Seems like it's the season for self-isolation, sitting at home and thinking about the status quo of various stuff.

Looking at the excellent images recently posted here by Darren at a focal ratio that was 7x the pixel size of his ASI290MC camera made me wonder, why should the 5x pixel size "rule" hold for colour cameras?

I have always been a little, lets say, uncomfortable by the maths that was used to come to this rule of thumb. The working below is probably the easiest to understand version of this rule, which relies on the seemingly arbitrary assumption that the Full Width Half Maximum (FWHM) needs around 2.5 - 3 pixels to completely resolve between peaks. This leads to the well accepted value for f-number to be around 5x the pixel size of the camera, at least for green light.

But what about blue light with a wavelength of around 460 nm? The formula recommends an f-number of around f/6.

What about a colour sensor with a RGGB bayer matrix? The red and blue "pixels" are actually separated by 2x the actual pixel size. Even the green pixels are separated by sqrt(2) times the pixel size, leading to a value of f-number around f/7, not f/5.

Am I missing something here? Or am I sitting at home being bored and trying to find something to do?

Thanks,

Andrew

**Edited by Tulloch, 05 April 2020 - 08:22 PM.**