Minimum Blank Size, Induced Prism, and a Progressive in a Shallow Frame
A 58-year-old woman selects a metal frame marked A 54, DBL 18, B 34, ED 58, temple 140. Her monocular PDs are 33/31 (64 total). Her Rx is −4.00 DS OU with a +2.00 add, and she has chosen a standard progressive with an 18 mm corridor. Her pupil height, measured with the frame adjusted on her face, is 20 mm from the lowest point of the lens. The lab has only 68 mm finished-diameter semi-finished blanks in the 1.60 index she wants. The optician is told: 'just cut it, there is 10 mm of decentration, it will be fine.' The frame's boxing centres sit temporal to both pupils.
1. Calculate the frame PD, the decentration per eye, and the minimum blank size. Can the 68 mm blank be used?
Read the worked answer
Frame PD = A + DBL = 54 + 18 = 72 mm, so each boxing centre is 36 mm from the bridge centreline.
Decentration = |frame PD/2 − monocular PD|:
- OD: 36 − 33 = 3 mm
- OS: 36 − 31 = 5 mm
Minimum blank size = ED + 2 × (decentration, using the larger eye) + edging allowance: MBS = 58 + 2(5) + 2 = 70 mm
A 68 mm finished blank is 2 mm short. If it is cut anyway, one of two things happens: the optical centre is pulled toward the frame centre (inducing prism), or the edge is machined too thin and breaks out at the nasal or temporal extreme. Correct actions: order a 70–72 mm blank in 1.60, choose a frame with a smaller A or DBL, or accept only a modest reduction in decentration. 'Splitting the difference' is not a lab instruction - it is a dispensing error waiting to be returned.
2. If the lenses were edged with the optical centres on the boxing centres, what prism would each eye get and what would the patient report?
Read the worked answer
Prentice's rule: P (Δ) = c (mm) × F (D) / 10, where c is the distance from the optical centre.
- OD: 3 × 4.00 / 10 = 1.2Δ
- OS: 5 × 4.00 / 10 = 2.0Δ
Direction: a minus lens is thickest at the edge, so the prism base lies away from the optical centre. Here both OCs sit temporal to their pupils, so the pupil is looking through the nasal side of each lens and the base is nasal → base in, both eyes.
What the patient reports: 3.2Δ total of base-in prism that she never had before, and - worse - it is asymmetric (0.8Δ difference between eyes). Symmetric errors are often adapted to; unequal ones usually are not. Expect asthenopia and 'my eyes ache by the end of the day,' worst with near work, with distance vision possibly described as 'fine.' The patient will blame the progressive design, and the real cause is decentration.
3. The frame has a B measurement of 34 mm and her pupil sits 20 mm from the bottom of the lens. How does that interact with the progressive choice, and what do you recommend?
Read the worked answer
Fitting height is measured from the lowest point of the lens to the centre of the pupil, and it must cover the corridor plus a usable near zone. A standard 18 mm-corridor design typically needs about 22 mm of fitting height; 20 mm is roughly 2 mm short, which truncates the reading zone and makes the near area feel 'small and low.'
Options:
- Change the design to a short-corridor progressive (minimum fitting height typically ~17–18 mm) in the same frame - usually the cleanest fix.
- Change the frame to one with a deeper B (roughly 38–40 mm), which raises the measured fitting height into the standard-design range; this may also change A/DBL and therefore the minimum blank size, so re-run the MBS calculation.
- Do not simply raise the fitting cross to make the numbers work - that steals from the distance zone and creates a new complaint.
Also flag the interaction with task 1: if the frame is changed to a deeper or larger one, the decentration changes and the 68 mm blanks may still be inadequate. Solve blank size, decentration and fitting height together before the order goes to the lab.