It's all about light
1. Qualities of light
b. Degree of diffusion
2. Kinds of light
a. Natural light
b. Existing light
c. Artificial light
B. Adjusting the exposure
1. Aperture: controlling light intensity
2. Shutter Speed: controlling exposure length
I'm going to be talking to you about light and exposure, and illustrating some of my presentation with photographs I have taken. Most of what I will be saying applies to taking pictures with digital and film cameras.
It has been said that photography is the language of light. Most of the equipment photographers use deals with light. Lenses focus it. Meters measure it. Film records the light image. Or in the case of digital cameras CCD's or CMOS's, electronically record the light image and transmit it to the camera's memory card. [CCD. Short for charge-coupled device. CMOS . Short for complementary metal oxide semiconductor.]
Light in photography comes from a variety of sources, and has various qualities and characteristics that affect the entire process - from taking the pictures to making the final print. Using light effectively is what makes your photographs work.
So let's talk about the qualities of light.
There's intensity, which is the brightness or dimness of the light reaching the subject you are photographing. If the metering system in your camera is working correctly, then the intensity of the light shouldn't directly affect the brightness or dimness of the photograph. Most cameras are able to deal with a huge range of dim to bright lighting situations and deliver well-exposed photos.
If your camera is set to full automatic mode then that program will set the shutter speed and aperture to match the light levels read by the internal light meter, and you should get a well-exposed photograph.
The light intensity, which can be thought of as the amount of the light, has an impact on depth of field and the action-stopping ability of the shutter. The greater the intensity of the light, the greater depth of field you will have, and the more easily will you be able to clearly photograph a moving subject. More about this later.
Another light quality is degree of diffusion. Light can be very sharp and concentrated, or at the other end it can be very soft and diffuse. Sharp light usually produces well defined shadows. It generally comes from a single point such as the sun in a cloudless sky or a flash unit. Diffuse light, on the other hand, gives hazy, ill-defined shadows or no shadows at all. Diffuse light emanates from large light sources, like the sun being filtered and diffused through clouds on an overcast day, or from several light sources such as an indoor scene which may be lit by light coming through windows along with light from lamps. The sharper the light, the more your images will show a high degree of contrast. My own experience and that of other photographers is that you can take beautiful pictures on cloudy, overcast, foggy and misty days.
Now let's talk about the Direction of light. The direction of light is the angle at which the light strikes your subject. It affects the way texture and shape are depicted in the photograph. Sidelight which rakes across the subject at right angles, tends to enhance texture. Frontlight minimizes textures and creates few shadows. As a result a front lit subject can appear rather flat. Backlight throws shadows to the front of the subjects, and in outdoor photography you can use backlight to create some interesting effects. Backlight can also give the photograph a hazy appearance and may result in a silhouette - which may be just what you want. Light that comes from above the subject tends to appear natural to us because that's where sunlight comes from. Light that comes from below tends to make some subjects, particularly if they are people, look unnatural and sinister, but on the other hand, it can make flowers look interesting and beautiful.
Color. I'm talking here about natural light. Sunlight has different colors depending upon the time of the day, the season, and the amount of diffusion from clouds, fog or rain, and where your are geographically in the world. The light may be cool, that is blue or blue gray, or warm (reddish or yellowish), or other colors.
Natural light is the light from the sun, usually outdoors. It takes many forms from the soft, diffuse light of an overcast day to the harsh, contrasty light of bright direct sun. Natural light may create no shadows, soft shadows or deep shadows. Sunlight is rich in possibilities, but sometimes unpredictable and impossible to control directly. Sometimes it is a matter of waiting -- for the angle of the light to change, for the clouds to shield and diffuse the sun's light, or waiting for another day when conditions are more to your liking.
Existing light is the light found at the location of your photograph, usually indoors (to distinguish it from natural light). Existing light is often created from a variety of light sources - that is a mixture of window light, light fixtures, and reflections.
Artificial light is the light that you add to the existing light on the scene. It may be as simple as a single flash or as complex as banks of studio light.
I've talked about light. Now I'm going to talk about how you can control the amount of light entering your camera so that you get the right amount of light to the film or the CCD or CMOS with the result being a well-exposed image. In a digital camera and in a film camera with negative or print film, too much light will result in a light image, and too little light will result in a dark image.
