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That distance is sometimes given on the filter in millimeters. A +3 close-up lens has a maximal working distance of 0.333 m or 333 mm. The magnification is the focal distance of the objective lens (f) divided by the focal distance of the close-up lens; i.e., the focal distance of the objective lens (in meters) multiplied by the diopter value (D) of the close-up lens:
A dioptre (British spelling) or diopter (American spelling), symbol dpt or D, is a unit of measurement with dimension of reciprocal length, equivalent to one reciprocal metre, 1 dpt = 1 m −1. It is normally used to express the optical power of a lens or curved mirror , which is a physical quantity equal to the reciprocal of the focal length ...
In optics, optical power (also referred to as dioptric power, refractive power, focusing power, or convergence power) is the degree to which a lens, mirror, or other optical system converges or diverges light. It is equal to the reciprocal of the focal length of the device: P = 1/f. [1] High optical power corresponds to short focal length.
Rear, rotating diopter drum sight of a SIG SG 550 service rifle.The viewing aperture above the "3" (denoting the 300 m setting) can be seen. The diopter is in principle a vertically and horizontally (elevation and windage) adjustable occluder with a small hole (aperture), and is placed close in front of the shooter's aiming eye.
Lens clock. A lens clock is a mechanical dial indicator that is used to measure the dioptric power of a lens.It is a specialized version of a spherometer.A lens clock measures the curvature of a surface, but gives the result as an optical power in diopters, assuming the lens is made of a material with a particular refractive index.
The prescription consists of all the specifications necessary to make the lens. Prescriptions typically include the power specifications of each lens (for each eye). Strengths are generally prescribed in quarter-diopter steps (0.25 D) because most people cannot generally distinguish between smaller increments (ex. eighth-diopter steps / 0.125 D).
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For example, to change refraction by 4 diopters with an optical zone of 3 mm would require ablation of 12 μm. As the depth of ablation is proportional to the square of the optical zone, changing the refraction by 4 diopters but with an optical zone of 6 mm would require a much deeper ablation of approximately 48 μm. [ 2 ]