Vertical Imbalance and Slab-Off: When Each Lens Gives a Different Prism
The problem only appears when you look down
A pair of glasses is measured at the distance optical centre. That is the point your pupil sits over when you look straight ahead, and it is the only point where the prescription you were given is literally realised. Read a book, though, and your eyes rotate downward, so the line of sight crosses the lens several millimetres below that point. In a single-vision lens this changes very little. In a bifocal or a progressive it is the entire purpose of the design, because that lower region carries the reading addition.
What is easy to miss is that every point on a lens away from its optical centre behaves like a weak prism, whether or not your prescription contains any prism at all.
Prentice's rule: prism is induced, not prescribed
The relationship is one of the simplest in ophthalmic optics. The prismatic effect at a point on a lens equals the distance from the optical centre, measured in centimetres, multiplied by the power of the lens in dioptres. Move twice as far from the centre and you double the prism; use a lens of twice the power and you double it again. No prism has to be written on the prescription for this to happen, and no error has to have been made in the laboratory.
An example makes the scale concrete. Suppose the right eye needs +2.00 D and the left +3.00 D, a one-dioptre difference between them. Read at a point 10 mm below each optical centre, which is 1 cm: the right lens induces 1.00 prism dioptre and the left induces 1.50. Each eye is now looking through a vertical prism, but the two prisms do not match, and the mismatch is half a prism dioptre.
If both lenses carried the same power, both would induce the same prism and the difference would be zero. This is why vertical imbalance is almost always a story about unequal prescriptions rather than about a mistake, and why it is most familiar in anisometropia, where the two eyes need meaningfully different powers.
Why unequal prisms feel worse than they sound
Binocular vision works by fusing two slightly different views into one percept, and the visual system is markedly better at tolerating horizontal differences than vertical ones. Horizontal disparity is the raw material of depth perception. Vertical disparity is not useful; it is an error to be suppressed or overcome.
When the two lenses deliver different amounts of vertical prism on downward gaze, each eye is presented with an image displaced by a different amount, and the extraocular muscles must absorb a disjunctive vertical movement each time the wearer looks down to read. The technical name for this state is spectacle-induced anisophoria. It shows up as eye strain concentrated on near work, a sense that one image rides slightly above or below the other, a dull frontal headache, and in more marked cases frank double vision.
Slab-off: one lens ground as a bi-prism
Slab-off is the classical answer. The name describes the method: a quantity of material is ground away across the lower part of one lens, effectively making that lens a bi-prism with two optical centres. The upper, distance portion keeps its original optical centre, and the lower portion acquires a second one, so the reading zone carries compensating prism while the distance zone is left undisturbed.
Because the correction is achieved by asymmetric removal of material, the lens it is applied to is chosen deliberately, and plenty of material has to remain afterwards for the lens to stay sound. On a high-minus lens that is not trivial. This is ordinary thickness arithmetic, the same arithmetic that decides how a strong prescription looks inside a given frame, and it can be estimated in advance rather than discovered at collection. Our lens thickness calculator works through that estimate from your prescription, frame and index.
Slab-off is not the only option. Reverse slab-off does the same job by grinding the opposite lens, dissimilar segment heights or designs can shift where the imbalance appears, and Fresnel press-on membranes can be trialled before anything is ground permanently. Progressive lenses, being power-variation lenses rather than two distinct zones, approach the same problem by other means.
One technique is worth separating out, because it is easy to confuse with the treatment. Prism thinning adds a matching base-down prism to both lenses in order to reduce thickness, which is a cosmetic and comfort measure rather than a correction of imbalance.
How much vertical prism is too much
The figure usually quoted is around 1.5 prism dioptres of unwanted vertical prism at the reading level. Below that figure, symptoms are rare, and the slab-off line itself becomes difficult to produce cleanly at low prism values. Some clinicians set their own working threshold nearer 2 prism dioptres, and the decision is routinely based on how the wearer describes their vision rather than on the number alone.
Common misconceptions
- "Prism in my glasses must have been prescribed." Induced prism appears in every off-centre gaze and is a consequence of geometry. Prescribed prism is written on the prescription; induced prism is not written anywhere.
- "If I am not seeing double, there is no imbalance." Many wearers adapt, developing a gaze-specific tolerance rather than a null imbalance. The adaptation costs effort, and effort is what strain is.
- "A slab-off means my glasses were made wrongly." A slab-off is a deliberate design decision, visible as a faint horizontal line. It is not a defect.
- "Everyone with an unequal prescription needs one." If binocular vision is impaired, for instance with amblyopia, a large imbalance may cause no symptoms at all, and correcting it brings no benefit.
When to ask about it
The pattern worth reporting is specific: discomfort that concentrates on reading or close work, that started or worsened with a new pair, and that eases when you close one eye. Measure the reading distance and note which eye you tend to use. An optometrist or dispensing optician can calculate the imbalance from the prescription and the reading level, and decide whether a slab-off, a different segment, or a repositioned frame is the better answer.