Introduction

• Images are imprinted on film depending on how light rays reach the film to ignite a chemical process. However, light rays reflect of most surfaces in many different directions. Without some way of controlling the light rays, the image would result in a blur over the entire surface of the film. Lenses are the devices that help control light rays on their passage through the shutter toward the film. In reality, the simplest of lenses is explained by the pinhole phenomenon. Assuming the exposure time was long enough, if you removed the lens of a camera and put a piece of black cardboard with a pinhole in it over the lens mount, then a picture would be allowed on the film. However, the image would be upside down and the sides would be reversed. With a crafted lens, the image is reversed vertically and horizontally, well built to provide for shorter exposure times and a sharper overall image.
  
  

Light control with a lens

• Lenses are built to control the amount of light that reaches the film. The maximum amount of light a lens is able to process to the film is dependent on the diameter of the lens and the focal length. The focal length is the optical center of the lens to the film plane if the lens is focused at infinity. The focal length divided by the diameter of the lens is what the lens speed is rated at. This number is designated on the aperture ring in f-stop numbers. So, for instance, if a lens is rated at f/2 (f/2 being the lowest f-stop on the aperture ring), then the lens could be one inch in diameter and have a focal length of two inches because the focal length of two divided by the diameter of one would equal 2 or f/2. This lens speed rating is the maximum f-stop the lens can transmit. An iris is placed in the lens to reduce the amount of light reaching the film. By closing the iris, the effective diameter of the lens is reduced. Remember the lower the f-stop the more light passes through, the higher the f-stop the less light passes through.
  
  

Focus control on lenses

• Lenses are also built with calibrated rings that specify various focusing distances. However, some very wide angle lenses known as "fisheye" lenses can have a fixed focus that is non-adjustable. Focus can be set one of two ways. If the camera is equipped with an accurate reflex viewing system, then the focus can be set by looking through the viewfinder by eye. If working with a zoom lens (a lens with a variety of focal lengths), zoom in on the subject's eyes and pull the focus ring until the eyes are in sharp focus and then zoom back out to you're preferred focal length. For older cameras without well-adjusted reflex viewing systems, measure the distance from the subject to the film plane and adjust it with the markers on the focus ring accordingly. The film plane is denoted on the camera with a symbol that resembles a "o" with a slash through it.