Depth and binocular vision
Short answer
A depth perception test checks how you judge distance; clinical stereopsis tests assess depth from both eyes working together. Frisby, Titmus and Randot use controlled targets.
This guide explains the methods: a flat online picture cannot replace them or certify your depth perception.
In detail
Stereoacuity describes the smallest difference between the two eyes’ views that you can reliably detect as depth. Clinical results commonly use seconds of arc.
The test design, viewing distance and instructions matter, so a score from one method should not be treated as interchangeable with another. Choosing a nearer-looking object from shading or size alone does not measure stereoacuity.
Measurement background: Frisby — Near Stereotest setup and scoring (PDF); Clinical stereotest comparison — original study (2020).
Depth perception is the broader ability to judge spatial relationships. Stereopsis is the contribution from combining slightly different views from two eyes. Size, overlap and perspective also convey depth, including in a picture viewed with one eye. A useful stereopsis test must separate binocular depth discrimination from these other clues.
That distinction explains why a task can look convincingly three-dimensional without testing stereo vision. Choosing the larger ball or the darker circle may answer a picture puzzle, but it does not establish the smallest binocular disparity you can detect. When choosing an assessment, look for a method that measures the ability you actually want to understand.
Method reference: Stereo Optical — Stereo Fly instructions (PDF).
Each method creates and controls binocular depth differently. This comparison describes the materials and their limits, so you can recognize the test named in an appointment.
In a clinical circle stereotest, you identify a circle that appears nearer than its neighbors while wearing the specified viewers. The target’s binocular disparity becomes smaller through the sequence. The clinician records performance according to that booklet’s rules; the answer is not supposed to come from a darker outline or a larger circle.
The standard Randot circle task presents three choices per group; the Stereo Fly circle task presents four. These are different materials with different scoring sequences. A web screenshot, photocopy or memorized answer key cannot preserve the controlled presentation or establish a valid result.
Sources: Stereo Optical — Randot instructions (PDF); Stereo Optical — Stereo Fly instructions (PDF).
A purpose-built online stereoscopic task can present binocular depth, but the equipment and viewing method are part of the test. McGill’s research experience, for example, asks about monitor size and viewing distance and requires amber-and-blue 3D glasses. Its setup is quite different from looking at an ordinary flat image.
Based on those setup requirements, an unvalidated online experience is best treated as a demonstration. Before trusting a claimed clinical score, check how each eye receives a different image, how distance and image scale are controlled, and which published validation applies to that exact implementation.
Sources: McGill University — stereo vision research experience; Clinical stereotest comparison — original study (2020).
Seconds of arc describe an angular difference, not reaction time. A smaller reliably detected disparity represents finer stereoacuity under that test’s conditions. Frisby uses plate-and-distance combinations; the standard Randot booklet uses fixed graded targets. The number must stay attached to the test name and viewing conditions.
Do not read a single number as a universal measure of depth perception in daily life. Comparative research evaluates agreement and variability between stereotests precisely because the methods differ. Ask which test was used, whether its response criteria were met, and whether follow-up with the same method is appropriate.
Sources: Frisby — Near Stereotest setup and scoring (PDF); Clinical stereotest comparison — original study (2020).
Depth, contrast, sharpness and color are different aspects of seeing. Browse our tools and guides to choose the topic that matches your question.
For a measured stereopsis result, ask an eye-care professional about an appropriate clinical test.
Last reviewed September 12, 2026. Educational guide only. No clinical stereotest materials are reproduced and no stereoacuity or fitness assessment is provided.