Ophthalmic Optics and Formulas Flashcards

The standard fitting height for a flat-top 28 bifocal segment is measured from:

Answer: The lowest point of the pupil to the top of the bifocal segment

Segment height is measured from the lowest limbal margin of the pupil down to the top of the bifocal segment.

Spectacle magnification (SM) is the product of which two factors?

Answer: Power factor and shape factor

SM = Power Factor × Shape Factor; power factor depends on back vertex power and vertex distance, shape factor on front curve and thickness.

A near addition of +2.50 D allows a patient to focus at what working distance?

Answer: 40 cm

Working distance = 1 / add power = 1 / 2.50 = 0.40 m = 40 cm.

Monocular PD for bifocal segment centration should be measured with the patient:

Answer: Looking straight ahead at distance fixation

Distance monocular PD is measured with the patient looking at a distance target, ensuring proper optical center placement for the distance portion.

Image jump at the top of a bifocal segment is determined by:

Answer: The add power multiplied by the distance from the segment top to the segment's optical center

Image jump = add power × distance from segment top to segment optical center; for flat-tops the OC is below the top, causing jump.

The minimum blank size (MBS) formula for lens ordering is MBS =

Answer: Effective diameter (ED) + 2 mm safety margin + 2 × decentration per side

MBS = ED + 2 × decentration + safety margin (typically 2 mm) to ensure the blank covers the entire edged lens.

Slab-off prism is used to correct vertical imbalance in anisometropic bifocal wearers. The amount of slab-off is determined by:

Answer: Prentice's rule applied to the difference in add powers at the near reference point

Slab-off corrects the differential vertical prism induced by the add; calculated using Prentice's rule on the add power difference at the reading level.