why is it that one needs more illumination when using high levels of magnification?

Question

why is it that one needs more illumination when using high levels of magnification (pertaining to the microscope)

Answer 1

Magnification involves expanding the light path reflecting off the image. You don't increase the number of photons this way, so they get spread further apart or "diluted". That means the light intensity drops.

Answer 2

Generally the aperture and the first magnifying lens are smaller and smaller as the magnification increases and therefor less total light is allowed into the tube. Then the refracting or collimation effects of the internal lenses/mirrors waste some light in the process of delivering an enlarged image to the eyepiece because the field of view is narrowed (angularly). So more light must be available to see the enlarged image.

Answer 3

You're looking at a smaller area because of the magnification so you're gettting less light for the viewing area of interest.