Why do sunglasses work?

[budfox]

Possibly a silly question, but isn't the point of the pupil in your eye to open and close, allowing for different light levels? If that's the case then how come putting on a pair of shades doesn't just force your pupil to open a bit and negate the effect?

[Simpo Two]

If you think of the iris as the aperture in a camera lens, it can only close so much. If that's still too bright, you need shades.

[budfox]

Yes, but even in conditions where my iris would be within opening and closing range, sunglasses stil make things darker.

[tapkaJohnD]

The Eye isn't just a camera, it uses a large amount of processing power to give you sight, in the retina and in the brain.
So when you put on shades, your pupil will dilate a bit, maybe to a point where it would be when the light it 'sees' was as low, but your brain registers that the light is dimmed. It has the sceen around the lenses as a comparison.

This processing is even more pronounced with colour, as taking photographs with film under natural and artifical light wil show. That green shirt will look quite different in the tow pics, but your eye will see no difference, in life.

JOhn

[Xerstead]

Don't forget the benefit of polarising lenses to reduce glare and reflections.

[budfox]

Thanks all.