To answer the original question, I would say that you need to decide what matters to you. In front of that scene, I would want the garlic in the foreground sharp, but I would be happy to have them go fuzzy as they disappear in to the yonder: this implies image depth and gives a 3D feel to the pic. I then like the trees and would choose to have some of them in focus. So a tilted plane of focus to include the foreground garlic, the trunks and the first curtain of foliage seems about right. After that, you can play with DoF to control the amount of fuzziness in the woodland context. That's just me and maybe a bit obvious. It's a nice setting so why not be simple and accept Nature for what it is, rather than going all arty? That's a personal choice.
Tim: I like your new webpage - fantastic to show the degradation of the circle of confusion. I knew those web skills would come in handy one day
. I think the main problem with it is practical use. I prepared a few years ago the following graph, but have frankly found little use for it, as I have been too busy trying to nail the damned picture before the proverbial tortoise hared off (and yes, Emmanuel, I did laminate it before packing it to protect it from our fickle weather):
It shows the circle of confusion at various apertures, based on the variation there is in the bellows extension. Each curve is for a different extension difference. By "extension difference", I mean: you focus at one point & measure the bellows extension; then change the focus for another point in your field of view & measure the new bellows extension - you then calculate the difference between the two extensions. Those are the values in blue. You then decide what CoC you would like and then read the aperture you should dial. I believe this works whatever the angle of the plane of focus. And translates fairly readily to the real world. Sort of, as you will see later...
So, quite naturally, you see in the graph that when you need more bellows movement to have two objects in focus, you need a smaller aperture to get the same circle of confusion. The red line shows the minimum f/number. Emmanuel, focal length does not come in to the equations directly, since we are talking about differences in bellows extension which will, however, increase with focal length for a given difference in the object distance - according to the thin lens rule. So a longer lens will need a smaller aperture to achieve the same depth of field. But I don't see why the film format - 35mm or LF - would alter the DoF. After all, 35mm is only selecting a smaller part of the image, isn't it? Let's say, that 35mm is 10x smaller than 5x4: a factor of 10 is about 3.3 stops. So that takes you from F/11.3 to f/4, which is a lot lower than Emmanuel suggests
. Might have misunderstood you here, but I now suspect that the optimal f/stop is independent of the format...One thing which is interesting here is the range of the optimal lens stops, which runs from f/16 to f/45. I seem to recall that Kerry Thalmann measured the optimal apertures of various lenses and came up with smaller values - typically around f/11, which tallies with a very small bellows extension difference in the above graph. So the lenses blur well before we use them in practice. I can't find the exact reference, but this page is a start. I think this leads us from optical sharpness to perceived (or artistic?) sharpness, where the increased DoF makes us think that the pic is better resolved. I hope this helps.
Regards,
Charles
