Tadeobiologo :)

I keep looking over and over again how photographers -people who otherwise produce astonishing images- talk about technical stuff with no real idea of what they are saying.

Respecto to DOF and sensor size the theme seems to repeat over the same circle of consensed nonsense.

If you go to a serious optics textbook you´ll encounter that DOF is affected by two main things: apperture (as diaphragm physical diameter, not the damn f-stop)  and lens-to-subject distance. they never ever mention that the film or sensor has any part on it.

so the DOF is determinded this way and there is nothing more (and if you want to blame do some research first)

Now pay close attention to this:

the larger the lens aperture, the shallower the DOF.

the closer to the subject the shallower the DOF.

So what the hell happens with the sensor/medium size??

if you put any (mean any) sensor/film to record the image formed from an arbitrary lens you are catching the exact same image in any case. what are the differences? well, you have sensors or films of different sizes so what you have are different crops from the same image. the other thing is that as you use diferent recording media each one of them has different sampling frequencies (they record at different spatial resolutions -take it as real life physical measurement in whatever units you like)but this has no effect on DOF.

Why on earth then images look so different when you compare them between formats?

if you want the same scene to be recorded with an equivalent perspective from the same lens but using different capture media, the smaller your sensor/film the more cropped image you´ll have. if the sensor/ film is smaller, in order to keep on the frame the same scenery elements you have to move backwards but oh! if you move farther from your subject your DOF gets bigger!

if you use a bigger sensor/film the crop you get is bigger and you get more things included so if you want to keep your composition you move closer (so DOF gets narrower)

if you don´t get it yet, try this out using the same lens on two different bodies using the same aperture (f-stop) and keeping the same composition.

The other side of the coin comes when you want to cover the same scene with different recording medium sizes but at the same distance. what you need to do then is to change your focal lenght. the smaller the focal lenght, the wider you go and for a smaller crop, a wider perspective can give you an approximately equivalent composition (of course affected by distortions propper of the lenses)

as you go smaller in format, you go smaller on focal lenghts and if you keep the same f-stop, well, remember how f-stop is the ratio between focal lenght/diaphragm diameter. to keep the same f-stop if you go at smaller focal lenghts you need smaller diaphragm diameters.

when the diaphragm diameter goes down, the DOF increases. that´s why angular lenses get almost everythibg in-focus while telephoto lenses don´t.

So in the end is a geometry and perspective matter and not sensor/film size magic/obscure/metaphysical/technological characteristics.