There are many out there, and I found this one particularly helpful. I liked this site, Cambridge in Colour, perhaps because it's written by a scientist with a more theoretical leaning than some other photo articles I have read (but not too technical!).
So I looked through the other articles and found this delightful one on diffraction. I liked this because it explained, in simple enough terms for me, the mystery of why it becomes counterproductive in the search for both depth of field and sharpness to close down your camera aperture to near pinhole size.
I'd read before advice against using an aperture setting of f/11 or above (aperture numbers and aperture size are inversely proportional [confusing me for years], in other words, a large number e.g. f/22 means a very small hole). The explanation was that diffraction interference would reduce the effective sharpness of your image. Right, yes – so what does that mean?
Thankfully, Sean McHugh, explains it very nicely through a series of graphics. As the aperture hole diminishes in size, light passing through it becomes more and more constrained and bends – diffracting – so that formerly parallel waves of light begin to shift in relation to each other, becoming out of phase.
The net result of these out of phase waves is a diffraction pattern, where light is either reinforced or reduced by the interaction of these shifting waves. That produces the Airy disk shown here in all its beauty. The size of this disk depends on your aperture setting; a smaller hole, a bigger Airy disk. For digital photographers, this has an important bearing on the effectiveness of pixel size. For once an Airy disk becomes larger than an individual pixel, resolution begins to be lost. Add to that the loss of resolution that overlapping Airy disks bring and you are heading down blur road.
Yet another argument against the trend to more and smaller pixels, and one I was completely ignorant of until last night.
As to the tilt-shift lens, well, that helps avoid the problem of wanting both great depth of field and sharpness by allowing you to angle the focal plane of your image to increase depth at much wider aperture settings than you would need with a conventional lens. But discovering that for myself will have to wait until the UPS man delivers my package.
Airy disk photograph, Colin Eberhardt, from Wikipedia