The magnification often accompanying an image in a book, usually expressed in the form as (say) x1,500 is included purely as a guide to give the reader an indication of the degree of primary enlargement used to reveal the given amount of detail. However, in a sense it is both meaningless and misleading. Consider the two micrographs below.
Scent scale or androchonium of male Large White butterfly (Pieris brassicae).
The upper micrograph is twice the size of the lower and the original greyscale image, directly from the microscope, is considerably larger than either. The original magnification is x9,820 but neither image on this page has a magnification this high. Many publications, hard copies or digital, will have the original magnification quoted but unless the image has been reproduced at exactly the same size at which it was originally captured the figure will be incorrect. Scientific papers usually have a bar mark on the image representing a given length (e.g 250um) and no matter at what size the image is reproduced the bar mark will always represent that stated length.
A very convenient way of determining magnification is to state the width of the horizontal field of view (FOV) of the micrograph. In the examples above both micrographs have the same FOV of 61.4um (0.061mm) and providing the image does not get cropped during subsequent publication this always remains true. It makes no difference if they are being viewed on a laptop, tablet, mobile phone or have printed them out, both images still have the same FOV. It is clear however the magnification of the larger image must be greater than the smaller one. The field of view gives the width of the image as it was in real life before being magnified.
As an example, FOV of 100um (0.1mm) means the full width of the image being viewed was in real life 0.1mm. The image on your laptop might be 7.5cm or, more conveniently, 75mm wide but this still represents the original FOV of 0.1mm and is therefore 75mm / 0.1mm times larger than life size, a magnification of x750. If the same image was being viewed on a wide-screen television at one metre wide the magnification would equate to x10,000 (1000mm / 0.1mm) whilst the FOV remains the same.