A camera is an optical instrument used to record images. At their most basic, cameras are sealed boxes (the camera body) with small holes (the aperture) that let light in to capture an image on a light-sensitive surface (usually photographic film or a digital sensor). Cameras have various mechanisms to control how the light falls onto the light-sensitive surface. Lenses focus the light entering the camera, the size of the aperture can be widened or narrowed to let more or less light into the camera, and a shutter mechanism determines the amount of time the photo-sensitive surface is exposed to the light.
The still image camera is the main instrument in the art of photography and captured images may be reproduced later as a part of the process of photography, digital imaging, photographic printing. The similar artistic fields in the moving image camera domain are film, videography, and cinematography.
The word camera comes from camera obscura, which means “dark chamber” and is the Latin name of the original device for projecting an image of external reality onto a flat surface. The modern photographic camera evolved from the camera obscura. The functioning of the camera is very similar to the functioning of the human eye. The first permanent photograph was made in 1825 by Joseph Nicéphore Niépce.
All cameras use the same basic design: light enters an enclosed box through a converging or convex lens and an image is recorded on a light-sensitive medium (mainly a transition metal–halide). A shutter mechanism controls the length of time that light can enter the camera.
The aperture, sometimes called the diaphragm or iris, is the opening through which light enters the camera. Typically located in the lens, this opening can be widened or narrowed to control the amount of light that strikes the film. The aperture is controlled by the movements of overlapping plates or blades that rotate together and apart to shrink and expand the hole at the center. The diameter of the aperture can be set manually, typically by adjusting a dial on the camera body or lens, or automatically based on calculations influenced by an internal light meter.
The size of the opening is set at standard increments, typically called “f-stops” (but also “f-numbers”, “stop numbers”, or simply “steps” or “stops”), that usually range from f/1.4 to f/32 in standard increments: 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, and 32. As the numbers increase, each increment (or “stop”) halves the amount of light entering the camera. Conversely, the lower the number, the larger the opening, and so the more light that is let into the camera.
The wider opening at the lower f-stops narrows the range of focus so the background of an image is blurry when focusing on the foreground, and vice-versa. This “depth of field” increases as the aperture closes, so that objects that are at differing distances from the camera can both be in focus; when the aperture is at its narrowest, the foreground and background are both in sharp focus.
The shutter, along with the aperture, is one of two ways to control the amount of light entering the camera. The shutter determines the duration that the light-sensitive surface is exposed to light. The shutter is opened, light enters the camera and exposes the film or sensor to light, and then the shutter closes.
There are two types of mechanical shutters. The leaf-type uses a circular iris diaphragm maintained under spring tension inside or just behind the lens that rapidly opens and closes when the shutter is released.
More commonly, a focal-plane shutter is used. This shutter operates close to the film plane and employs metal plates or cloth curtains with an opening that passes across the light-sensitive surface. The curtains or plates have an opening that is pulled across the film plane during an exposure. The focal-plane shutter is typically used in single-lens reflex (SLR) cameras, since covering the film rather than blocking the light passing through the lens allows the photographer to view through the image through the lens at all times except during the exposure itself. Covering the film also facilitates removing the lens from a loaded camera (many SLRs have interchangeable lenses).
Digital cameras may use one of these types of mechanical shutters or they may use an electronic shutter, the type used in the cameras of smartphones. Electronic shutters either record data from the entire sensor at the same time (a global shutter) or record the data line by line across the sensor (a rolling shutter).
In movie cameras, a rotary shutter opens and closes in sync with the advancing of each frame of film.
The duration is called the shutter speed or exposure time. The longer the shutter speed, the slower it is. Typical exposure times can range from one second to 1/1,000 of a second, though durations longer and shorter than this are not uncommon. In the early stages of photography, exposures were often several minutes long. These long exposure times often result in blurry images, as a single object is recorded in multiple places across a single image for the duration of the exposure. To prevent this, shorter exposure times can be used. Very short exposure times can capture fast-moving action and completely eliminate motion blur.
Like aperture settings, exposure times increment in powers of two. The two settings determine the exposure value (EV), a measure of how much light is recorded during the exposure. There is a direct relationship between the exposure times and aperture settings so that if the exposure time is lengthened one step, but the aperture opening is also narrowed one step, the amount of light exposing the film or sensor is the same
In most modern cameras, the amount of light entering the camera is measured using a built-in light meter or exposure meter. Taken through the lens (and so called TTL metering), these readings are taken using a panel of semi-conductors that are sensitive to light. They are used to help calculate the best exposure settings. These settings are typically determined automatically as the reading is used by the camera’s microprocessor. The reading from the light meter is combined with aperture settings, exposure times, and film or sensor sensitivity to calculate the optimal exposure.
Light meters typically average the light in a scene to 18% middle gray. More advanced cameras are more nuanced in their metering, weighing the center of the frame more heavily (center-weighted metering), considering the differences in light across the image (matrix metering), or allowing the photographer to take a light reading at a specific point within the image (spot metering).