Segment Inset: Why the Reading Zone Sits Closer Together Than Your Pupils
The near zone is not under the pupil
Look closely at a pair of bifocals or at the corridor of a progressive lens and you will find that the near zone is not centred on the distance optical centre. It sits inward, toward the nose, by a millimetre or two. That offset is called inset, and it exists because of a fact that is easy to state and easy to forget: when you look at something close, your eyes turn inward. The reading zone has to be where the eyes actually point at reading distance, not where they point at the horizon.
Inset is a horizontal decentration, specified per lens, and it is one of the few numbers in dispensing that can be calculated from first principles rather than guessed from a table.
Convergence, in one line of geometry
Take the distance between your pupils as PD, measured at the eye. Now look at a target at distance d. The two lines of sight meet at the target, so at the plane of the spectacle lens, which sits a distance h in front of the eye, the separation between those lines is smaller than PD. Similar triangles give the relationship directly: the separation at the lens plane equals PD multiplied by one minus h divided by d.
Substitute the usual values and the size of the effect becomes clear. With a 63 mm PD, a 13 mm vertex distance and a 40 cm reading distance, the separation at the lens plane is 63 times 0.9675, which is 60.95 mm. The near pupils are about 2 mm closer together than the distance pupils. Half of that difference, about 1 mm per lens, is the inset that would put the segment's optical centre under the line of sight at that distance.
What the number actually is
- 63 mm PD at 50 cm: separation 61.4 mm, inset about 0.8 mm per lens.
- 63 mm PD at 40 cm: separation 61.0 mm, inset about 1.0 mm per lens.
- 63 mm PD at 33 cm: separation 60.5 mm, inset about 1.2 mm per lens.
- 66 mm PD at 40 cm: separation 63.9 mm, inset about 1.1 mm per lens.
Two conclusions follow. First, inset is small: it is measured in tenths of a millimetre and rarely exceeds two. Second, it depends on the working distance, which means a lens optimised for a desk at 70 cm wants less inset than one optimised for a book at 33 cm. A single inset value is therefore always an approximation of a range, and the range is what the dispenser is really choosing.
What happens when the inset is wrong
An error in inset means the near line of sight passes through the reading zone at a point away from its optical centre. Prentice's rule then applies to the addition: prism dioptres equal distance from the centre in centimetres multiplied by the power of the near zone. With a +2.50 D add, a 1 mm error produces 0.1 times 2.50, or 0.25 prism dioptres per lens, and half a prism dioptre across the pair.
Half a dioptre of horizontal prism at near is not enormous, but it is continuous: every minute of reading carries it, and the eyes must converge or diverge against it for as long as the task lasts. The subjective result is not blurred vision but a specific kind of fatigue, usually described as pulling, aching or a reluctance to keep reading, and it typically appears after twenty minutes rather than immediately. Whether the residual prism sits base-in or base-out depends on which way the inset is wrong, but the consequence is the same: the eyes are being asked to hold a posture they did not choose.
Why progressives can hide a bad inset and flat-tops cannot
A digitally surfaced progressive lens can carry a variable inset: the corridor widens and shifts as the addition rises, and the software places the near zone according to the working distance the dispenser specifies. That flexibility is why progressive lenses tolerate a range of reading habits. A flat-top bifocal cannot do this: the segment is a fixed moulded insert with a fixed optical centre, and the only variables are its height and its horizontal position in the frame. If the wearer reads at a distance the lens was not built for, the prism is simply there.
Three misconceptions
Your PD is one number. Distance PD and near PD are different quantities, and the difference is a function of working distance, not of the person. Ordering a lens with the distance PD applied to the near zone guarantees a small error for every wearer, in the same direction.
Inset is cosmetic. It is a prism specification in disguise. A lens with the near zone in the wrong place is not badly styled, it is optically off-axis at the exact point where the wearer spends the most time.
More inset is safer. Overshooting puts the near zone nasal to the line of sight and produces the opposite prism. Inset is a target, not a margin, and adding to it for comfort usually moves the problem rather than removing it.
When to raise it with your optician
Ask about inset if reading is comfortable at one distance and effortful at another, if one eye tires before the other in a way that follows the frame rather than the eye, or if a pair of glasses built for a desk is being used for books. Those are fitting questions with numeric answers. Bring the working distance you actually use, measured rather than estimated, because it is the input the calculation needs and the one most often guessed.
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