The examples might be interesting. How much closer? The standard basic formulas break down as the magnification become significant, with texts typically limiting their utility to m<1/10 or even m<1/20, meaning subject distance should be at least about ten or twenty times the focal length. At closer range, lens extension means that both the effective focal length and effective aperture diameter shift, and more precise formulas are needed.
Another part of the approximation is that the formulas break down when the aperture is small so that the DOF is very large: large enough to be substantially greater behind the focal plane than in front of it. (The rule of thumb about 1/3 of DOF in front, 2/3 behind has no quantitative scientific basis: DOF is close to equally distributed fore and aft when the DOF is shallow enough, but grows more behind than in front as aperture size is reduced, eventually reaching infinity behind in the hyperfocal case.) The formulas are more oriented to choosing an aperture that ensures enough DOF in situations where DOF is potentially quite limited.
In the comparisons below, the focussing distances, although close, are significantly greater than 20x the longest focal length used, which was 40mm. The distance to the focus point would have been about 2 metres, or 50x the longest focal length. In order to be pedantic, I've downsampled the 50D image to the 5D size. However, whether the 50D image is downsampled or the 5D image is upsampled makes no difference to the conclusion. In these samples, the 50D still retains a slightly greater DoF edge, even with a 2 stop difference, although I'm prepared to accept that such miniscule differences fall within the margin of experimental error.
As I mentioned before, my purpose in doing these tests, shortly after receiving my brand new 50D, was to check out the noise, comparing it with my 5D at equal shutter speeds and equal DoF, which involved comparing 50D noise at ISO 100 with 5D noise at ISO 320, and ISO 200 with ISO 500 etc. It was as a consequence of such tests that I discovered that the 1 1/3rd stop difference for equal DoF did not seem to apply in those circumstances and that nothing less than a 2 stop difference would produce the desired results.Here's the over all scene:
[attachment=13510:The_scene.jpg]The general area of focus at 100%:
[attachment=13511:General_...us_point.jpg]The specific area of focus at 200% (the pale mauve, artificial flower):
[attachment=13512:precise_..._at_200_.jpg]The closest points in the foreground:
[attachment=13513:Nearest_points.jpg]The background, centre right:
[attachment=13514:Centre_right.jpg]The background centre left:
You'll notice that I should have used ISO 400 with the 5D instead of ISO 640. However this apparent advantage to the 5D (regarding noise) is at least partly offset by the slightly faster shutter speed (1/15th as opposed to 1/13th for the 50D) and is not as great as it might at first seem. I was simply trying to get a good ETTR. Checking out DXOmark figures later, I discover that at ISO 100, the sensitivies of both cameras is equal at ISO 93. However, at the nominal ISO of 400 the 5D is actually ISO 357, according to DXOmark, but it should be 4x93=372, so it does appear to be slightly understated. Nevertheless, according to EV compensation adjustments in ACR, the 5D has about a 1/3rd stop ISO advantage in these comparisons, that is, it requires -1 EV adjustment whereas the 50D image requires -0.67 EV adjustment.
The noise comparison images below were converted with zero black, zero contrast, linear tone curve, no sharpening, and no noise reduction of either luminance or color. Both images have been lightened to the same degree in 'levels'. The 5D comes out quite well. I was surprised that at ISO 640 the 5D would be on a par with the 50D at ISO 100. One should also bear in mind that the slight ISO advantage I've given the 5D does not translate to more photons per unit area of sensor, but a slightly greater analog amplification of the signal.Noise comparison:
All this "fine print" is why one needs to understand the physics, mathematics, and applicability of the formulas, or otherwise use them cautiously, in the range of conditions for which they were intended. More detailed formulas are available in more advanced optics texts for the harder cases!
The problem here is that, in the field where capturing the moment is often of the essense, we don't have time to engage in complex calculations. Outside of studio conditions, it's also very difficult to get precise measurements of distance to subject, so DoF calculations, however precise theoretically, can be no more precise in practice than the precision with which can one measure distances in the field.