Photographic Lenses

 Understanding Camera Lenses

Since optical designs of photographic lenses (wide angle to telephoto) differ so radically we will not go into detail about optics. This lesson will cover a basic discussion of lenses and some good tips on lens use and care. Lenses are designed to refract (bend) light rays. Here is a basic convex lens (crude drawing below). Notice that this bends the light towards the center of the film plane. A magnifying glass is a basic convex lens.

Convex Lens
A concave lens bends the light away from the center. In modern camera lenses, you will find a variety of these lenses with some individual elements oddly shaped.

concave lens illustartion

Below is a cutaway illustration of a typical camera lens. Notice the different shapes of the elements. The science of optical design has become quite sophisticated and as it has progressed we, the photographers, have benefited with sharper, faster, lenses with greater color purity.

 

Photographic Lenses

How a lens works

 

Depth of field is a term we use in dealing with the sharpness of objects in our field of view. We know the subject we focused on will be sharp, but what about objects in front of, and behind? We control the sharpness of those objects by using THE APERTURE which is built inside each lens.

The aperture is used primarily for controlling the exposure (the lesson on Exposure follows this page, be patient) but it also controls the depth of field (depth of sharpness would have been a better term). Words cannot explain this as well as visuals so take a look below at how different aperture settings can control the sharpness of objects in front of, and behind, the subject you are focusing on:
Aperture and hoses



I like to think of the aperture as a hose and how far the hose will reach if it is covered or not. If the lens aperture is “wide open” ,like a hose that you don’t cover the end at all, the depth of field is very shallow or the hose doesn’t spray very far… notice how the runner (below) is sharp and the other players are out of focus. However, if you close the aperture all the way down, or cover the end of the hose so the water can only escape through a small hole, objects in front of, and behind, will appear much sharper (see below).

aperture wide open girl kicking soccer ball
By keeping the aperture wide open, we have a shallow depth-of-field giving the girl playing soccer a “3D” look.

 

aperture closed down family portrait
If you close the aperture down, you get a larger depth-of-field so all of your subjects are in focus and the runner gets lost in the image.

Tips On Lenses

Now to some other tips on lenses:

  • UV FILTERS – We always use a UV filter in front of the lens (instead of bothering with lens caps). The filter protects the lens and blocks out unwanted, hazy UV light.
  • LENS CLEANING – The glass used in photo lenses is covered with a coating (to correct some color problems) therefore, you should be very careful when cleaning the lens. We suggest that you not use lens cleaning fluids, rather a gentle blowing (your breath has moisture) followed by a gentle wiping with a clean, soft cloth or lint-free napkin. If the lens is seriously smudged then put a drop of lens cleaning fluid on the cloth and gently wipe. DO NOT apply lens cleaning fluid directly onto the lens… the fluid could get into the edges and destroy the adhesive that keeps the element attached to the barrel.
  • COLD WEATHER – Don’t bring a lens in from extreme cold … the moisture inside will instantly freeze up the whole lens. It is best to carry the camera under your