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Resolution
In my last post I commented on the need to properly define the maximum possible resolution for an optical system. Only after that is defined can we calculate the proper focal length to provide 'critical sampling' ( 2x the maximum resolution as per Nyquist).
Attempts have been made to define maximum resolution based on the visual observation of double stars (Rayleigh limit and Dawes limit). These definitions have suited visual observers of double stars but are not suited to high resolution digital imaging of planets. In the "Handbook of Astronomical Image Processing" Richard Berry and James Burnell nominate the diameter of the Full Width Half Maximum (FWHM) as "a realistic measure of the smallest detail contained in an astronomical image". The diameter of the FWHM is the region within the bright core of the diffraction disk where the "light has fallen to half its central intensity".
I have adapted a formula given in Berry and Burnell to give the focal length for critical sampling given the aperture of an optical system, the size of the pixels in your webcam, and the wavelength of light you are interested in. I have attached a PDF file with the formula.
This, I think, is an answer to the question "what focal length should I use with my webcam to achieve critical sampling". Not quite the question that was asked, but a step closer!
Oversampling
If you image at focal lengths below those calculated by the formula attached you are undersampling. If you image at focal lengths above you are oversampling. The comment from HAIP regarding oversampling is that sampling at 3 to 5 times the maximum resolution "insures that none of the information present in the telescopic image is lost because the image is broken into discrete samples".
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