Find the maximum and minimum focal lengths of the eye lens
Answers
Answer:
Explanation:
I think an anology to a camera lens is confusing here. We are accustomed to thinking that if we put a 50mm focal length lens on a full frame camera that it is always 50 mm focal length even when we adjust the focus to a near object. Like taking a magnifying galass and repositioning it to focus an image of something closer.
This most definitely is not true. For example, Nikor lenses dramatically change effective focal length when refocused. Depending on what type photography you are doing, this could be an issue. Canon lenses tend to preserve focal length, but not completely. I know photographers who greatly prefer Canon lenses for studio work for this reason. A fixed magnidying lens has the same EFL no matter where you move it focus.
The lens in the eye does not move. It refocuses by actually changing its shape and curvature. In other words, it changes effective focal length. In fact it changes by as much as 20 diopters.
Lets assume an eye length of 22.2 mm. The cornea has about 40 diopters of power (focal length 25 mm). The lens has to add another 5 diopters to focus a distant object on the retina. This is a focal length of 22.2 mm.
Now a near object only 10 cm from the eye lens needs another 10 diopters of focusing power to arrive focused on the retina. So the eye has to have a total of 55 diopter to focus on it. This is a focal length of 18 mm.
Very young people can focus as close as 6.5 mm from the eye. This is an additional 5 diopters of focus. In this case, the total focusing power of the eye is 60 diopters. That is a focal length of 16.7 mm. If you froze the eye in this condition and then looked at a distant star, the light would in fact focus at 16.7 mm, which would be well in front of the retina.
The actual focal length of the lens in the eye does change from 16.7 mm to 22.2 mm which may not be intuitive to photographers, particularly Canon photographers who are used to having a nice constant EFL.
You actually specified the eye lens. Remember the cornea is 40 diopters. That means that the actual crystalline lens varies from 5 to 20 diopters. That is a focal length of 200mm to 50mm.
The cornea has about 40 dipolers of power. The lens has to add another 5 dipole to focus a distant object of light retina