What Are fstops

The f/stop goes by many names, including aperture, lens opening, f-number, relative aperture, and aperture stop. While each of these names don't always mean precisely the same thing, they all refer to the relative diameter of the opening, or entrance pupil that admits light to the lens. The larger the opening, the more light the lens can potentially transmit to the sensor; a smaller f/stop limits the amount of light that can "flow through" the lens. The size of the opening is variable, and controlled by a clever adjustable mechanism called an iris diaphragm (because it operates like the iris of the human eye) that uses a set of overlapping thin leaves (see Figure 1.13) that move to adjust the size of the gap in the center of the lens.

Figure 1.13 The size of the lens opening is changed by adjusing a set of overlapping metal leaves.

In olden times, aperture stops were actual metal plates with fixed-size holes that could be inserted into a slot in the side of the lens. To change the aperture size, you needed to remove the current stop and replace it with one having an opening of the desired size. The iris diaphragm, particularly as used in modern cameras (which can be resized on the fly automatically by the camera's exposure system) is a much more convenient component to use. Indeed, most digital SLR lenses used on consumer cameras lack a control, called an aperture ring, that can be used to set the f/stop manually. Most of the time, even in manual exposure mode, you'll use a dial on your camera body to specify the aperture. Some lenses, often "professional" lenses or those originally designed for older film cameras, do have an aperture ring on the lens barrel that can be unlocked and used to select the f/stop. (See Figure 1.14.) The ability to set the f/stop on the lens itself comes in handy when the lens is not mounted directly on the camera, such as when it is used with a close-up accessory such as a bellows, non-automatic extension tube, or reversing ring (described in Chapter 8).

F/stops are called relative apertures because an opening of a given size provides more or less light for the sensor, depending on how far it is located from the sensor. That distance is calculated using the lens's focal length, which you'll recall, is the distance from the optical center of the lens to the sensor (or focal plane). An opening with a diameter of, say,

Figure 1.14 Although the aperture is usually set by a control on the camera, some lenses have their own aperture ring to allow manual adjustment.

AWor/Jof

12.5mm (about half an inch) located inside a lens with a 50mm focal length yields an f/stop of f/4 (50mm divided by 12.5mm). Yet, an opening of the exact same size on a 100mm lens (or a zoom lens at its 100mm setting) would provide an f/stop off/8 (100/12.5). Figure 1.15 shows a simplified example (in the illustration, both apertures are exactly the same diameter; the top version looks larger thanks to an optical illusion) .

You can see from the way f/stops are calculated that they can be considered fractions. And, as fractions, values with a smaller lower number (denominator) are actually larger than those with smaller denominators, just as 1/8 is smaller than 1 /4, which is smaller than 1/2. So, in the example above, f/4 (produced when the aperture is 50mm from the sensor) is larger than f/8 (generated when the sensor is 100mm from the lens opening).

But wait! There's more. F/stops are calculated using the square root of 2 (1.414), so that f/4 doesn't admit twice as much light as f/8; it transmits four times as much light. If you want to calculate the sequence of f/stops, each half the size of the previous aperture, the correct sequence includes some odd numbers:

f/1.0, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32

Figure 1.16 shows how the iris diaphragm changes as the f/stop is adjusted. There are also so-called "half-stops" and "third-stops" between the whole number f/stops, with values like f/3.5, f/4.5, orf/6.3. These are often used to represent the maximum aperture of lenses when that maximum falls between one of the "regular" f/stops, or displayed in your digital camera's read-out when you or the camera select an f/stop that falls between one of the traditional whole number aperture settings. For example, if your digital camera is set to jump in one-third-stop steps, the individual fractional f/numbers between f/4 and f/1 6 would be (with whole f/numbers in bold):

f/4, f/4.5, f/5.0, f/5.6, f/6.3, f/7, f/8, f/9, f/10, f/11, f/12.5, f/14, f/16

Figure 1.15 The effective f/stop is calculated by dividing the focal length by the diameter of the aperture.

A World of Lenses

Figure 1.16 The relative size of the lens opening at (top row) f/2.8, f/4, f/5.6, and (bottom row) f/8, f/11, and f/16.

Although your digital SLR may choose an f/stop for you much of the time, there are situations when you will want to select the lens opening yourself to control the range of sharpness (depth-of-field) or to provide the best image quality. To work with f/stops, you'll need to keep the following concepts in mind:

♦ The larger numbers represent smaller lens openings and less exposure; smaller numbers represent larger lens openings and more light admitted to the sensor.

♦ Each standard whole f/number allows twice as much light, or half as much, depending on whether you are opening up (changing to a larger f/stop) or stopping down (switching to a smaller f/stop).

♦ While f/stops are based on powers of 2, each power of 2 allows four times as much light as the last: between f/2 and f/4 is an intermediate whole stop, f/2.8, which admits half as much light as f/2, and twice as much as f/4.

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