resolution

Imagine looking at a car's headlights from very far away at night: the two lights blur together into a single glow, and only as the car approaches do they separate into two. The distance at which two points stop looking like one is the heart of resolution. In microscopy, resolution is the smallest distance between two objects at which you can still tell that there are two of them, not one. It is the real measure of how much fine detail a microscope can show.

Resolution is set chiefly by the wavelength of the light or beam used to make the image, not by how much you magnify. A rough rule says you cannot resolve things much closer than about half the wavelength. Visible light has wavelengths of 400 to 700 nanometres, which caps an ordinary light microscope at roughly 200 nanometres of resolution. Electron beams have far shorter wavelengths, which is exactly why electron microscopes can resolve down to fractions of a nanometre and reveal structures light can never separate.

Resolution matters because it draws the hard boundary of what any microscope can ever show. If two organelles sit closer than the resolution limit, no amount of magnification, sharpening, or zooming will pry them apart — they will always look like one blob. A common confusion is mixing up resolution with magnification: magnification makes the image bigger; resolution determines whether that bigger image actually contains more genuine detail or just a larger smear.