Prism Base Direction: Base In, Out, Up, and Down Explained

Prentice’s rule (Δ = cF) gives the amount of induced prism; the lens sign gives the base. A plus lens is thickest at its center, so the base points toward the optical center. A minus lens is thickest at its edge, so the base points away. Base in with base in adds; base up in both eyes cancels.

Last checked Nov 2, 2026. Confirm fees and dates with the certifying body. optical.education is independent and is not affiliated with or endorsed by the American Board of Opticianry and National Contact Lens Examiners (ABO-NCLE). Disclaimer.

Magnitude recap

The amount of induced prism comes from Prentice’s rule, Δ = cF, with c in centimeters. That part is covered on the Prentice’s rule guide. This page is about the other half of the answer: which way the base points, and what happens when you put the two eyes together. Working out prismatic effect falls under Ophthalmic Optics (25%) in ABO-NCLE’s test specifications, checked Nov 2, 2026.

Plus vs minus: the thick-part rule

A prism’s base is its thickest edge. Standard texts describe a plus lens as two prisms joined base to base, and a minus lens as two prisms joined apex to apex. That gives one rule you can apply anywhere on a lens:

• Plus lens: thickest in the middle, so the base points toward the optical center (OC).
• Minus lens: thickest at the edge, so the base points away from the OC, toward the nearest edge.

Put another way: if you move a plus lens, the base follows the direction the OC moved. If you move a minus lens, the base goes the opposite way.

Horizontal: base in, base out, add or cancel

Base in means toward the nose and base out means toward the temple, whichever eye you are on. So for a right lens, base in points to the wearer’s left; for a left lens, it points to the wearer’s right.

• Same label in both eyes adds. Base in OD plus base in OS is a larger total base-in effect. The same goes for base out with base out.
• Opposite labels work against each other. Base in in one eye and base out in the other partly cancel, so you subtract the smaller from the larger.

Vertical: base up, base down, and imbalance

Vertical prism works the other way round. Base up in front of both eyes moves both images the same way, so it cancels. Base up in one eye with base down in the other adds. Base down before the right eye affects the wearer the same way as base up before the left.

Reading below the optical centers: a plus lens gives base up (the thick middle is above the reading point), and a minus lens gives base down. When the two lenses differ in power, the eyes get different amounts of vertical prism, and the difference is the vertical imbalance.

Check it with the calculator

The Prentice’s rule calculator asks for the lens power, the distance, and which way the optical center sits from the pupil. It shows the base direction with the same thick-part reasoning used here. Work each example below by hand, then check it.

Worked examples

1. Minus lenses set too far in

Both lenses −4.00 D. Each OC sits 3 mm nasal to the pupil, so the wearer looks through a point 3 mm temporal to each OC.

1. Amount: 0.3 × 4.00 = 1.20Δ per eye.
2. Direction: minus, base away from the OC. The OC is nasal, so the base is temporal: base out.
3. Both eyes base out, same label, so they add: 2.40Δ base out total.

2. Plus lenses set too far out

Both lenses +3.00 D, each OC 2 mm temporal to the pupil.

1. Amount: 0.2 × 3.00 = 0.60Δ per eye.
2. Direction: plus, base toward the OC, which is temporal: base out.
3. Total: 1.20Δ base out.

3. Vertical imbalance at near

OD −5.00 D, OS −2.00 D, reading 10 mm below both optical centers.

1. OD: 1.0 × 5.00 = 5.00Δ, minus below the OC, so base down.
2. OS: 1.0 × 2.00 = 2.00Δ base down.
3. Both base down, same vertical direction, so they cancel in part: 5.00 − 2.00 = 3.00Δ imbalance, base down OD (the same as 3.00Δ base up OS).

Practice problems

Original problems. Find the amount, then the base direction.

1. +2.00 D lens, right eye. The optical center sits 4 mm nasal (in) from the pupil.

   Show answer

   Δ = 0.4 × 2.00 = 0.80Δ. Plus: base toward the OC, so the base is in. 0.80Δ base in

2. −6.00 D lens, left eye. The optical center sits 2 mm temporal (out) from the pupil.

   Show answer

   Δ = 0.2 × 6.00 = 1.20Δ. Minus: base away from the OC, so the base is in. 1.20Δ base in

3. Both lenses −3.00 D. Each optical center sits 3 mm out from the pupil. Total horizontal prism?

   Show answer

   Each eye: 0.3 × 3.00 = 0.90Δ base in. Same label in both eyes adds: 1.80Δ base in total.

4. OD +1.00 D, OS +3.00 D. The wearer reads 8 mm below both optical centers.

   Show answer

   OD 0.8 × 1.00 = 0.80Δ base up; OS 0.8 × 3.00 = 2.40Δ base up. Same direction cancels: 1.60Δ imbalance (net base up OS).

5. OD −2.00 D and OS +2.00 D. The wearer looks 3 mm above both optical centers. Total vertical prism?

   Show answer

   Above a minus OC the base is up: OD 0.60Δ base up. Above a plus OC the base is down: OS 0.60Δ base down. Opposite vertical bases add: 1.20Δ total.

Reading prism on a lensometer is a separate skill; see the lensometer simulator.

Standard texts used for this page: Brooks CW, Borish IM. System for Ophthalmic Dispensing, 3rd ed. Butterworth-Heinemann, pp. 62–63 (Prentice’s rule, base direction), pp. 356–358 (base in, out, up and down; both-eye effects), pp. 377–378 (decentration and base direction) and p. 501 (vertical imbalance); the University of Iowa Optics Review and the National Academy of Opticianry’s Advanced Prism course, linked under sources.

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