If you've ever driven with the blazing sun facing you (and who hasn't), you know that glare can greatly affect your quality of vision -- reducing contrast and even blurring what you see. At times glare can be intense enough to cause momentarily blindness. But it can also affect your eye health, causing eye fatigue and even cataracts.
Damage from the sun accumulates over time, so protecting the eyes should begin as early as possible. Because of this, lenses that filter glare are essential. Photochromic lenses are ideal because they not only absorb 100% of UVA and UVB light, but they change as the intensity of light changes. They are light enough to wear indoors, but darken enough to shade the eyes if exposed to the sun (which means carrying around prescription sunglasses for bright light conditions is unnecessary).
The popularity of the Transitions brand of photochromatic lenses has led to the common practice of referring to these lenses as "transition lenses."
Some worry about the lenses intensely tinting at undesirable times. The lenses are made of certain substances, including silver halide, that undergo a chemical reaction -- causing the lenses to fully darken -- only when exposed to UV light.
If they do change color while you're indoors, they do so in only a slight way -- you likely won't even notice. If you walk outdoors, they will automatically darken if UV radiation is present. The brighter the sun, the darker the lenses will get, usually as dark as regular sunglasses.
There is a catch, however. If there's something blocking UV rays, such as the windshield of your car, the lenses will be prevented from darkening. Because of this, most photochronic lenses are tinted.
Photochromic lenses promote healthy eyesight and protect your eyes and vision every day. Because they automatically adjust to different light conditions, these lenses increase your ability to distinguish objects of different sizes, brightness and contrast, so they will actually improve your overall quality of vision.
Fowler, Colin and Keziah Latham Petre. “Spectacle Lenses.” Elsevier Health Sciences, 2001. Pp 121.