Aperture or f/stop
The aperture stop of a photographic lens can be adjusted to control the amount of light reaching the film or digital sensor (CCD or CMOS). In combination with variation of shutter speed and film speed, the aperture size will regulate the film's degree of exposure to light. Typically, a fast shutter speed will require a larger aperture to ensure sufficient light exposure, and a slow shutter speed will require a smaller aperture to avoid excessive exposure.
A device called a diaphragm controls the aperture. The diaphragm can be considered to function much like the iris of the eye—it controls the effective diameter of the lens opening. Reducing the aperture size increases the depth of field, which describes the extent to which subject matter lying closer than or farther from the actual plane of focus appears to be in focus. In general, the smaller the aperture (the larger the number), the greater the distance from the plane of focus the subject matter may be while still appearing in focus.
Aperture is usually measured in f-numbers. A lens will have a set of "f-stops" that represent doublings in the amount of light let through the aperture. A lower f-stop number denotes a greater aperture opening which allows more light to reach the film. A typical lens will have an f-stop range from f/16 (small aperture) to f/2 (large aperture) (these values are approximate and may vary). Professional lenses for 35mm cameras can have f-stops as low as f/1.0, while professional lenses for some movie cameras can have f-stops as low as f/0.75 (very large aperture). These are known as "fast" lenses because they allow much more light to reach the film and therefore reduce the required exposure time. Large aperture prime lenses (lenses which have a fixed focal length) are favored especially by photojournalists who often work in dim light, have no opportunity to introduce supplementary lighting, and capture fast breaking events.
Zoom lenses typically have a minimum aperture of f/2.8 to f/6.3 through their range. A very fast zoom lens will be constant f/2.8, which means the aperture will stay the same throughout the zoom range. A normal zoom will be a constant f/4, and a consumer zoom will typically have a variable diaphragm, normally being something along the lines of f/4.5 to f/5.6, or even f/4.5 to f/6.3 (rare). There are a few exceptions to this rule, as even high quality hyperzooms often have as slow of an aperture as f/5.6 throughout the whole zoom range. Such is the case with most lenses which have more than 4x zoom range, like a 100-400 mm f/5.6.
The reason for consumer zooms to have a variable aperture is that the f-number is proportional to the ratio of the focal length to the diameter of the diaphragm opening. This means that if you have a 75-300 mm lens, a physically bigger diaphragm opening will be needed at 300 mm than at 75 mm, to maintain the same f-number. More light is needed as the focal length increases, to compensate for the fact that light from a smaller field of view is being spread over the same area of film or detector.
Aperture priority refers to a shooting mode used in some semi-automatic cameras. It allows the photographer to choose an aperture setting but allow the camera to decide the correct shutter speed. This is sometimes referred to as Aperture Priority Auto Exposure, A mode or semi-auto mode.
ISO
The standard known as ISO 5800:1987 from the International Organization for Standardization (ISO) defines both a linear scale and a logarithmic scale for measuring film speed.
In the ISO linear scale, which corresponds to the older ASA scale, doubling the speed of a film (that is, halving the amount of light that is necessary to expose the film) implies doubling the numeric value that designates the film speed. In the ISO logarithmic scale, which corresponds to the older DIN scale, doubling the speed of a film implies adding 3° to the numeric value that designates the film speed. For example, a film rated ISO 200/24° is twice as sensitive as a film rated ISO 100/21°.
GOST (Russian: ГОСТ) is a pre-1987 standard used in the former Eastern Bloc. After 1987 GOST scale was changed to align with ISO scale. Only found on pre-1987 photographic equipment (Film, Cameras, Lightmeters) of Eastern Bloc manufacture.
The most common ISO film ratings are 25/15°, 50/18°, 100/21°, 200/24°, 400/27°, 800/30°, 1600/33°, and 3200/36°. Consumer films are generally rated between 100/21° and 800/30°, inclusive.
