Human eye, focal length

[Bengo]

I never could understand this: the lens focuses light onto the retina so shouldn't the distance between the lens and the retina be the focal distance? It does not seem to be the case while I was trying to look around for the answer on the internet. And since light is focused on the retina, the image forms behind the retina? Thank you in advance!

 

[A.T.]

shouldn't the distance between the lens and the retina be the focal distance?

It is, when you look at distant objects.

 

[Drakkith]

I never could understand this: the lens focuses light onto the retina so shouldn't the distance between the lens and the retina be the focal distance?

Sure, for light that is parallel to itself when it enters the pupil. But for closer objects the light is still diverging and it will be focused further from the lens. That's why your lens has to bend and change its focal length to accommodate objects at a huge variety of distances.

And since light is focused on the retina, the image forms behind the retina?

No, the image forms on the retina, just as a glass lens will form an image on a screen.

 

[Andy Resnick]

I never could understand this: the lens focuses light onto the retina so shouldn't the distance between the lens and the retina be the focal distance? It does not seem to be the case while I was trying to look around for the answer on the internet. And since light is focused on the retina, the image forms behind the retina? Thank you in advance!

Most of the refractive power of your eye is generated at the cornea due to the large change in refractive index, the lens is a secondary element. A 'standard' model of the human eye, the Gullstrand model, is described here:

http://www.google.com/url?sa=t&rct=...VyevD3H5h1j7poHsQ&sig2=Hz0wcl5g022HPf12ZItuQg

 

[sophiecentaur]

the image forms behind the retina?

Where did that come from`? Do you have a reference?

 

[Bengo]

Thank you all for the help! But I'm still very confused. I keep thinking the image has to form behind the focal point because in my physics class we drew these diagrams similar to those seen here with the giraffe http://www.olympusmicro.com/primer/anatomy/magnification.html .

So if an object is infinitely far away, the image is basically at the focal point, which is at the retina?

In the case that the object is closer, the focal length is shorter than the distance between the lens and the retina but the image still forms on the retina?

 

[Drakkith]

Thank you all for the help! But I'm still very confused. I keep thinking the image has to form behind the focal point because in my physics class we drew these diagrams similar to those seen here with the giraffe http://www.olympusmicro.com/primer/anatomy/magnification.html .

Oh god, that image is terribly misleading. Rays emanate from each point on an object and spread out in a large cone. Part of this expanding cone is caught by the lens of the optical device and the rays are then focused down to a spot on the focal plane. However, each spot on the object has its own expanding cone of rays emanating from it. The image here is only showing two rays, each from opposite edges of the image, as they pass through the eye, and even then they aren't shown well.

So if an object is infinitely far away, the image is basically at the focal point, which is at the retina?

Let's be specific here. The rays from each point on the object are focused down to a point on the focal plane. The focal plane lies at some distance from the lens. (To simplify things let's assume that the lens is doing all the focusing, even though it really isn't) If the object you are looking at is in focus, then this focal plane lies at the same distance from the lens as the retina, so that they coincide. (In reality there is a little bit of 'play' in that if the focal plane is just a little too far forward or backward the image will not be noticeable out of focus)

The focal length of any simple lens is usually given by assuming that the rays from a point on an object are parallel to each other. In reality the rays from an object are always diverging when they hit the lens. The closer the object is to you, the more the rays are diverging when they enter the lens and the further back the focal plane lies. In other words, the closer the object is to the lens, the longer the focal length is for an object at that distance. Luckily, the lens in your eye can change its shape so that the focal length changes. This is why far away objects that were in focus now appear blurry when you focus on a nearby object. The rays used to be focused on the retina, but the since the focal length of the lens has changed, they are now focused too strongly. They pass through each other and spread out before hitting the retina, causing the image of a far away object to appear blurry.

In contrast, nearby objects appear blurry when you are focused on a far away object because the focal plane for an object at that close distance lies behind the retina. In other words, when focused on far away objects, the lens doesn't bend the light rays enough to focus them down onto the retina.

 

[sophiecentaur]

In the case that the object is closer, the focal length is shorter than the distance between the lens and the retina but the image still forms on the retina?

The image is always adjusted so that it lands on the retina because the focal length of the system is adjusted by muscles which alter the focal length of the lens. You will notice that you can only focus for one distance at a time. Your brain, of course, is so smart that it 'remembers' much of the detail of a scene after the eye has re-focussed at a different depth. You can read a book and study ships on the horizon quite happily, without being aware of what's going on in your eye. It also works when you close your eyes and the whole input from your eyes is switched off. You are still aware of the scene you were looking at.