Focal length: The distance from the focal point to the optical center is called focal length. Represented by "f"
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Lens refractive power: the length of lens focal length f indicates the refractive power. The shorter the focal length, the greater the refractive power. Usually, the reciprocal of the focal length of a lens is called the lens power. It is expressed by φ, that is, φ =1/f.
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Lens power: the power of glasses
=
100× reciprocal of focal length. Namely: degree
=
Φ× 100
=
(1/ layer) × 100. The degree of hyperopia lens (convex lens) is represented by a positive number; The degree of myopia lens (concave lens) is expressed by negative number. For example: +300 degrees, -200 degrees.
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If the focal length of a lens is 0.5m, its focal power is φ =1/0.5m = 2m-1; Usually, the lens power of glasses is the value of lens power multiplied by 100. For example, the lens magnification of an original lens with 100 degree is 1m- 1, and the focal length is 1m.
Such as the focal length of 500-degree glasses.
According to the formula, 500 =100×1/f.
Therefore, f=0.2m=20cm.
If it is a concave lens, it is the virtual focus, f=
-20cm； If it is a convex lens, it is the focus, f=20cm.