- Dec 20, 2002
2.2 Aperture and F-Stops
If you have experience in film or photography, you should certainly know about aperture. Simply put, the aperture is an opening, generally a circular hole or similar shape, within a camera lens that controls the amount of light coming into the camera and onto the film or computer chip. The aperture is controlled by f-stops. The "f" refers to the focal length of the lens divided by the aperture. Camera operators are always concerned about aperture and f-stops, as these can make or break a shot. The aperture on a real camera is a mechanical diaphragm that operates like the iris of an eye (see Figure 2.1).
Figure 2.1 An aperture on a camera lens works like the iris of your eye, opening and closing to allow more or less light into the camera.
The Origins of F-Stop
The origin of the lower case "f" in f-stop goes back to 1932 and renowned photographer Ansel Adams. He and a few others, Willard Van Dyke, Imogen Cunningham, Edward Weston, Hentry Swift, Sonya Noskowiak, and John Paul Edwards, formed a group called f.64. This group was dedicated to pure photography, including portraits and landscapes. Van Dyke originally proposed the name US256, but Adams thought it sounded like a highway. He simply wrote an "f" and then put a dot similar to the old aperture settings. They soon updated this to an f with a slash mark to read Group f/64.
The "f" in f-stop is a number or value that represents the ratio between the size of the focal length of the lens to the aperture. The "stop" portion of an f-stop is determined by a division of a lens' focal length and the aperture's diameter. This division is in millimeters. An example might be a 50mm lens with an aperture of 25mm in diameter; the f-stop is f2.
When you talk about f-stops, no matter what the measurement, the same measurement of light is reaching the film or digital chip. When a change in f-stop occurs, the light is either doubled or cut in half.
If this diaphragm is open wide, you have a low f-stop, which allows more light into the camera. Conversely, an aperture that is small (the diaphragm is closed to a tight opening) is a high f-stop, allowing very little light into the camera. It's confusing at first, but just think opposites:
Low f-stop, more light (larger lens opening)
High f-stop, less light (smaller lens opening)
In addition to aperture and f-stops, real-world photographers also have to consider shutter speed and film speed. Think of the shutter as the eyelid of the aperture (the iris). When a photographer pushes the button on a camera to take a picture, the shutter opens and closes quickly, exposing either film or a digital chip to the amount of light specified by the aperture opening. Shutter speeds are measured in fractions of seconds, such as 1/30 sec or 1/125 sec. Higher shutter speeds are used for fast-moving action, such as sports. Lower shutter speeds are used for shots with less movement. Generally, you don't have to worry about shutter speed when working in the digital 3D environment. However, you may consider the animation frame rate to correspond to motion blur in 3D animation. Frame rates will be discussed more in detail in Chapter 10, "Resolutions, Compression, and Rendering."
Along the lines of f-stops are t-stops. Often thought of as the same thing as f-stops, t-stops are actually quite different. The "t" stands for transmission and is a "theoretically perfect" f-stop. However, because of intermediums like the glass lenses, f-stops are never actually perfect, and there is always a loss of light. A t-stop's numbers are always higher than f-stops. A t-stop factors in the loss of light from a camera lens's optics. While t-stops are not used too much in photography, they are used in filmmaking and scientific work for more accuracy.
Aperture indirectly plays a role in digital cinematography because many 3D applications include settings for f-stops. Varying f-stops in the digital environment directly play a role when applying depth of field effects.
The "stop" portion of f-stop comes from original photographic technology in which the aperture was selected by turning a wheel that had various sized holes. Each hole let in twice as much light as the previous hole. With this in mind, you can understand what it means when someone says to "stop down" your lens. You can think of f-stops in terms of your own eyes. When you squint, you decrease the opening (larger f-stop) and block light from entering. Or, if you want to see more, you open your eyes wide (smaller f-stop) and let more light in. F-stops can range anywhere from f/1.4 to f/28. The lower f-stop number is considered "shooting wide open," meaning the aperture is at its largest opening, allowing more light to enter the camera lens. The higher f-stop number closes the aperture, allowing less light into the camera (see Figure 2.2). F-numbers are ratios. An f-stop is the ratio between the focal length of the camera lens and the diameter of the diaphragm (aperture) opening. For instance, f/2 means that the aperture diameter is 1/2 the focal length of the lens.
Figure 2.2 With a wide aperture set to f/2, the focus point is concentrated to a specific point. As the camera lens is stopped down to f/16, an increased depth of field is created. A typical 3D animation without depth of field turned on will always render everything in focus, similar to a small aperture like f/16 or higher.
So when do you allow more or less light into the camera and why? And how does the f-stop play a role? More importantly, why in the world is this important to digital cinematography? A basic understanding of photographic principles can help you assess your 3D situation in terms of both lighting and depth of field.