>Ok, so to have more details it is not just in more mpix thing. >Things are connected to each other. More mpix mean more noise >at high iso which destroy details , etc.. > >What about low iso then? Low iso = less noise = more details?
A certain number of megapixels (10-12) is needed in a certain size sensor (FX, DX/APS-C, Four-Thirds), combined with a certain type and level of color accuracy, and whole range of other controls, all processed by a CPU capable of handling the data load presented by the interdependent features and functions applied by the designers and engineers of the camera, in order to achieve a specified quality level.
10-12 megapixels, given the high quality of sensor designs today, provide data capture which records detail sufficiently fine to satisfy the vast majority of 35mm photographers. That's the baseline. That's why we're no longer seeing large increases in pixel density as new models are released. All technology incorporates practical limits - that is, limits beyond which it is no longer cost effective to do the engineering needed to, say, achieve higher pixel density and lower noise on a given size CMOS sensor. When the engineers bump their heads on that sort of ceiling, the next step has to be a larger form factor, but even then there's still a large cost increase.
Once again though, it's not a matter of capturing more objective photographic detail, but rather a matter of improving the clarity, visibility, and editability of existing capturable details by reducing noise, improving color accuracy and other things in order to create a piece of hardware capable of producing better quality photos containing subjectively greater detail.
Your equation is much too simple to be of use in your own photography. Every good camera and every objectively good photographic composition incorporates innumerable considerations before the shutter is pressed. If creating a good camera and a good, detailed photo was as simple as your equation, I think everyone would already be doing it.