations corrected to the point that residual wavefront errors are substantially less than one-quarter the wavelength of the energy being acted upon.

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