Visual acuity: how it's tested and what the results mean

Visual acuity is the threshold for recognizing small optotypes under given lighting conditions and distances. This measurement serves as a vital sign in ophthalmology: it allows for a rapid assessment of central vision function, allows for comparison of results between visits, and allows for an understanding of the contribution of the optic nerve, retina, and visual pathway. A properly performed test yields reproducible numerical values, which are used to guide decisions about correction and treatment. [1]

Several notations are used in clinical practice: decimal, Snellen, logarithmic (logMAR), and letter counting based on the Early Treatment Diabetic Retinopathy Study principles. For comparison of results between patients and studies, logMAR and tables based on the Bailey-Lovie and ETDRS principles are preferred because the size of the characters and the spacing between lines vary uniformly on a logarithmic scale. [2]

Visual acuity is only part of a functional assessment. In many conditions, patients with "good" visual acuity complain of "flat" or "dim" vision. In such cases, contrast sensitivity completes the picture and helps explain the complaints, especially in early macular pathology and after surgery. [3]

Another practical challenge is to distinguish uncorrected refractive error from organic pathology. A pinhole diaphragm can be helpful here: improved reading through the pinhole indicates a role for refraction, while a lack of improvement suggests a diagnosis of medial and retinal diseases. [4]

Table 1. Basic visual acuity designations and relative reference points

Notation	Example of a recording	Example in decimal	Example in logMAR	Comment
Snellen metric	6 by 6	1.00	0.00	Household and historical records
Snellen Anglo-American	20/20	1.00	0.00	Metric equivalent
Decimal	0.5	0.50	0.30	The higher the number, the better the vision.
logMAR	0.30	0.50	0.30	The lower the number, the better the vision.
Visual Acuity Score	85 points	0.30	0.30	Conversion from ETDRS 1 point per letter [5]

Why are ETDRS and logMAR more accurate than Snellen?

The classic Snellen chart is uneven: the number of letters per row varies, the geometric progression of sizes is not maintained, and the spacing between lines varies. This reduces accuracy and makes it difficult to compare results. The Bailey-Lovie and ETDRS logarithmic charts eliminate these problems: 5 digits per row, equal spacing, and a 0.1 logMAR step between lines make the test sensitive to small changes. [6]

In large comparisons, ETDRS scores are more stable and yield "better" values in patients with low visual acuity than Snellen, due to its uniform scale and frame-by-frame letter counting. This is why clinical trials and recorded outcomes are based on ETDRS, and in practice, conversion between scales is permitted. [7]

For interpretation, it is convenient to remember a simple rule: 1 row in the logMAR table equals 0.1 logMAR, and 1 letter equals 0.02 logMAR. This allows for correct interpolation of a partially read row and obtaining accurate numerical differences between visits of the same patient.

If necessary, equivalents can be used: decimal, Snellen metric and Snellen imperial, VAS scale. Recent validation studies for near vision also publish tables for correct conversion between recording systems. [9]

Table 2. Equivalents between scales

logMAR	Decimal	Snellen 6 on X	Snellen 20 on X
0.00	1.00	6 by 6	20/20
0.10	0.80	6 to 7.5	20 to 25
0.30	0.50	6 by 12	20 to 40
0.50	0.32	6 to 19	20 to 63
1.00	0.10	6 to 60	20 to 200

Standardized conditions: distance, illumination, optotypes

The adult distance test is performed at a fixed distance and under photopic lighting conditions. For ETDRS charts and their analogs, uniform retro-illumination of approximately 85 cd/m² is recommended, ensuring consistent contrast and repeatability between rooms and visits. [10]

The choice of optotypes depends on the task. For clinical measurements, Sloan letters, preschool symbols, and Landolt rings are used. The international standard ISO 8596 describes the method of using Landolt rings under photopic conditions for certification and licensing purposes. [11]

The room lighting must be comfortable and stable, without glare on the media. The distance to the table is recorded on the examination card, as is the type of optotypes and the recording method. Even a slight variation in distance without recalculation leads to pseudodynamics. [12]

Retro-lit cabinets approved by regulatory authorities are used to document outcomes in studies. This reduces inter-cabinet variability and makes the data comparable. [13]

