Diffraction Limit Discussion Continuation

Started 8 months ago | Discussions thread
Jonny Boyd
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Re: Diffraction Limit Discussion Continuation
In reply to Anders W, 8 months ago

Anders W wrote:

Jonny Boyd wrote:

I said that at low resolutions it's more of a plateau that a peak, so you effectively get the same resolution at smaller apertures.

No you didn't say that. You said the peak would occur at different apertures depending on sensor resolution (just as Cambridge in Colour). Do you want me to look up the specific posts for you?

I forgot to add the following quotes from CiC:

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. One can think of it as the smallest theoretical "pixel" of detail in photography. Furthermore, the onset of diffraction is gradual; prior to limiting resolution, it can still reduce small-scale contrast by causing airy disks to partially overlap.

In practice, the diffraction limit doesn't necessarily bring about an abrupt change; there is actually a gradual transition between when diffraction is and is not visible. Furthermore, this limit is only a best-case scenario when using an otherwise perfect lens; real-world results may vary.

This should not lead you to think that "larger apertures are better," even though very small apertures create a soft image; most lenses are also quite soft when used wide open (at the largest aperture available). Camera systems typically have an optimal aperture in between the largest and smallest settings; with most lenses, optimal sharpness is often close to the diffraction limit, but with some lenses this may even occur prior to the diffraction limit. These calculations only show when diffraction becomes significant, not necessarily the location of optimum sharpness (see camera lens quality: MTF, resolution & contrast for more on this).

n MTF of 1.0 represents perfect contrast preservation, whereas values less than this mean that more and more contrast is being lost — until an MTF of 0, where line pairs can no longer be distinguished at all. This resolution limit is an unavoidable barrier with any lens; it only depends on the camera lens aperture and is unrelated to the number of megapixels. The figure below compares a perfect lens to two real-world examples:

The aperture corresponding to the maximum MTF is the so-called "sweet spot" of a lens, since images will generally have the best sharpness and contrast at this setting. On a full frame or cropped sensor camera, this sweet spot is usually somewhere between f/8.0 and f/16, depending on the lens. The location of this sweet spot is also independent of the number of megapixels in your camera.

I don't think CiC says what you think it says, if you believe it claims that peak aperture depends on the sensor resolution rather than just the lens resolution.

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