The great diffraction delusion
Are more megapixels actually a disadvantage when using small apertures? Prof Newman dispels this myth
In recent years, there has been a run of camera releases featuring high-pixel-count sensors. The least pixel-rich of these is the Sony Alpha 7R Mark III at 42MP – somewhat overshadowed by the 45MP Nikon D850 and the 100MP and 400MP releases in the Hasselblad H6D range.
Whenever such a release occurs, it is accompanied by comments on web forums that these high megapixel counts have restricted usefulness owing to a phenomenon called ‘diffraction limiting’. The thesis put forward is the small pixels cause such cameras to perform worse at large f- numbers. I think this particular meme started on a very popular photographic website, which even has an interactive calculator to let worried photographers predict when their camera will become ‘diffraction limited’. While the internet is a wonderful and rich source of information, it is also responsible for false information spreading like wildfire and this is, fortunately, false. There is indeed a phenomenon known as ‘diffraction limiting’, and it describes the state in which the resolution of an optical system is essentially determined by diffraction. Indeed, it can occur in small-pixel cameras with good lenses, but that is a good thing – it means the camera is extracting all the information that the lens can provide.
High-MP versus low-MP cameras
Diffraction is a phenomenon whereby an aperture will ‘spread out’ a light ray due to the interaction of the light waves at different parts of the aperture (another false idea that is common is that it is due to photons ‘bouncing off’ the edges of the aperture). The smaller the aperture, the more the light is spread. A point source is spread into a characteristic pattern called ‘the Airy disc’. To understand how this interacts with pixellation, it is necessary to know how different sources of blur interact. The ‘diffraction-limiting’ thesis is based on the idea that Airy discs become invisible if the pixels are large enough to hide them; hence with large pixels diffraction is not so visible. This isn’t the manner in which pixellation and diffraction interact, which in reality is described by a mathematical process called convolution, or one mathematical function ‘running over’ another. This is shown in the illustrations below left. In the upper two, we can see the ‘point-spread’ function ( just a slice through the brightness profile) of the Airy disc and the pixel. In each chart the point-spread function for the Airy disc is shown in blue and that for the pixel in red. The convolution of the two together is shown in green. As can be seen, the effect of the pixellation blur is to spread out the point spread function of the Airy disc. At no time does the Airy disc simply ‘hide’ behind a pixel. The larger the pixel, the more the disc is spread. This is very noticeable at f/4, where the large pixel results in a significant loss of resolution. It still occurs, but with less effect at f/11, where the diffraction blur is greater. The lower two charts compare the total blur for the small and large pixel at f/4 and f/11. As can be seen, the small pixel produces less blur in both cases, though its advantage is marginal at f/11.
So, in summary, there is no need to fear that high megapixel cameras will produce worse results than low megapixel ones at small apertures. It just isn’t true.