Clever lenses

I’ve just bought a new pair of glasses. The prescription is a little different from my old pair, meaning that although everything is slightly sharper the world seems to curve downwards a bit towards my right. That’s just my brain getting used to the new way in which the world is projected onto my retina – give it a few days and it will figure out what the image is supposed to be and things will look level again.

We’ve got a talk coming up at cafe scientifique on the human visual system. The particular question John Perrone will talk about is how the brain can make sense of an image that is continually jumping around on the retina – you move your head, and the image moves on the back of the eye – but your brain still makes sense of it all and you don’t see a blurry, shaky world out there.

I once saw a demonstration of a piece of real-time camera-shake elimination software. A video camera was stuck on the end of a long, flexible pole, and its picture was fed live to a screen. The guy demonstrating it then struck the camera hard, and it bounced quite violently on the end of the pole, but the image stayed almost stationary. I don’t know what processing was going on in the computer (presumably it wasn’t specific to the type of shake at the end of a pole – but I don’t know) but it looked pretty impressive.

A camera and the eye are similar in many respects, but there are a few differences. For a start, the eye has a pretty impressive bit of image processing ‘hardware’ behind it, namely the brain. The iris is pretty well the same – except in the eye it is in front of the lens, whereas for the camera it is behind the lens. Then there’s the focusing method. For a camera, the focus is changed by changing the distance between the lens and the sensor (or ‘film’ for people who remember what that is) – to focus on close objects you increase the distance of the lens from the sensor – while the eye focuses by changing the shape of the lens – to look at nearer things the lens is squashed to a lens with greater power – to look at far away objects it reverts to a lower power lens.

A short-sighted person like me has an lens to retina distance that is too long – which means the image from far-away objects comes into focus before the retina. To correct for this my glasses are made from diverging lenses – that counteract slightly the power of the lenses in my eyes, so that the image slips backwards a bit and lands nicely on my retina in focus. And I can see again.


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