Photochromatic lenses feature some of the most advanced technology found in sunglass lenses today. Photochromy works when there is an increase in light intensity and the lenses react by darkening, ideally by 20%-30%, or there is a reduction in light intensity so the lenses lighten resulting in a lens which reacts to light.
The benefits of photochromatic or react-to-light lenses are phenomenal. A prime example and benefit of photochromatic lenses is that of when driving. Light conditions will be constantly changing and there may be times when your sunglass lenses are too dark or too light. This could be a result of overhead cloud cover, sudden glare from the road, driving through a wooded grove. With these constant and rapid changes it is very hard to take off and put on sunglasses, as well as altering sun visors and mirrors. If you were wearing photochromatic sunglasses then you would not have had to take off and then put on your sunglasses. The lenses would have reacted to the change in light conditions and would keep your eyes well protected throughout your journey.
A photochromatic lens will only alter in the area subjected to change in light, therefore if only one part of peripheral vision is subjected to an increase in light intensity then only that part will change. Every area of the lens is fully independent from all other areas.