A film speed is converted from the linear scale to the logarithmic scale by this formula (plus rounding to the nearest integer):
Conversion from the logarithmic scale to the linear scale is analogous, except that results must be rounded to the conventional values of the linear scale listed in the table above.
Film speed is found by referencing the Hurter & Driffield curve for the film. This is a plot of density vs. exposure (lux-s). There are typically five regions in the curve: the base + fog, the toe, the linear region, the shoulder, and the overexposed region. Following the curve to the point where it exceeds the base + fog by 10%, find the corresponding exposure. Dividing 0.1 into that yields the speed.
The speed is used in the Exposure Index equation to find the appropriate exposure. Four variables are available to the photographer to obtain the desired effect: light, film speed, f/#, and exposure time (shutter speed). The equation may be expressed as ratios, or, by taking the logarithm (base 2) of both sides, by addition. In this form, it was easier at the time it was proposed to put into a nomograph (slide rule) so non-scientists could obtain good results. As a result, every increment of 1 is a doubling of light intensity, known as a "stop". The f/# is proportional to the ratio between the lens focal length and aperture, which in turn is proportional to the lens area by the square root. Thus, a lens set to f/1.4 allows twice as much light to strike the focal plane as a lens set to f/2. Therefore, each increment of the square root of two (approximately 1.4) is also a stop, so lenses are typically marked in that progression: 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, etc.
Speed is roughly related to granularity, the size of the grains of silver halide in the emulsion. Fine-grain stock, such as portrait film or those used for the intermediate stages of copying original camera negatives, is "slow", meaning that the amount of light used to expose it must be high or the shutter must be open longer. Fast films, used for shooting in poor light or for shooting fast motion, produce a grainy image. The image actually consists of a mosaic of developed and undeveloped areas of the emulsion, and each grain of silver halide develops in an all-or-nothing way. Thus, film is a threshold detector rather than a linear detector. If the subject has an edge between light and darkness and that edge falls on a grain, the result will be an area that is all light or all shadow. An accumulation of such areas breaks up the visible contours of the object, the effect known as graininess (or grain). Fast films are also relatively contrasty, for the same reason. That is, an area of the image will consist of bright areas and dark ones with few transitional areas of midtones.
Kodak used to use a Granularity Index (GI) to characterize film grain. Alternating images of the film under test and a standard grain were shown to test subjects who indicated when they perceived a match. The standard grain samples were the index. More recently, Kodak switched to a measurement of grain using an RMS measurement. Granularity varies with exposure - underexposed film looks grainier than overexposed film.
In the early 1980s, there were some radical improvements in film stock. It became possible to shoot color film in very low light and produce a fine-grained image with a good range of midtones. In advertising, music videos, and some drama, mismatches of grain, color cast, and so forth between shots are often deliberate and added in post-production.
Certain high-speed black-and-white films, such as Ilford Delta 3200 and Kodak T-Max P3200 (TMZ), are marketed with higher speeds on the box than their true ISO speed (determined using the ISO testing methodology). For example, the Ilford product is actually an ISO 1000 film, according to its data sheet. The manufacturers are careful not to refer to the 3200 speed as an ISO speed on the packaging. These films can be successfully exposed at EI 3200 (or any of several other speeds) through the use of push processing. The most sensitive sensor common in commercial photography may be the Silicon Intensified Target vidicon, at ASA 200,000, used in TV cameras.
Digital camera sensitivity
The photographic sensors in digital cameras can in most cases be adjusted in sensitivity. These sensitivity settings are normally calibrated using ISO film speed numbers, because they are familiar to photographers. ISO Standard 12232:1998 addresses the measurement of digital imaging speeds. Just as in photographic film, greater sensitivity comes with some loss of image quality, though this is visible as noise rather than grain. The best digital cameras as of 2005 exhibit no perceptible noise at ISO 100 sensitivity, and some produce useable results up to ISO 3200.
Setting ISO, F Number, Shutter Speed
Linear Mode
