How do I calculate the power of a magnifier?

Magnifiers form images that are not real and cannot be focused onto a screen for instance. The image is called 'virtual' and appears larger and un-inverted when compared to the object being viewed. The magnifying power (MP) of a simple magnifier (handheld magnifier, comparator, pocket magnifier, base magnifier or loupe) is the ratio of the angular size of the image seen with the magnifier to the angular size of the object viewed without the magnifier. The angular object size is the maximum value that the eye can see without any assistance. As an object gets closer to the eye, its angular size becomes larger. However there is a distance at which the relaxed eye cannot focus any closer, this is called the 'near point' and is generally accepted as 10 inches (250mm).

As an example, if a single positive (converging) lens is used as a magnifier and the object is placed at the lens' length (EFL) on one side of the lens, the image will appear to be an infinity. Your eye then focuses that image onto the retina. The generally accepted formula to determine the magnifying power of a simple magnifier is:

MP = 250mm/EFL

The above equation is for small magnifiers, i.e. base magnifiers and pocket magnifiers. The larger magnifiers are viewed so that the virtual image of the object is placed at the near viewing distance of the eye. This is the case for a single positive lens when the object is placed just within the lens' focal length (EFL) on one side of the lens. The image will appear to be formed at the near point distance from the lens when viewed from the other side of the lens. In this configuration the equation changes to:

MP = (250mm/EFL) + 1

Note that angular magnification and is used with magnifiers rather than lateral magnification.