Originally posted by GUB
Naaaa wipe that egg off your face Dave -- I haven't heard a fat lady singing yet. Firstly I agree with Photoptimist it is not to do with pixel size but it still has nothing to do with format either. (except in the practical analogue area)
Let me present an example;
A FF sensor. Lets say we go and put a Nikon lens on and we all know don't we that, because we have payed three times as much for it, it has twice the resolution of the glunky ol pentax one

So we now have a FF image with twice the resolution of a normal FF. Should we double the coc on the calculator accordingly.?
I say yes and so it is not format based but resolution based.
Your pixels were too small in relationship to the coc to have any real input but the practical lpm is much closer to the size of the coc so lpm has a large effect.
For me the only question in all this has been whether or not the viewing size of a digital photo can be physically scaled back to the physical size of the sensor
in any way at all. For my money, online depth of field calculators are so simplistic in the assumptions they make they are never going to be much use. My own approach has just been to have the final viewing size in mind when I shoot, and go by what I know works from experience with the lens I happen to be using. That's dependent on the CoC projected onto the sensor by the lens, the size of the sensor pixels, and also on the pixel size on my 24" 1920x1080 monitor -- which is about 0.25mm and of course displays a downsampled image.
And downsampling an image is not a simple arithmetical process. I've only got a very basic layman's understanding of it, but I have checked that understanding with my father, who designed computer graphics hardware for a living and was a member of mpeg before he retired. So I like to think he's a fairly reliable sounding board when I want to double-check if I'm understanding something correctly. My understanding is that in most cases, when you're looking at a digital photo, it's done by scaling the waveform and most definitely not by simple processes like skipping pixels. Which is why I still think that @Photoptimist is oversimplifying downsampling in his examples, even though I'm happy to go along with him in general that the CoC at the final viewing size does have a physical size, and so it is scalable back to the physical size of the CoC on the sensor.
I do accept your point that the pixels on a current generation sensor are small enough that we can think in terms of the physical size of the CoC on the surface of the sensor. So once we know the final viewing size, resolution, and distance we can then scale back in a meaningful way. But if people start asserting again that the viewing size of a digital photo is an optical enlargement in the same way that a print from a negative is then I'll be right back in the fray.