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What Is an F-Stop?

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What Is an F-Stop? revised 2006/11/15 What is an f-stop? An Introduction to the Technical Side of Photography © Copyright 2006 by John Cornicello Introduction and scope This article introduces some of the technical issues in photography. Most of the content is directed to users of Single Lens Reflex (SLR) cameras with interchangeable lenses, but it can also be used by someone with a point and shoot camera that offers manual control settings. It is assumed that the student already has a camera (film or digital) with at least one lens and is familiar with its basic operations; this article will not help you decide which camera to purchase (but will help you in selecting lenses for your camera). When mention is made of film (film speed, film plane, etc.) it can be assumed that the text refers equally to traditional film or to a digital sensor unless it is otherwise noted. What is photography? Photography translates to Light Writing—producing images of objects on sensitized surfaces by the chemical reaction of light on film or a digital sensor—the process or technique of rendering optical images on photosensitive materials. To master photography you must learn to control exposure. Exposure The basis of photography is exposure. Though it might not appear so at first, exposure is an important creative tool. Equivalent exposures (by changing aperture and shutter speed to compensate for each other, which will be discussed later) can render radically different images. Learning how shutter speed and aperture affect your image will help guide you to getting your perfect exposure. Some subjects can be obviously over- or under- exposed, but the correct exposure can be subjective and you may want to use a “non” technically-correct exposure to convey a mood or for other artistic reasons. Exposure is a by-product of four variables: Light intensity Film/sensor sensitivity Aperture Time Light intensity is the amount of light falling on your subject. Light can come from the sun, from a lamp, from a flash unit built into your camera, or from a separate flash unit, etc. The amount of light is measured by a light meter. Most of today’s cameras have a built-in meter and that should suffice for the scope of this article. Film/sensor sensitivity measures how the film reacts to the light striking it. It is measured in ISO. Old timers might call it ASA (or even DIN). “Slower” films have lower numbers (25 to 100) and require more light. Medium films have mid range numbers (200 to 400). High speed films respond to lower light levels and have higher numbers (800-1600). Some digital cameras can be set as high as 3200. Each doubling of film speed is said to be one “stop” faster than the previous. It is twice as sensitive as the previous setting. Film speed is fixed in manufacturing, but can be modified through custom film processing (not directly applicable to digital). In digital, the sensor is built to have a certain light sensitivity. This sensitivity can be changed on a digital camera—one of the great advantages of digital over film; you don’t have to change to a different roll of film if you want to change film speed for one (or a series of) image(s). Higher film speed settings on a digital camera apply more amplification to the signal recorded by the processor. In both film and digital there is a trade-off in using higher speeds. Higher speed films have more grain and higher digital speeds introduce electronic noise (which is somewhat similar in appearance to film grain). John Cornicello Page 1 Lenses have an adjustable opening called the aperture. Aperture size is controlled by an iris-shaped diaphragm and various size openings are referred to as f-stops—which is the relationship between the size of the adjustable opening and the focal length of the lens when it is focused at infinity. The aperture controls the amount of light passing through a lens to the film. It also controls the depth of field (the range of depth between the nearest and farthest objects that appear to be acceptably sharp). For more information about lenses please see the Lenses section of this article. Shutter speed controls how much time the light is allowed to reach the sensor. It controls image blur (both from the subject moving and/or from the camera moving). A slower shutter speed emphasizes the blur. A faster shutter speed will do better at stopping action and cancelling camera motion. See Controlling Time below for more information about shutter speed. Lens Speed: What is the speed of the lens? It tells how much light gathering power a lens has? Speed is determined by the largest aperture (smallest f/stop because f-stops are actually fractions) available. A lens with a maximum aperture of f/2.8 is faster (lets through more light) than one that has an f/4 maximum aperture. Fast lenses are larger, heavier, and more expensive than slower lenses. F-stop is determined by dividing the size of the aperture into the focal length of a lens. For example, a 50mm lens with a 25mm aperture will have an f-stop of 1/2, or f/2. A 100mm lens with a 25mm aperture will be f/4 and will allow less light to pass through to the film than the 50mm lens with the same 25mm aperture. A 400mm lens with a 25mm aperture will be f/16. Working this backwards you will see that to get a 400mm f/2 lens the aperture would have to be 200mm (about 8 inches). That would require an immensely large, heavy, and expensive set of lens elements. There are some 400mm f/2.8 lenses. The Canon version sells for around $6500, is 6.4 inches wide (the aperture is 5-1/2”), and weighs more than 11-1/2 pounds. The aperture scale is made up of a series of numbers where each major stop allows 2x the light to pass as the next number (or 1/2 the light when going in the opposite direction). The numbers are actually the denominator of a fraction. F/2 means Focal Lenght divided by 2, or 1/2, or the aperture is 1/2 the size of the focal length of the lens. So a larger f number denotes a smaller lens opening. When going from a smaller number (large opening) to a larger number (small opening) you are said to be “stopping down” the lens. Conversely, going from a larger number (small opening) to a smaller number (large opening) is called “opening up” the lens. The standard scale includes the following numbers: 1.4 (larger opening) 2 2.8 4 5.6 8 11 16 22 32 (smaller opening) The lens pictured above has a maximum aperture of f/4. These examples show what the aperture looks like when stopped down to 5.6, 8, 22, and 32 John Cornicello Page 2 Most lenses allow you to select an intermediate aperture between the major steps listed above. Some systems let you set 1/2 stops, others allow 1/3 stops. So you may see intermediate numbers like 3.2, 6.3, 7.1, 13.5, etc. You normally can not see the effect of the aperture while you are looking through your camera. The lens stays at maximum aperture to maintain a bright viewfinder to make focusing easier. But when you press the shutter button the lens aperture closes to the f/stop you have set just before the shutter opens to expose the frame. Most cameras, though, have a depth of field preview button. Check your camera manual to find out if yours has this feature. When you press the button the lens will stop down to the set aperture. This causes two things to happen—the viewfinder gets much darker as there is less light being let in and you will be able to see what is in focus in the viewfinder (once you get used to the darkened screen). The maximum aperture of a lens may not be on the standard scale. It is typical to see lenses with a maximum aperture of 1.8 or 3.5. This does not affect the rest of the scale. A lens with a maximum aperture of 1.8 will have its next stop listed as f/2 (1/3 stop difference) and then would fall into the standard progression. You may notice that some zoom lenses list two maximum f/stops. For example, the Canon 28-135mm F/3.5- 5.6 lens. When you see this it means that the maximum aperture of the lens changes as you zoom. When you are set at its shortest focal length (28mm) the lens is f/3.5. When you zoom in to its longest length (135mm) the lens lets through less light and becomes a f/5.6 lens. This is done to keep down the size, weight, and cost of the lens. Again, this doesn’t affect the rest of the scale. These lenses still stop down to 8, 11, 16, etc. And you don’t have to worry about it. The marked f/stops on the lens are accurate, no matter which focal length the lens is set to. Typically, lenses perform best in the middle of their scale (5.6 to 11). At the larger openings (2.8, 1.4) there could be some softness because of optical errors. And at the other end of the scale (16, 22) there is loss of image quality due to something called diffraction (where the light rays are bent dramatically because of the tiny opening).
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