Photography Lesson #1
Light is the primary working tool of the photographer. Light is the visible portion of the electro-magnetic spectrum. The spectrum starts from red, the weakest of rays, all the way to violet which is the strongest of rays.
Light rays from the sun are considered white and they contain all the colors of the spectrum. Black is the absence of any color.
When we place a prism in front of a white ray, the prism will diffract (separate) that light into
all the colors of the spectrum. Stop for a minute and watch this:
When we place a LENS in front of any ray, that lens will simply refract (bend) that ray.
So, being in focus simply means moving the lens elements forward and backward until all the rays coming toward the camera from the subject meet at one point; the focal point on the film.
In the illustration above, you will see a point where the rays meet. That point is also called the film plane. At that point, all light rays of the subject must meet to accomplish sharp, focused pictures