What is a dioptre?
A dioptre (UK), or diopter (US) is a unit of measurement of the optical power of a lens or curved mirror.
As the dioptre increases, the lens becomes thicker, making the object look bigger. The higher the dioptre, the more magnification the lens provides. A flat window has an optical power of zero dioptres, and does not converge or diverge light.
It is also important to note that the power of the lens is inversely related to the focal distance (working distance). Meaning, as the power of the lens increases, so the focal distance decreases, making it more difficult to perform tasks with high magnification lenses.
See Mantis Compact low magnification stereo microscope if higher magnification is required.
Magnification is calculated by simply dividing the dioptre by 4, then adding 1.
e.g. 3 dioptre lens = 3/4 + 1 = 1.75x
Therefore, objects viewed through a 3 dioptre lens will appear 175% bigger than normal.