Cataracts are diagnosed through a comprehensive eye exam that typically includes a vision test, a slit-lamp examination, and a dilated eye evaluation. There’s no single test that confirms a cataract. Instead, your eye doctor pieces together findings from several steps to determine whether a cataract is present, how severe it is, and whether it’s the actual cause of your vision changes.
When to Get Screened
The American Academy of Ophthalmology recommends a baseline eye disease screening at age 40, even if your vision seems fine. Cataracts develop slowly, and early clouding of the lens often causes no noticeable symptoms. If you have risk factors for eye disease, such as diabetes, a family history of cataracts, or long-term steroid use, your eye doctor may recommend exams earlier and more frequently. After the baseline screening, they’ll tell you how often to come back based on what they find.
Visual Acuity Testing
The exam usually starts with a standard eye chart reading, where you identify letters from across the room. This measures your best corrected visual acuity, meaning how well you see with your current glasses or contact lens prescription. A score of 20/40 is a significant threshold: it’s the minimum for unrestricted driving in the United States, and research shows it marks a tipping point where about half of people start struggling with common vision-dependent tasks.
But a standard eye chart only tells part of the story. Cataracts often affect how well you see in low contrast or glare conditions well before they change your letter-reading ability. Your doctor may also test contrast sensitivity, which measures your ability to distinguish objects against similar-toned backgrounds at varying levels of detail. This test can be performed with and without a glare source under both day and night lighting conditions. Contrast sensitivity testing is especially useful for detecting early cortical and posterior subcapsular cataracts, the types most likely to cause problems with oncoming headlights or bright sunlight.
The Slit-Lamp Exam
The slit-lamp is a specialized microscope with a thin, adjustable beam of light. Your doctor focuses this beam through your eye layer by layer, starting at the front surface and moving deeper. When the beam reaches your lens, it creates what’s called an optical section, essentially a cross-sectional view of the lens in real time. This reveals the distinct internal layers of the lens: the outer capsule, the cortex, and the denser central nucleus.
In a healthy lens, most of these layers appear clear. A cataract shows up as cloudiness, discoloration, or opaque spots in specific zones. Your doctor can see exactly where the clouding is located and how dense it has become. Nuclear cataracts appear as yellowing or browning deep in the center of the lens. Cortical cataracts show spoke-like opacities radiating from the edges. Posterior subcapsular cataracts form a plaque-like haze on the back surface of the lens. Each type affects vision differently, which is why pinpointing the location matters.
How Doctors Grade Severity
Eye doctors don’t simply label a cataract as “mild” or “severe.” They use a standardized grading system called the Lens Opacities Classification System (LOCS III), which compares what they see in your lens to a set of reference photographs showing increasing stages of cloudiness. The system evaluates three zones of the lens separately: the nucleus, the cortex, and the posterior subcapsular area.
For the nucleus, two features are graded: how opaque it has become and how much it has changed color. Each is scored on a scale from 0.1 (completely clear) to 6.9 (advanced clouding and deep brown discoloration). Cortical and posterior subcapsular cataracts are graded from 0.1 to 5.9, with the highest score indicating complete opacification. These numbers help track progression over time. If your cortical cataract was graded a 2.0 last year and it’s a 3.5 now, your doctor has an objective measure of how much it’s advanced.
Dilated Eye Exam
To get the fullest view of your lens and the structures behind it, your doctor will use dilating drops that widen your pupils. Think of it like opening a door to let more light into a dark room. With the pupil fully open, the doctor can examine the entire lens from edge to edge, including peripheral areas that aren’t visible through an undilated pupil.
Dilation also allows your doctor to check the retina and optic nerve at the back of your eye. This step is critical because cataracts aren’t the only condition that causes blurry or dimmed vision. Diabetic retinopathy, glaucoma, and age-related macular degeneration can all produce overlapping symptoms. If one of these conditions is also present, removing the cataract alone may not fully restore your vision, and your doctor needs to know that before recommending surgery.
Catching Hidden Problems With Imaging
Sometimes the cataract itself makes it harder for your doctor to see the retina clearly during a standard exam. This is where advanced imaging comes in. Optical coherence tomography (OCT) uses light waves to create detailed cross-sectional images of the retina, and it works even when a cloudy lens or a poorly dilating pupil limits the view through a traditional microscope.
Research has shown that experienced ophthalmologists can miss subtle retinal problems when a cataract is obscuring their view. In one study, routine OCT scanning before cataract surgery detected macular issues in a considerable number of patients that had been missed during standard examination. Finding these conditions beforehand helps set realistic expectations for what surgery can and can’t fix.
Clues From Your Changing Prescription
One early diagnostic clue is a noticeable shift toward nearsightedness in your glasses prescription. As a nuclear cataract develops, the lens thickens and its internal density increases, which bends light more strongly. This change in the lens’s refractive power can produce an average shift of nearly 3 diopters toward nearsightedness. Some people actually find their reading vision temporarily improves, a phenomenon sometimes called “second sight,” even as their distance vision worsens. If your eye doctor sees a significant and unexpected prescription change, particularly toward nearsightedness, it raises suspicion that a nuclear cataract is forming.
Ruling Out Other Causes
Not all lens or corneal cloudiness is a cataract. Corneal scarring, swelling, or dystrophies can also reduce vision and cause glare. During the slit-lamp exam, your doctor can distinguish between cloudiness in the cornea (the clear front window of the eye) and cloudiness in the lens (which sits deeper inside). If there’s any question about whether the cornea is contributing to poor vision, a simple test can help: placing a contact lens on the eye. If vision improves significantly with the contact lens, the cornea is likely the culprit rather than the lens behind it. Corneal topography and light scatter measurements can further clarify how much each structure is contributing to the problem.
Measurements Before Surgery
Once a cataract is confirmed and you and your doctor decide surgery makes sense, additional diagnostic measurements come into play. Your natural lens will be replaced with an artificial one, and its power needs to be calculated precisely. This process, called biometry, involves measuring two things: the curvature and power of your cornea (using a device called a keratometer or corneal topographer) and the length of your eye from front to back (measured with ultrasound or a laser-based optical instrument).
These two measurements, corneal power and axial length, are plugged into a formula that determines the correct power for your replacement lens. Accuracy here directly affects your vision after surgery. Even small measurement errors can leave you more nearsighted or farsighted than intended, which is why this step is done carefully and sometimes repeated to confirm results.