The Queen Mother had an operation to remove a cataract from her left eye in 1995 when she was 95 but, unlike her daughter, she spent a night in King Edward VII's Hospital after the surgery.
When cataracts develop, they can cause vision to become blurry or hazy.
Cataracts are more common among elderly people.
Although she turned 92 in April, the queen traditionally holds an official birthday on the second Saturday of June - this weekend - to enjoy more favourable weather.
Cataracts obviously affect the vision, but symptoms can vary between sufferers.
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What happens during cataract surgery?
Cataract surgery is the most common operation in the United Kingdom, and involves replacing the cloudy lens with an artificial one.
During the operation, a tiny cut is made in the eye to remove the cloudy lens and replace it with a clear plastic one.
Cataract surgery typically takes just 30 to 45 minutes under local anaesthetic, but it can take an individual four to six weeks to fully recover. During recovery, patients have to protect their eyes from bright light for at least a few weeks.
We've always known Queen Elizabeth is one tough lady, whether she was greasing elbows as a mechanic and truck driver during World War 2 or taking up the mantle of Queen at a young age.