<P> The magnification of a magnifying glass depends upon where it is placed between the user's eye and the object being viewed, and the total distance between them . The magnifying power is equivalent to angular magnification (this should not be confused with optical power, which is a different quantity). The magnifying power is the ratio of the sizes of the images formed on the user's retina with and without the lens . For the "without" case, it is typically assumed that the user would bring the object as close to one eye as possible without it becoming blurry . This point, known as the near point, varies with age . In a young child, it can be as close as 5 cm, while, in an elderly person it may be as far as one or two metres . Magnifiers are typically characterized using a "standard" value of 0.25 m . </P> <P> The highest magnifying power is obtained by putting the lens very close to one eye and moving the eye and the lens together to obtain the best focus . The object will then typically also be close to the lens . The magnifying power obtained in this condition is MP = (0.25 m) Φ + 1, where Φ is the optical power in dioptres, and the factor of 0.25 m represents the assumed near point (1⁄4 m from the eye). This value of the magnifying power is the one normally used to characterize magnifiers . It is typically denoted "m ×", where m = MP . This is sometimes called the total power of the magnifier (again, not to be confused with optical power). </P> <P> However, magnifiers are not always used as described above because it is more comfortable to put the magnifier close to the object (one focal length away). The eye can then be a larger distance away, and a good image can be obtained very easily; the focus is not very sensitive to the eye's exact position . The magnifying power in this case is roughly MP = (0.25 m) Φ . </P> <P> A typical magnifying glass might have a focal length of 25 cm, corresponding to an optical power of 4 dioptres . Such a magnifier would be sold as a "2 ×" magnifier . In actual use, an observer with "typical" eyes would obtain a magnifying power between 1 and 2, depending on where lens is held . </P>

Where should an object be placed in order to use a convex lens as a magnifying glass