Table 3. Reference conditions for the distance test

Parameter	Recommendation	Why is this necessary?
Table highlighting	About 85 cd/m²	Stable contrast and repeatability
Optotypes	Sloan, Landolt, children's symbols	Comparability with guidelines and standards
Distance	Fixed and recorded	Eliminate re-sorting of results
Recording the result	logMAR and an additional convenient form	Convenience of comparison and monitoring [14]

Protocol for adults: step-by-step and without pitfalls

The test is performed first without correction, then with the usual correction. The results are recorded for both eyes separately and, if necessary, binocularly. On each attempt, the patient reads lines from top to bottom until the last one, where they recognize at least three of the five characters. The partially read line is recalculated using the "letter" scheme. [15]

If reading is limited, a pinhole diaphragm is used. A significant improvement indicates a refractive error, while a lack of effect suggests opacities, macular pathology, or a neuro-ophthalmological cause. This is a quick triage method in the office and during mass examinations. [16]

Contrast sensitivity is also assessed when complaints are not explained by visual acuity measurements. The Pelli-Robson test is informative for mid- and low-frequency spatial measurements and correlates better with functional complaints of reading and driving at twilight than visual acuity alone. [17]

It's important to avoid common mistakes: unstable lighting, incorrect distance, prompts, and "memorizing" the table. During repeat visits, it's helpful to change the optotype layout while maintaining a 0.1 logMAR step. [18]

Table 4. Pinhole diaphragm: what does improvement mean?

What's happening	Interpretation	The next step
Improvement by 1 line or more	Uncorrected refraction is likely	Correction selection, retinoscopy
No improvement	An organic cause is likely	Biomicroscopy, OCT of the macula as indicated
The difference between the eyes	Anisometropia or asymmetry of pathology	Refinement of refraction and structure

Childhood screening and amblyopia: age thresholds and tools

In children aged 3-5 years, at least one visual acuity screening has strong evidence of benefit for the early detection of amblyopia and risk factors. Pediatric societies and ophthalmological associations recommend direct visual acuity measurement in cooperative children and instrumental risk screening in young children. [19]

When cooperating, age-appropriate optotypes are used: HOTV or LEA symbols at a distance of 3 meters or 10 feet. The essence of the "critical line": the child must recognize at least 3 of the 5 symbols on a given line. The "spiral" format of multi-sided tables for children's groups facilitates work in classrooms and schools. [20]

The "passed" threshold varies by age: benchmarks of 20 over 50 for 3-year-olds and 20 over 40 for 4-year-olds are widely accepted in guidelines and recent review publications on preschool screening. Any failure to meet the threshold is grounds for referral to an ophthalmologist. [21]

If cooperation is lacking, instrumental screening for amblyopia risk factors is used at early visits and progresses to direct measurement as the child matures. Standardization of distance, lighting, and reward is critical for reliability. [22]

Table 5. Childhood screening: what is considered “passed”

Age	Optotype	Distance	The threshold has been passed
3 years	HOTV or LEA	About 3 meters or 10 feet	No worse than 20 to 50
4 years	HOTV or LEA	About 3 meters or 10 feet	Not worse than 20 to 40
5 years	Sloan or HOTV	About 3 meters or 10 feet	Not worse than 20 to 32

Near Vision and Reading: Standardized Flashcards

Near vision testing is necessary for all patients with reading complaints and in presbyopic age. For comparability, reading cards with a logarithmic font size progression and standardized text are preferred, allowing for measurement of not only the "minimum legible font" but also the critical print size and reading speed. [23]

Calibrated kits such as MNREAD, RADNER, and Bailey-Lovie Word Reading Charts comply with the recommendations of the International Council of Ophthalmology and provide reproducible reading parameters. This is particularly useful in macular pathology, neuro-ophthalmological diseases, and in evaluating surgical outcomes. [24]

Near notations include "N-size," "M-units," and decimal equivalents at a fixed distance of 40 centimeters. Recent publications provide consistent conversion tables, facilitating data exchange between clinics and research centers. [25]

