Ray is right and Howie is wrong.
Howie simply isn't using the commonly accepted definition of DOF.
I think the notion of HFI DOF is at the crux of the matter. You can call a horse a donkey and then debate about how your donkey runs faster, and looks better and is taller than all the other donkeys. But the problem is that you have misunderstood the definition of donkey.
COC is dependent on film format.
The DOF calculations use COC as a variable.
When you change format you have changed your COC, and so you will change DOF.Wiki entry - Circle of ConfusionWiki entry - Depth of Field
In film photography, the circle of confusion criterion is sometimes defined as the largest circle of blur on a film negative that will still be perceived by the human eye as a clean point when printed at 30 cm diagonal size and viewed from a normal viewing distance of 50 cm (and variations thereon).
Accepted values for circle of confusion based on d/1500
Film format Frame size CoC
APS-C 22.5 mm x 15.0 mm 0.018 mm
35 mm 36 mm x 24 mm 0.029 mm
645 (6x4.5) 56 mm x 42 mm 0.047 mm
6x6 56 mm x 56 mm 0.053 mm
6x7 56 mm x 69 mm 0.059 mm
6x9 56 mm x 84 mm 0.067 mm
6x12 56 mm x 112 mm 0.083 mm
6x17 56 mm x 168 mm 0.12 mm
4x5 102 mm x 127 mm 0.11 mm
5x7 127 mm x 178 mm 0.15 mm
8x10 203 mm x 254 mm 0.22 mm
Of course these values are for d/1500. If your lenses and film/sensor have higher resolution and you vary print sizes and viewing conditions etc you might want to change the value, but for whatever values you use you can draw up a similar chart for COC of different formats. A list of my favourite Howie quotes from this thread:
(Taken somewhat out of context for humorous effect )
One can prove so many things true if you start with assuming they are true.
A variable is something that can be changed. "Changing a variable" is when I change something that can be changed.
"Image capture size" (format?) isn't a variable at all.
I don't care what many or most people think. The truth is the truth regardless of who or how many believe it. DoF is not a popularity contest.
Because I don't own a pixel, I need not know anything about its size.
I have not yet run into a case where my CoC was smaller than the lens/film resolution. In fact, I don't even know what that is for my lenses and film selections.
All that I have been trying to say is that if you use two cameras of different formats but with the same focal length lens, same f/stop, same CoC and same focus distance, to make equal prints (same sizw focused image), DoF is the same for bothrints. The only variable changed is camera format. DoF does not change. The logical conclusion is DoF is independant of format.
It isn't that hard. I'm thinking you are either being obstenant or putting me on.
If I make an 8x10 print, then cut it in half, the characteristics of each half are not changed. Only the print format has changed. The DoF of each half is unchanged. It is so simple as to approach absurdly so.