Lesson 4: Understanding Photographic 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.
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.
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.
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:
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).
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 winter coat when not in use.
- HOLDING CAMERA – When making photos you should make sure the lens is in the palm of your hand, freeing up the fingers to focus. This takes the pressure of the lens mount AND provides for more steadiness. When using a telephoto lens you should also tuck your elbow into your side … which offers you more camera support (i.e., less shake).
- CARRYING CAMERA – When carrying the camera on your shoulder (using the strap) the lens should point inward to protect the glass and keep it from banging into other things … like little people. You’ll find that the camera is now positioned in such a way that when you reach for it, it will go into your hand easier and feel more natural.
- FILTERS – There are numerous filters out there, some designed to correct color/light problems … others add special effects. We will discuss three basic filters (which you probably should have).
- UV/HAZE filter: It cuts out ultra-violet light rays. Although invisible to the naked eye, ultra-violet rays give bluish tint and haziness in color photographs. UV/HAZE filters make the picture clearer. This filter does not block enough light to cause you to make an exposure compensation, therefore it is should be kept on the lens at all times to protect the lens surface.
- POLARIZING filter: This filter subdues undesired reflections from non-metallic surfaces such as water, windows, etc. When used in color photography it will darken blue sky by blocking atmospheric haze. By eliminating that haze skies will be bluer, red objects will be redder …. in fact all colors will be much more saturated and true.
- CLOSEUP filters: These are simple lenses that, when attached to the front of your lens, will allow you to focus much closer … allowing you to make full-frame images of very small objects. The more powerful the closeup filter, the more you will have to “open up” the lens, because these filters do “absorb” some of the light rays. The filter should come with instructions that will tell you how much exposure compensation you should make. If not, simply experiment … AND take notes.