The Basics of Digital Photography: The Exposure Triangle - Part 1

Today's digital cameras make it possible for even the rank beginner to take good pictures using one of the cameras fully programmed modes. That's the way my grand daughter started out in digital photography when I gave her, her first digital camera on her tenth birthday a few years ago. Many people never move from using the fully programmed shooting modes and from letting their camera make all the decisions for them. Those people are satisfied with controlling nothing more than the zoom lens setting, focusing, and tripping the shutter. These people are those people that the serious photographers refer to as snapshot takers. Then there are people like my grand daughter who want to learn how to move beyond the automation so they can take advantage of all the creativity that their camera has to offer them.

When my grand daughter asked me to teach her all about digital photography she already knew about focusing, composition, and how to trip the shutter without introducing camera shake. She wanted me to teach her how to use the "Aperture Priority," "Shutter Priority," and "Manual" modes but she needed to learn some basics first. She needed to have a good grasp on basics concepts like the "Exposure Triangle.

The exposure triangle involves an understanding of ISO, Aperture, and shutter speed. She not only needed to know what these things were; she needed to know how they each affected the other.

In this, the first of a four part series, I will introduce you to the concepts of lens aperture, lens speed, f-stops, and depth of field. Each of these lessons will build upon the previous lessons so you may want to print them out and keep them in a loose-leaf binder.

Lens Aperture refers to the opening that allows light to enter the camera. The size of the aperture is measured in f-stops and the actual size of the opening is inversely proportional to the f-stop number. In other words, the larger the f-stop numbers the smaller the actual opening in the lens diaphragm. The f-stops on a typical lens might be 1.0, 1.4, 2.8, 4.0, 5.6, 8.0, 11.0, 16.0, 22.0, 32.0, 45.0, and 64.0.

This is a good point to introduce another basic concept, the concept of "Lens Speed." Lens speed is not to be confused with shutter speed. Lens speed refers to and is determined by the lens' largest aperture. In other words, a lens with an aperture of f-1.0 is faster than a lens with an aperture of f-1.4 because it will allow twice as much light to reach the camera sensor.

The difference between any two f-stop numbers on the lens' f-stop ring is one full stop. You need to understand what this means. By changing the f-stop setting from f-1.4 to f-1.0, you increase the aperture opening by one full stop, which doubles the amount of light reaching the camera sensor. By reducing the size of the aperture by one full stop, by changing the lens' f-stop setting from 1.4 to 2.0, as an example, you reduce the light reaching the camera sensor by half. If you were to reduce the aperture by two full stops, say by going from f-1.4 to f-2.4, you would reduce the amount of light reaching the sensor by a factor of 4. One-fourth the amount of light that reached the sensor at f-1.4 reaches the sensor at f-2.8. In other words, there's a constant factor of two between any two full f-stops.

"Depth of Field" is another important concept that is directly related to f-stop setting. Depth of field refers to how much of the foreground and background is in focus. The larger the f-stop number, the greater the depth of field. The smaller the f-stop numbers the shallower the depth of field. Lenses used on film cameras have a depth of field scale behind the f-stop ring that show in feet and/or meters how much of the foreground and background will be in focus. Digital cameras lack this scale because the f-stop settings are made by menu on the LCD screen or in the viewfinder. Mastering depth of field provides the photographer with a great deal of creative control over the pictures that they make. Great pictures are made. Snapshots are taken.

For the inquiring minds, here is a little extra information about f-stop numbers. The f-stop number of a lens is calculated by dividing the focal length of the lens by the diameter of the lens. In other words, a 200-mm lens with a diameter of 50 mm will have a maximum aperture of f-4.0. f-stop = fl/d where fl = lens focal length and d = lens width or diameter.

In part 2 of this series we will talk about shutter speed and how shutter speed relates to f-stop selection.