Lecture 5 single vision dispense and collection

What is a single vision lens?

A lens with the same power across its entire surface, prescribed for distance, intermediate, or near vision.

What are the three main ophthalmic lens categories?

Single vision lenses

Bifocal lenses

Progressive (PPL) lenses

What factors affect lens curvature?

Lens power

Lens thickness

Lens shape

How does lens curvature affect vision?

Steeper curvature → shorter focal length

Steeper curve + thicker lens → greater magnification

Incorrect curve may reduce comfort

How does lens power affect lens thickness?

Minus lenses → edge thickness increases

Plus lenses → centre thickness increases

How can lens thickness be reduced during dispensing?

Choose a frame size close to the patient's PD to minimise decentration.

What is decentration?

The difference between the Frame PD and the Patient PD.

What is the MSU formula?

MSU = (Frame PD − Patient PD) + Widest box lens measurement + 2 mm.

What is the rule for selecting lens blank size?

Always choose the next largest 5 mm blank size.

Example: MSU 67 mm → use 70 mm blank.

What is PD (IPD)?

The distance between the centres of the pupils, used to position lens optical centres correctly.

Why is PD important in dispensing?

Correct PD places the optical centres in line with the visual axes and prevents unwanted prism in primary gaze.

What is the difference between anatomical and physiological PD?

Physiological PD uses corneal reflexes and aligns visual axes.

Anatomical PD uses anatomical landmarks and is typically 0.5-1.0 mm larger

When should monocular PD be measured?

For multifocal lenses and patients with facial asymmetry.

What is Near Centration Distance (NCD)?

The position of the optical centres for near vision when the eyes are converged. NCD is typically 3-4 mm less than distance PD.