What does magnification specifically measure?

Magnification specifically measures the height of the image relative to the object. In optics, magnification is defined as the ratio of the size of the image produced by an optical instrument, such as a microscope or telescope, to the actual size of the object being observed. This means that magnification provides insight into how much larger (or smaller, if less than one) the image appears compared to the original object. It is calculated by dividing the height of the image by the height of the object, which directly reflects how much the image has been enlarged or reduced.

Other choices relate to different aspects of image formation or perception, such as the angular size of an object, which concerns the angle that the object subtends at the lens, rather than a direct measure of size; brightness, which refers to how luminous an image appears; and field of view, which pertains to the extent of the observable area that can be seen at one time through the device. Each of these is an important concept in optics, but they do not define magnification itself.