Why am I still able to see if I take off my glasses and
look through a small hole that I make with my index finger and thumb?
An object is "out of focus" because the lens of the eye converges some
rays behind your retina, some rays in front of the retina and some right
on the retina. These different focal depths correspond to light that goes
through different regions of the lens (moving outward from the
center). If you limit the light to only go through the center of the lens
then it has only one focal depth.
Think about what has to happen for you to see some object clearly: light
from each spot on the
object has to land on one particular spot on your retina, and not on any
other spot. If you are
looking through a small hole, this is guaranteed, because there is only
one path from any given
spot on the object to your retina. Light that does not go in exactly the
right direction does not make
it through the pinhole, and so cannot blur the image you see.
But when the hole you are looking through gets bigger, you need some
other way to make light from
a spot on the object land all on one spot on your retina. A lens can do
that. So, if you are looking
through a large hole, you need a good, well focused lens; if you are
looking through a small hole, the
lens is irrelevant.
You have reinvented the pinhole camera! It works because, at least if the
hole is infinitesimal, every point in the scene you are looking at
illuminates just one point on the retina, To understand this, draw a retina
(or a flat screen) and an object (perhaps an arrow) with a tiny hole between
them, Drawing straight lines (because light travels in straight lines) from
points on the object, through the pinhole and onto the screen will
illustrate the principle.
There are disadvantages to pinholes, which is why lenses are popular. First
a small hole lets through only a small amount of light whereas you would in
general like to have a bright image. Second, light going through holes is
diffracted, more so as the hole is made smaller. This means the light ray
can be bent as it goes through the pinhole (light does not always travel in
straight lines) and makes the image fuzzy and badly focused.
Best, Dick Plano, Professor of Physics emeritus, Rutgers University
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Update: June 2012