Two jobs, checked
| Everyday | Aviator | |
|---|---|---|
| Frame | 52□18 | 56□16 |
| ED | 52 mm | 61 mm |
| PD | 64 mm | 58 mm |
| Blank | 65 mm | 70 mm |
| Frame PD | 70 mm | 72 mm |
| Decentration | 6 mm | 14 mm |
| Minimum blank | 60 mm | 77 mm |
| Verdict | Will Cut Out (65 mm blank is sufficient) | Will NOT Cut Out — Need 77 mm blank |
The aviator fails because a narrow PD in a wide frame moves each optical centre 7 mm inwards. The choices are a larger blank if the lens is made in one, a frame whose A + DBL is closer to the PD, or a second look at the PD measurement.
The formula
MBS = ED + (A + DBL − PD) + 2 mm
- ED
- Effective diameter: twice the longest radius from the boxing centre to the edge of the lens shape.
- A, DBL
- Boxed eye size and bridge, as printed on the frame. A + DBL is the frame PD.
- PD
- The wearer's distance PD for both eyes. A + DBL − PD is the total decentration, twice the amount per eye.
- 2 mm
- Allowance for edging the bevel or groove and small centring errors.
The calculator uses the size of the difference, so a PD wider than the frame PD is handled the same way.
Measuring ED
ED is not printed on most frames and is always larger than A. Find the point on the lens shape furthest from the boxing centre, usually at a lower outer corner, measure that radius and double it. A frame tracer gives it directly.
Limits
- Horizontal decentration only. For bifocals and progressives the fitting height also decentres the blank vertically; combine the two as √(horizontal² + vertical²) and use that in place of the horizontal figure.
- A blank can cut out on paper and still be too thin at the edge for a groove or drill mount.
- The check is only as good as the ED and PD you measured; see the medical disclaimer.
Watch: the same calculation in the Asaan Optics desktop app
This video works a blank size example in the desktop app's calculators, not in the calculator on this page. The calculator on this page uses the same blank size formula.
Video, 2 min 30 s
One lens order, checked from start to finish: a -4.00 / -2.00 x 180 prescription, PD 63, in a frame with eye size 54, bridge 17 and effective diameter 58. The minimum blank size comes to 68 mm (58, plus twice the 4 mm decentration, plus a 2 mm edging allowance), so a 70 mm blank is ordered, and the same job then goes through prism decentration, edge thickness by index, base curve and lens position.
Chapters
- 00:00The lens order
- 00:13Minimum blank size
- 00:52Decentration for prism
- 01:07Lens thickness by index
- 01:38Base curve starting point
- 01:54Lens position effect
What the video covers
- Frame PD 54 + 17 = 71, patient PD 63, so each lens is decentred 4 mm in
- Blank size: 58 + 8 + 2 = 68 mm, and why the app suggests a 70 mm blank
- Decentration for a prescribed prism: 1 prism dioptre base in on a -4.00 meridian moves the optical centre 2.5 mm out
- Edge thickness for -4.00 / -2.00 at 68 mm: about 8 mm at index 1.60, 7.3 mm at 1.67, an estimate for a flat, thin lens
- Base curve by Vogel's rule: +3.50 D as a starting point, to be checked against the manufacturer's chart
- Lens position effect: -6.00 moved 4 mm closer to the eye acts as -6.15 D
Transcript
Work out the blank size, the thickness and the base curve for a lens order, with five calculators. A customer has chosen a frame, and you are ordering the lenses. Open Vision Calculators.
In the list, choose Minimum Blank Size. Type the eye size of the frame, fifty-four, the bridge, seventeen, and the effective diameter, fifty-eight. Then the P D of the patient, sixty-three. The edging allowance stays at two millimetres. The frame P D is seventy-one, so each lens is decentred four millimetres in. This is the figure to order by: sixty-eight millimetres. That is fifty-eight, plus twice four, plus two. The app suggests a seventy millimetre blank. A smaller uncut lens would not cover the frame.
Decentration for Prism is for a prescription with prism. Take one prism dioptre, base in, on a minus four meridian. The optical centre moves two point five millimetres outwards.
Next, Lens Thickness. Enter sphere minus four, cylinder minus two, and the diameter from step one, sixty-eight. At index one point six, the edge is about eight millimetres. Choose one point six seven. Now it is seven point three, twenty-one per cent thinner than index one point five. This is an estimate for a flat, thin lens. The base curve, the lens design and the frame shape change the real thickness.
Base Curve uses Vogel's rule. Enter the same prescription. It suggests a base curve of plus three point five dioptres. That is a starting point: follow the base curve chart of the lens manufacturer.
Last, Lens Position Effect, for a lens that sits closer to the eye, or further away. Minus six, the strongest meridian here, moved four millimetres closer, acts as minus six point one five. You have the figures for the order: a seventy millimetre blank, the edge thickness, and a base curve to check. These calculators support your own judgement and the tables of your lab. They do not replace them.
Questions
What if the minimum blank is bigger than any stock lens?
Order a surfaced lens in a larger diameter if it is made, choose a frame with a smaller eye size or a frame PD closer to the wearer's PD, or re-measure the monocular PDs.
Why add 2 mm?
The edger needs a little extra material to form the bevel or groove, and the lens is never centred perfectly. Two millimetres covers both.