- Magnification refers to the ratio between the apparent size of an object (the size of the image of the object) and the object's true size. Thus magnification is a unit-less number. Magnification can be calculated for a variety of objects, though, even if the image's size cannot be directly measured. Magnification is used in many high-tech devices, including cameras, telescopes, magnifying glasses and microscopes. Calculating magnification depends on the device and on the type of lenses and mirrors used in the device to magnify.
- Calculating microscope magnification is a basic laboratory task. The magnification of a microscope is given by M = O x E, where O is the magnification of the objective and E is the magnification of the eyepiece. The magnification of the objective depends on the focal length (f ) and on the distance (d) between the object and the "back focal plane" of the eyepiece, called the tube length, such that O = d/f. Technically speaking, these equations produce inverse magnifications (inverted images) and as such should more correctly include minus signs.
- At a certain point, there is a maximum magnification on any instrument (such as a microscope or a telescope) at which point any further magnification will create a bigger image, but not one with any further detail. This occurs when the finest detail the instrument can pick up is magnified to match the finest detail the eye can see. For an optical microscope using oil immersion, the best possible magnification is 1200x. Any increases in magnification beyond this point will result in "empty magnification," or the enlargement of the image with no increase in resolution. Some cheap optical instruments take advantage of this, by including eyepieces that give magnification higher than is useful.
