What is the magnification of a field if its field size is 5mm?

To determine the magnification of a field, you need to know both the field size and the actual size of the object being viewed. If the field size is 5 mm, the magnification can be calculated by comparing the field size to the size of the image produced by the optical system. Without additional information about the actual size of the object or the image size, it’s impossible to determine the magnification accurately.