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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 ...
The aim of an accurate intraocular lens power calculation is to provide an intraocular lens (IOL) that fits the specific needs and desires of the individual patient. The development of better instrumentation for measuring the eye's axial length (AL) and the use of more precise mathematical formulas to perform the appropriate calculations have significantly improved the accuracy with which the ...
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.
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.
For example, if a person has NP = 1 m and the typical near point distance at their age is D = 25 cm, then the optical power needed is P = +3.24 diopters where one diopter is the reciprocal of one meter.
c is decentration (the distance between the pupil centre and the lens's optical centre, in millimetres) f is lens power (in dioptres) The primary use of Prentice's rule is that under certain circumstances, the prescribed prism can be obtained without grinding prism into the lenses, by decentering the lenses as worn by the patient.
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]