If you shoot a lot of megapixels, let's say 21MP, what image improvements/changes will occur when you downsample the image to a resolution around 1920x1080 for display on a normal 1080p HD television? (assuming you are shooting uncompressed raw)
What effect will this have on DOF, perceived sharpness, and noise?
When you downsample a 4000×3000 image to 400×300, you are "discarding" 11.9 million of the 12 million pixels. This clearly reduces "image quality", depending on what exactly you mean by that term.
If you go from 1000×750 to 400×300, you're reducing the area by about 6 times. Again, data is discarded, but not as much.
Now, if your original image had a lot of defects — noise, aberrations, and focus error — the resolution may exceed the "effective image quality", and the extra detail is contributing nothing. In that case, downsampling to some degree doesn't "really" reduce image quality.
What exactly happens when your image has "a lot of defects — noise, aberrations, and focus errors" and you downsample? Can this improve?