[Leica] Re: Focal length of human eye

[Chris Bitmead <chrisb@ans.com.au>]

Shawn London wrote:

> The human eye itself is exactly like a lens.  That's what it IS.  There's
> no mystery there.  You are confusing perceptual issues in the brain and
> retinal anatomy which have nothing to do with the actual focal length of
> the eye.

Just like a 50mm lens on a large format camera is different to a
50mm lens on a 35mm camera which is different to a 50mm lens on
an APS camera, so it is that you need to know the "film area" of
the human eye in order to make comparisons. Yes, you could work
out the focal length of the human eye, but that wouldn't help
much without a number representing the film area.

The "film plane" in the eye isn't even flat, it is curved. But
forget that. How will a scientist measure it? Do you measure the
small area called the fovea that has maximum perception yet can
only record a few degrees of sight? Or will you measure the whole
retina right up to 240 degrees that can barely see, but can
really only perceive movement?

As you move from the centre to the edge the percentage of rod vs
cone receptors changes radically, such that you can barely
perceive colours. As you move even further you can barely
perceive shapes, only movements. Where exactly to draw the line
to make your measurement?

> When trying to get an idea of the approximate focal length of your eye,
> it helps to look through a rolled up piece of paper to keep you from
> confusing the wide angle of view of the human eye with its actual focal
> length, which is the issue here.  

How big a roll of paper, and what do I look for once I'm looking
through it?

> The human eye can have such a huge
> angle of view at a given focal length when compared to the equivalent
> lens in front of a 35mm film frame because the "film" of the eye is a lot
> larger, proportionally.
> 
> >Tell us how a scientist might
> >determine what length lens corresponds to the human eye?
> 
> One of the many ways of determining the focal length of a lens is:
> 1/f=1/s+1/s' where f= focal length in cm, s=distance from object to lens,
> and s'= is distance from image produced to lens.  Thus, s would be the
> distance from the object you're looking at to the front of your eye, and
> s' is the distance from the lens of the eye to the image produced on the
> retina (this changes as your eye focuses).

Yes, and then what? Let's say we figure out the focal length is
10mm. Now what can we say about the human eye vs camera lenses?