you still have nagging questions in your mind about lenses, aperatures
and exposure? Are you confused about circles of confusion?
You might be surprised to discover that even many published textbooks
present misleadingly kooky
information on the subject of basic lens optics.
you like to have a more intuitive grasp of how a lens works?
this simple thought experiment:
two points in space, Point A on a brightly colored
object, and Point B, on a white wall.
B receives light rays from billions of rays coming from billions
of places. Its final color is the sum total of all of the incoming light
from each ray.
A is a point on a well lit, brightly colored object. (In this
case, a bright red chili pepper.) It gives off rays of red light in
billions of different directions, but only one of those billions of
rays travels from Point A to Point B. Because (in our illustration)
only one of the billions of rays reaching Point B is red, there is no
chance for a human observer to perceive Point B as having any sort of
reddish color to it.
we could position a lens between Point A and Point
B in such a way as to take millions of red rays from Point
A — rays never originally destined for Point
B — and redirect them so that they could contribute their
light to Point B.
B still receives billions of rays of light, but now, thanks
to the lens, millions of those rays come from Point A.
You can imagine that with those proportions in effect, Point
B now takes on a reddish color.
the same can be said for any point on the object, and since any adjacent
points on the object "map" to adjacent (but upside-down and
backwards) locations on the screen, the lens is "projecting"
an image of the object onto the screen.