|The near zone of progressives are nasally displaced to ensure that the fields of view of the two converging eyes coincide. This lateral displacement of the near zones is known as inset.|
In near vision the horizontal prismatic effects of the lens require deflection of the lines of fixation, the extent of which is dependent on the dioptric power. The higher the vertex power of the lens, the greater the convergence required by hyperopic eyes in near vision, while less convergence is needed by myopic eyes.
Carl Zeiss gives special consideration to this by the principle of variable inset. The inset of Gradal Top progressives varies from 2.0 mm to 4.5 mm, depending on the distance power and the addition. Variable inset ensures that the wearer always experiences the largest possible binocular field of view and is an important component of horizontal symmetry.
Lines of fixation and pricipal rays for myopic wearers
Lines of fixation and pricipal rays for hyperopic wearers
Variable inset in Gradal Top E progressives (simplified)
Thickness reduction prism
Engravings and stamp
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