When 20/20 Isn't the Whole Story
A patient may read the smallest letters on a standard eye chart yet report difficulty recognizing faces in poor lighting or seeing low-contrast details. A 20/20 result is valuable, but it does not describe every aspect of visual function.
Best-corrected visual acuity (BCVA) and low-contrast visual acuity (LCVA), also called low-contrast letter acuity (LCLA) in some research, answer related but different questions. One measures how well a patient resolves high-contrast detail with appropriate refractive correction; the other examines letter recognition when the difference between the letters and their background is reduced.
What Is BCVA?
BCVA describes the best visual acuity achieved with the appropriate optical correction under the specified testing conditions. It differs from uncorrected visual acuity, measured without correction, and presenting visual acuity, measured with the glasses or contact lenses a patient is wearing at the time.
BCVA remains central to eye examinations, clinical decision-making, and ophthalmic research. However, a normal high-contrast acuity result does not exclude other problems with visual function, particularly when a patient reports symptoms that the acuity chart does not explain.
Why ETDRS and logMAR Scoring Matter
ETDRS-style charts use a consistent logarithmic progression of letter sizes, proportional spacing, and five letters per line. When the established testing protocol is followed, letter-by-letter scoring helps clinicians document changes more consistently than traditional line-based Snellen reporting.
In the logMAR scale, lower values indicate better acuity, and a five-letter line represents approximately 0.1 logMAR. For a comparison of chart formats, see Snellen vs. logMAR visual acuity testing. ETDRS methods are widely used in clinical studies because they support repeatable measurements, but a study's required chart, distance, illumination, and endpoint definitions depend on its specific protocol. Neither a single chart design nor a single testing distance is mandatory for every clinical examination or regulatory study.
Changes of 15 ETDRS letters, or three lines, are important endpoints in several major retinal disease trials. That does not mean every change below 15 letters is clinically irrelevant, or that 15 letters is the universal action threshold for all patients and conditions.
What Is Low-Contrast Visual Acuity?
LCVA assesses how well a patient can identify letters when the contrast between the optotypes and the background is lower than on a standard high-contrast chart. Sloan letter charts with fixed contrast levels such as 2.5% and 1.25% are used particularly in multiple sclerosis (MS) and optic neuritis research, although other levels and chart designs are available.
The size of the letters still changes across the rows, as it does in high-contrast acuity testing. LCVA therefore measures acuity at a specified low-contrast level; it is not interchangeable with contrast sensitivity testing, which evaluates the ability to detect progressively smaller differences in contrast, often using letters of constant size or other targets.
Results depend on the chart, contrast level, illumination, viewing distance, correction, and scoring method. Very faint charts may also produce floor effects, where a patient can identify few or no letters, making further decline difficult to measure.
What Can These Tests Reveal About Your Patients?
A patient with normal or near-normal BCVA can still experience difficulty with low-contrast visual tasks. Examples include distinguishing subtle edges, reading faint print, or recognizing features under challenging illumination. LCVA may help document an aspect of that difficulty, although no single chart reproduces the complexity of everyday vision.
Low-contrast letter acuity has been studied extensively in MS and optic neuritis. Research has demonstrated associations between low-contrast letter scores, patient-reported visual function, and measures such as retinal nerve fiber layer thickness on optical coherence tomography (OCT). These relationships make LCVA useful as a complementary functional measure in appropriate clinical and research settings, not as a standalone test that diagnoses MS or determines the cause of visual symptoms.
The 2024 revisions of the McDonald MS diagnostic criteria recognize the optic nerve as a fifth anatomical location relevant to dissemination in space. Supporting evidence of optic nerve involvement may come from appropriately interpreted OCT, visual evoked potentials (VEP), or MRI; LCVA should not be described as an independent formal criterion.
Reduced contrast performance may also occur in other ocular conditions, but the clinical evidence, preferred testing method, and interpretation vary by disease. When a patient reports problems despite satisfactory BCVA, the next evaluation should be guided by their symptoms and examination findings rather than assuming that an abnormal LCVA score identifies a particular diagnosis.
Testing Conditions and Meaningful Change
Interpretation depends on consistency. To compare results across visits, clinicians should document and, where possible, reproduce the same chart model, test distance, chart luminance, room lighting, refractive correction, monocular or binocular testing mode, and scoring procedure.
Traditional ETDRS protocols commonly use a four-meter viewing distance, but other validated distances and chart arrangements may be appropriate for a specific setting or trial. Luminance targets also vary by chart and protocol, so approximately 85 cd/m² should not be presented as a universal BCVA standard. Equipment should be selected and maintained according to the measurement protocol being used.
In MS low-contrast letter acuity research, a seven-letter loss has been used as a meaningful change threshold based on test-retest variability and associations with other outcomes. It should not be applied automatically to other diseases, contrast levels, or chart systems. Similarly, interpretation of an ETDRS letter change should account for test variability, the clinical context, and the specific study or practice protocol.
Tools for BCVA and LCVA Testing
Chart selection and illumination should follow the clinical or research protocol. The following Good-Lite tools support different approaches to low-contrast letter acuity assessment and standardized chart viewing.
Flip Book: Sloan Letter Low Contrast
Sloan letter flip book with five contrast levels from 25% to 1.25%.
ETDRS Sloan Letter Low Contrast Charts (13 ft / 4 m)
Translucent Sloan low-contrast charts for compatible ESV3000 and ESC2000 cabinets.
Good-Lite ESC2000™ Illuminated ETDRS Clinical Trial Cabinet
LED-illuminated viewer for compatible high- and low-contrast visual acuity charts.
For more on illuminated viewing systems and protocol-specific luminance choices, see BCVA and LCVA Testing: Why ETDRS Cabinet Illumination Matters. Chart compatibility and testing conditions should always be verified before use.
When Should Clinicians Use BCVA, LCVA, or Both?
Use BCVA to establish corrected high-contrast visual acuity and follow it over time as appropriate to the examination or clinical protocol. The result provides an important reference point, especially when standardized testing is repeated at subsequent visits.
Consider LCVA when low-contrast complaints are not explained by high-contrast acuity, or when the condition and assessment protocol call for a complementary low-contrast measure. For neurological and research applications, interpret LCVA alongside the broader clinical examination and other established outcome measures.
A 20/20 BCVA result and an abnormal low-contrast score are not contradictory. Together, the findings can help clinicians describe different aspects of a patient's visual performance and choose appropriate follow-up assessments.
