# Homework Help: Eye/Lens Question

1. Mar 28, 2005

### dranger35

I have a question concerning the eye but I need to know how many diopters of adjustment an adult eye has? I've been looking online alot but can't find what the answer is. Please help.

2. Mar 28, 2005

### Andrew Mason

The eye has to focus the image on the retina which is located about 17 mm behind the lens of the eye. The focal length of the lens of the eye adjusts to the range of the object being viewed. To view an object at a long distance away, the lens must have a focal length:

$$\frac{1}{f} = \frac{1}{o} + \frac{1}{i} = 0 + \frac{1}{i}$$ so

$$f = i = 17mm$$

A lens with a focal length of 17mm, has a diopter value of 1/.017 = 58.8

If the eye can also focus on an object up to a distance of 15 cm (6 inches), then:

$$\frac{1}{f} = \frac{1}{.15} + \frac{1}{.017} = 65.5$$

So the diopter range of the eye is from about 58.8 to 65.5 diopters.

AM

Last edited: Mar 28, 2005
3. Mar 28, 2005

### krab

It's an easy thing to measure. Can you focus to infinity? And as close as 1 foot (.3 m)? Then your range is 1/.3m - 1/infinity = 3.3 diopters.
I'm a totally different example: My closest focus without glasses for my right eye is 0.17m, farthest focus is 0.25m. So the range is 1/.17-1/.25= 2 diopters. (That's cuz I'm over 50.) And of course to correct my farthest focus to infinity requires a -6 diopter correction (6 diopters = 1/.17m). That's my prescription. When correected, my close focus is 1/2diopters = 0.5 metre.