FILM SPEED (ISO)
(This will help complete your understanding.)
In Chapter Two, we learned that each sequential lens opening (f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16) either doubles or cuts in half the amount of light that reaches the film. As a reminder, f/8 creates a lens opening twice as large as f/11; therefore, a setting of f/8 allows twice as much light to reach the film as f/11. Similarly, a lens opening of f/5.6 allows half as much light to reach the film as f/4.
Film speeds, as with f-stops, help photographers control light. They are the ability (sensitivity) of a particular film to absorb and record light. A film with a speed of 100 has twice the sensitivity to absorb light as a film with a speed of 50; therefore, 100-speed film will absorb the existing light twice as fast as 50-speed film. Likewise, 200-speed film has half the sensitivity to record light as 400-speed film; therefore, 200-speed film will absorb the existing light half as fast as 400-speed film. In low light situations where a flash cannot be used, a higher speed film is often utilized.
The numbering system of film speeds is established by the International Standards Organization (ISO) so the same film speed numbers are used in United States as in many other places around the world. This gives standardization to travelers buying film in different countries. Therefore, the speed of a particular film is often referred to as its ISO number. While buying film, you might be asked, “What speed do you want?” or “What ISO do you need?” If you have a digital camera, you will have various ISO settings on the camera to select the ISO you desire. Digital cameras have ISO setting that electronically mimic film speeds. Setting a digital camera to a different ISO speed will have the same effect as changing film to a higher or lower film speed in a film camera.
I should mention that ISO speeds have another important characteristic. The clarity or sharpness of your photographs depends on film speed, especially if you are going to make enlargements. I do suspect, after you master the techniques presented in this book, that you will be enlarging some of your wonderful bird flight photographs to 8” x 10” or larger. When you do, you want them to be as clear and sharp as possible.
The lower the ISO speeds, the sharper and clearer the photographs. Therefore, a bird photographed with 100-speed film or setting will be sharper and have more detail than a bird photographed with 400-speed. This may not be evident in small snapshots, but it quickly becomes apparent if the photograph is enlarged. During the enlargement process, the “grain” or individual dots making up the image in film also become larger. Slower films have smaller grains than faster films, thus the slower the film speed, the greater the clarity in your photographs. Photos taken with fast films often appear “grainy” after being enlarged. Their clarity or sharpness diminishes with each subsequent enlargement.
With digital cameras the quality of the print is also somewhat dependant on the ISO you selected, but instead of calling the poor quality connected with high speed ISO settings "grain," it is referred to as "noise." The same rules apply with digital cameras, the higher the ISO setting, the poorer quality of the finished photographs, especially if you are going to enlarge and them. Digital cameras also offer you much higher ISO settings than are normally available from common films. Many digital cameras go well past an ISO setting of 1000.
Just as f-stops can control the amount of light that reaches your film, they are more useful if used to control depth of field. Similarly, just as films with different speeds are used to take advantage of their sensitivity to the existing light, they are often more useful if used to control the sharpness and clarity of our bird photographs. Look at the following chart.
What Types and Speeds of Film Are Available?
Digital cameras have settings for tungsten, daylight, and sometimes fluorescent lighting. Many have automatic "white balance" settings as well. Refer to your digital camera's owners manual for assistance with your particular model. If your digital photo's color is incorrect because of an improper color setting, you can often fix these problems with various software programs on your computer (Corel Photo Paint, Adobe Photo Shop, and many other manufactures of digital photo manipulation.)