Stuffing more and more pixels onto a fixed sized sensor introduces diffraction issues at larger and larger apertures. Also, higher pixel densities usually lead to a poorer signal to noise ratio.
More pixels do not introduce diffraction issues or lens aberration issues, they just allow you to see those issues more clearly.
Diffraction is not a function of lens flaws. Even a perfect lens will have diffraction. When the size of the airy disk that even a perfect lens produces, exceeds the pixel pitch of the sensor, resolution starts to become diffraction limited.
this is high school physics. Look through a screen, then finer and finer screens. The finer the screen the more diffraction. Period. This is how they make diffraction gratings.
The aperture introduces diffraction, not the sensor. The aperture is the screen in your example. A lot of small pixels just allows you to image the diffraction better it does not create it.
Diffraction affects image detail by reducing contrast. The technical term for the contrast reduction is called the Modulation Transfer Function (MTF) and describes the contrast the camera delivers as a function of the spacing of lines (called the spatial frequency), or fine detail. Here the spatial frequency is expressed in terms of pixel spacing. As the f/stop increases, the diffraction spot becomes larger, and fine detail in the image becomes reduced in contrast. The red, green and blue lines show the diffraction effects for red, green and blue wavelengths of light for f/ratios 1, 2, 4, and 8.
We buy lenses based on 50% MTF's, then some people want to reduce their contrast by packing more and more megapixels on a sensor. Once you reach 4 microns pixel pitch, even at f/2.8 you start to lose contrast due to diffraction.
Eric Fossum, Inventor of the CMOS image sensor, explains it here -
Photons to Bits and Beyond: The Science & Technology of Digital
The resolution is limited by the aperture and the wavelength of the light. Once the pixels become smaller than is scientifically possible for the lens to resolve, the file size just becomes larger without a corresponding increase in resolution.
The extra megapixels resolution gained by Nikon going from 16 megapixels to 24 megapixels is a fraction of the file size increase. The only way to keep the marketing people from bloating our files, is for the consumer to know what they are getting from that 'number on the box'.
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