Diffraction Limit

Started Aug 28, 2013 | Discussions thread
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Detail Man
Detail Man Forum Pro • Posts: 16,420
Re: Diffraction Effects in a Real Lens-Camera System

LTZ470 wrote:

From a very credible source:

"Diffraction thus sets a fundamental resolution limit that is independent of the number of megapixels, or the size of the film format. It depends only on the f-number of your lens, and on the wavelength of light being imaged. "

Ian was rightly setting-straight the poster on that thread who stated that Diffraction was a function of aperture diameter - which it is not, it is an (approximate, for larger F-Number values) function of the Focal Length divided the aperture diameter as measured from the outside of the lens (the entrance-pupil diameter). Also known as the lens-system F-Number.

The poster corrected in Ian's post seemed to have Diffraction mixed up with DOF - which is not surprising, given how the two subjects are often lumped together in what is an incomplete explananation. Those explanations do not take into account the lens-system MTF curve "extinction" effect (see below).

Regarding DOF. Smaller image-sensor cameras use lower valued F-Numbers than do larger image-sensor cameras (to achieve the same DOF with the same image-framing), so that situation is a "self-compensating" one.

Diffraction MTF response curve "extinction" does indeed result in a "fundamental resolution limit" - but only when the product of Wavelength multiplied by F-Number is fairly high - higher than you, or most other people, typically talk about (although this does indeed become relevant for smaller sized photosites when Wavelengths and F-Numbers are of a high numerical value).

In that "extinction" limit of spatial frequency response, it is Wavelength, F-Number, Photosite aperture, shape, and "pitch", as well as the de-mosaicing algorithm used that result in the (RAW-level) image-data, that then is further processed (in-camera, or in an external processor) to the point where we as viewers look at it appearing at some physical viewing-size from some distance with some unknown visual acuity, and (may) exclaim, "dad-gum that shore lookie sharp" !" ... Every single one of the above things (all) affect perceived image "sharpness" ...

When we talk about how images from specific camera-lens system appear (at least to our own eyeballs), all of the above listed things matter (in addition to all image-processing). Of course, we are also assuming zero focus-error on the Plane of Focus (which is not reality), and we are also assuming an effective zero amount of camera-motion (which is not reality). These things will typically "swamp" most effects from lens-diffraction into completely obscure irrelevance ...

I myself have learned a lot from listening to others and considering their thoughts and their viewpoints. Joe and Anders know a lot about these subjects. They are worth reading and listening to. I myself might be worth reading and listening to as well (have I ever "steered you wrong" ?). Have in the last few months given quite a bit of specific thought surrounding these subjects (here):


Keep the Fun,

DM ...

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