It's important to document not only the threshold, but also the reading speed at a comfortable font size. These parameters correlate more strongly with quality of life than distance acuity alone. [26]

Table 6. Near-field cards: what they measure and when they are useful

Kit	What does it give?	Where it is especially useful
MNREAD	Read threshold, critical size, speed	Macular pathology, presbyopia
RADNER	Logarithmic step, speed, reading accessibility index	Dynamics after treatment
Bailey-Lovie Word	Comparability with logarithmic scale	Research and rehabilitation

Contrast sensitivity: when the numbers are good, but it's hard to see

Contrast sensitivity reveals a decrease in visual acuity with normal visual acuity and explains complaints about reading small, low-contrast text, glare, and difficulty in twilight. In practice, the Pelli-Robson and other validated tests are used, selecting an instrument based on the spatial frequency range of interest. [27]

The Pelli-Robson score evaluates the mid- and low-frequency range and often correlates with actual performance better than acuity alone. To improve reliability, a minute-by-minute "letter" score is recommended, which narrows the confidence limits and makes repeat visits comparable. [28]

Systematic assessment of contrast is indicated in diabetic retinopathy, after corneal and lens surgery, in neuro-ophthalmological diseases, and in cases where "numbers do not explain the complaint." This adds clarity to the treatment plan and rehabilitation. [29]

Table 7. Contrast sensitivity tests and clinical utility

Test	Frequency range	Where it helps in solving
Pelli-Robson	Medium and low	Complaints with "normal" acuity, twilight
Cambridge, Regan, UW	Different ranges	Selection for a specific task

Home and telemedicine solutions: When appropriate and how to interpret them

Smartphone tests based on ETDRS principles have demonstrated good agreement with clinical measurements in field and clinical studies. This expands access to primary screening and monitoring, especially in settings where retrofitted testing rooms are not available. [30]

However, limitations are important: distance, screen brightness, pixel size, and calibration can vary between devices. Therefore, for medical decisions and therapy, in-person measurements on logarithmic tables under controlled conditions are preferable, with apps used as a preliminary guide. [31]

A convenient "home monitoring plus in-office confirmation" system. The patient uses a proven app according to the instructions, and the clinic confirms the progress using a standard chart at 85 cd/m². This reduces the number of unscheduled visits and improves treatment adherence. [32]

Table 8. Home tests: where they are suitable, where they are not

Scenario	The application is sufficient	A desk review is required
Self-monitoring between visits	Yes, according to instructions	Yes, with any deterioration
Decision on therapy or surgery	No	Necessarily
Mass field surveys	Yes, as a screening	Yes, to confirm positive results

Common mistakes and how to avoid them

A common mistake is confusing scales and distances, recording "improvement by 1 line" without specifying which chart and under what conditions. Any layout change requires maintaining the 0.1 logMAR step, fixed distance, and highlighting description.

The second mistake is ignoring the pinhole test. Rapidly differentiating between optical and organic factors saves time, especially in emergency situations. Lack of improvement through diaphragm reduction requires an extensive investigation. [34]

The third mistake is to assess only acuity when complaints are typical of decreased contrast. Adding the Pelli-Robson test with a "letter" score increases the diagnostic value and better reflects actual function. [35]

The fourth mistake is the use of non-standardized cards for near reading. To ensure comparable results, distance is specified, calibrated sets are used, and reading speed and critical font size are documented. [36]

Table 9. Error - what it leads to - how to fix it

Error	Risk	Correction
Unspecified test conditions	Pseudodynamics	Fix the distance, illumination, optotypes
Ignoring pinholes	Waste of time and wrong tactics	Add a pinhole diaphragm to the protocol
Only sharpness without contrast	"Normal numbers" with severe complaints	Add Pelli-Robson
Non-standardized cards for near vision	Incomparable results	Use MNREAD, RADNER and similar [37]

Results

Modern visual acuity testing relies on logarithmic tables based on the Bailey-Lovie and ETDRS principles, standardized illumination of approximately 85 cd/m², and precise recording procedures. Pinhole testing and contrast sensitivity measurements enhance diagnostic value and bring interpretation closer to the patient's actual function. [38]