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Magnifying Terminology

1. Lens Magnification

2. Lens size

3. Focal distance

Lens Magnification and The Lens Size

The higher the magnification of a magnifying instrument, the smaller the magnifying lens diameter. The lower the magnification of a magnifying instrument the larger the magnifying lens diameter.

Magnification ratings of 2-3X power lens provide larger viewing areas and these are perfect for activities like enlarging the text in a book or newspaper. To reveal the details of a gemstone, stamp, or other items with extremely fine detail, smaller diameter lens are used. These small diameter lens instruments are usually called loupes. Loupes typically provide magnifications that range from 10-60X.

Focal Distance

Instruments with a smaller lens diameter typically have higher magnification while having a shorter focal distance. The focal distance of a magnifying lens is the distance between the lens and the object being viewed.Instruments with shorter focal distances like loupe's must be held close to the object being viewed. For these magnifier types they are usually held no more than an inch away from the object being examined.

More Information

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.[3] 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.

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 MP0 = (0.25 m) 1, where is the optical power in dioptres, and the factor of 0.25 m represents the assumed near point ( 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 = MP0. This is sometimes called the total power of the magnifier (again, not to be confused with optical power).

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).

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.

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