Optics/ the eye/ near, far-sightedness

Bengo

Hello all,

I'm having trouble understanding the human eye and what the focal length has to do with where light converges. Why do we need a longer focal length to see distant objects and shorter to see near objects?

Thank you

Simon Bridge

Draw an eye, with a vertical line in front of it representing the corrective lens, on an optic axis.

In the case of near sightedness ... there will be a maximum distance that the eye will see a clear image, which will be close to the lens. So mark that spot on the optic axis ... this is the spot the rays must appear to come from for the virtual image the lens presents to the eye.

Now mark in a very distant object.
How does the lens have to bend the rays from the very distant object to make them appear to come from the clear image?

Andy Resnick

Don't forget that the distance from the lens to your retina is (essentially) constant. In order to focus on objects at varying distances, the only parameter that can vary is the focal length. It's also worth noting that about 2/3 of the refractive power in your eye is located at the air-cornea interface; the lens provides the remaining 1/3.