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Stephen, if I may say, that's a very clear, concise and comprehensible definition of, among other things, the Circle of Confusion, or rather the issues involved in where you set the CoC. I know many photographers deliberately set the CoC at a level which is lower than the manufacturer's guidance for the camera in order to compensate for the less rigorous approach the manufacturer takes to who might be viewing and image and at what print size (and from what distance) - both Cambridge in Colour and the Outdoor Photography article take this view and it is one that I find sensible.
When looking at DOF/HFD charts/tables/calculators, the best I have seen, on Cambridge in Colour's website, takes into account the larger print size and viewing distance which effectively results in a longer HFD. An HFD on a 'normal' calculator, if values are changed to reflect a larger print size and (as the Pete Bridgwood puts it in his OP article) the 'pixel peeping elite' scrutinising the image closely, then the HFD changes - it goes from perhaps 2m to around 10m. This change is also reflected in DOF calculators if you change the CoC from say 0.019 (Canon EOS 60D - my camera) to say 0.010 - it does not go to 10m, instead to around 4m but the point remains the same and accords with the advice given in a lot of articles I have read - "focus beyond the HFD to be sure".
Having read a great deal on this now - which is useful for more than just depth of field, sharpness and optical law, but also to see how aperture reacts and to entrench some of the basic principles involved in photography - I think my approach will be as follows:
For landscapes with great foreground interest, focusing at or, more likely just beyond, the HFD when set with a CoC of no higher than 0.010 or, indeed, using the values in the Cambridge Calculator which imply a larger print size, will be what I do. This may well mean a focus point of between 6 and 10m depending on where the foreground interest is.
For landscapes, particularly in the mountains, where foreground interest is a little further away or not quite so important, I will focus on infinity. This is largely because, on my 17-40 f/4L USM, focusing at 10m+ is pretty much dead on the Infinity bar anyway.
The sweet-spot for sharpness on both my lenses, 17-40 f/4L USM, and 24-105 f/4L IS USM, is f/8. I imagine f/11 will not be much worse (if at all) but as I may well be handholding a lot, especially in the mountains, f/8 is preferably.
Thanks for the responses.
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