Basically the correct exposure is set with controls. In film and digital cameras the aperture ring and the shutter speed dial can directly adjust exposure. In digital cameras, there is a third control called the ISO index - which is a number expressing the light sensitivity of the light medium that receives the light image. In film cameras that ISO refers to the light sensitivity of the particular film you are using. Typical ISO sensitivity numbers for both film and digital cameras are 50, 100, 200, 400, 800, and 1600. The higher the number the greater the light sensitivity of the medium, and the less light is needed for a correct exposure. For example, if your camera is set at 200 ISO, it only requires ½ of the light for a correct exposure than would be needed if the camera were set at 100 ISO.
So how do you get a good, well-exposed image? One answer is to put your camera in Automatic or Program mode, and let the programs in the camera do their thing to give you a well-exposed picture - most cameras can do this very well. Many cameras now have additional picture taking modes such as sports/action, scenery (landscape), portraits/people, night scenes, and others. Using these pre-programmed settings should increase the variety of circumstances in which you can take well-exposed pictures without having to make a lot of individual settings in your camera.
[By the way, If you have a new camera, READ THE MANUAL. Read it many times, you will keep finding new settings that you can try out on your camera. My new digital camera came with a 171 page manual, and I'm still trying new settings out.]
Here are the ways in which you, and/or the program settings in your camera control how much light comes into the camera when you press the shutter button.
The aperture controls the intensity of the light passing through the lens. A large aperture lets through a lot of light, and a small aperture allows little light to pass through. The size of the aperture, also called the lens opening , is designated by the f-stop. F-stops are noted in a rather odd series of numbers like this 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32.
The larger f-stop numbers (like f/16) indicate smaller lens openings, the smaller numbers (like f4) represent larger openings. When you or your camera program change the lens opening from one f-stop to the next, the aperture is being adjusted "one stop". The f-stop series has been arranged so that each such change either doubles the intensity of light or cuts it in half. When you go down to the next smaller aperture (from f/4 to f5.6, for instance) you are "stopping down" one stop and letting in half as much light. When you move a stop up to a larger aperture (say from f/8 to f5.6) you are "opening up" the lens and allowing twice as much light in.
The shutter speed controls the length of the exposure. A closed shutter means than no light may enter the camera body and strike film or electronic medium. Opening the shutter allows light to enter the camera until the shutter closes again. Shutter speed is the length of time the shutter remains open. Very simple cameras such as one use film cameras often have a single fixed shutter speed. Adjustable cameras have a wide range of shutter speeds.
Shutter speeds are noted in seconds and fractions of a second and arranged in a series like this: B, 1, 1/2, 1/4, 1/8, 1/15, 1/30, 1/60 1/125, 1/250, 1/500, 1/1000, and higher.
Camera shutter settings are organized so that each one is either half or double the length of time of the one next to it. For example, moving from a setting of 1/30 to one of 1/60 lets in half as much light because the shutter stays open only half as long. Going from 1/500 to 1/250, on the other hand, allows in twice as much light because the shutter is open twice as long. Many cameras include longer shutter speeds like 2", 4", 8" or longer. In additions cameras usually include a "B" shutter speed which allows you to keep the shutter open as long as you want. Very useful for night pictures or other special circumstances - but you need to have a tripod for long exposures.
The shutter speed series and aperture series parallel one another. In both, each time you move a notch up or down the scale, you halve or double the amount of light reaching the camera light medium. The exposure controls were designed so that you can trade off shutter speeds and apertures. If, for example, you want to use a faster shutter speed at a sports event (faster shutter speed = smaller amount of light), you will need to compensate by increasing the size of the aperture (larger aperture or f/stop = greater amount of light).
Various combinations of shutter speed and f/stop will give you the same exposure, but not the same picture. Too slow a shutter speed may give you a blurred action picture when you really wanted to "freeze" the action. The size of the aperture creates differences in the overall sharpness of the picture from foreground to background. The overall sharpness of the image is called "depth of field", and the smaller the aperture, the greater the depth of field. The automatic programs that are built into almost all cameras these days have programs that completely automate the exposure, including setting the aperture and shutter speed. Other programs, often called aperture controlled or shutter controlled allow you to set and change the aperture or shutter settings to enable you to better control the outcome you want.