I don't understand this. I know what diffraction is but what does it have to do with high MP? Thanks.
In brief, the higher the MPX in a given sensor size (and lets talk full frame 35mm) the smaller the well size where the photons get captured. The smaller the well size, the harder it is for the photons to enter the well and thus the lower the signal to noise. Lower MPX sensors have better signal to noise ratios than higher MPX sensors. BSI sensors help a little with this, but ultimately you cant beat physics - smaller well sizes (higher MPX) = higher noise at a given ISO than a lower resolution sensor.
In terms of diffraction - the smaller the well size on the sensor the faster you run into diffraction. A 24 MPX sensor has much larger wells than a 50 MPX sensor so the 50MPX sensor will run into diffraction well before the 24 mpx sensor at a given f-stop. As such, a 50 MPX sensor will be diffraction limited far sooner than a 24 MPX sensor. What does this mean?
It simply means, once you hit the diffraction limit on the 50 MPX sensor you loose your resolution advantage. Its not that the 50 MPX sensor will look worse than the 24 MPX, its just that it wont look any better; you loose the advantage of the extra MPX. Exactly where diffraction occurs is complicated as it depends on the lens used, the sensor design and atmospheric conditions. You can certainly see it clearly though if you take the time to test this.
To be clear, Im not stating the 24 MPX sensor is better - it isn't (and the 50 isn't better than the 24 as it depends on the lens used, the f-stop the image is shot at and the particles in the air between the lens and subject). This is simply the physics of it.