4.2 Magnification and scale bars
When you see micrographs in older text-books, a magnification is usually stated in the legend (e.g. x100). Strictly, this should mean that the magnification of the illustration in the book is 100-fold larger than the original item. However, there is occasionally some ambiguity. For example, the statement can mean that the picture was taken using a microscope set with a 100x magnification (10x objective, 10x eyepiece). Since the light path to the camera is not the same as the light path to the eye (which passes through the eyepieces), these magnifications are not meaningful. Moreover publishers may increase or decrease the size of a micrograph to fit the available space. The stated magnification in the legend should then be corrected, but often it is not.
For these reasons, the use of scale-bars has replaced a statement of magnification. A scale bar, corresponding to a convenient unit of length, is added to the image taken by the camera, and is then an integral part of that image. If the image is increased or reduced in size thereafter, the scale bar changes in proportion, so that it is always possible to see the correct size of the cells or tissue.