Visual Acuity Test

What it measures

The sharpness of your central vision — how much fine detail you can resolve — expressed as a Snellen ratio such as 20/20. The ratio compares the distance at which you can read a line of letters to the distance a person with reference vision could read the same line.

How it works

You first calibrate the screen by sizing an on-screen card to a real credit card (85.6 mm wide), which tells us how many pixels make a millimetre. After you set your viewing distance, standardised Sloan letters appear at progressively smaller sizes and you select the letter you see. The smallest line you read correctly is converted into a 20/xx acuity estimate.

Tips for an accurate result

• 1Don't squint or peek — Squinting acts like a pinhole and flatters your result. Read naturally and only count letters you genuinely see.
• 2Check each eye separately — A difference between your two eyes can matter clinically even when both are individually 'good'.
• 3Re-test if results surprise you — Tiredness, dry eyes, or a smudged screen can drop a line. A repeat on another day is more reliable.
• 4Blurry vision deserves an exam — Reduced or recently changed acuity is easily corrected with glasses — but should be checked by an optometrist.