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The Camera Defined - Focus and Viewing, Exposure, Image Handling

cameras light film lens

VMI, Inc.

The camera is a tool that controls the transmission of light or other energy that is required to create a visual record using light-sensitive materials or digital sensors. The energy from a scene is transmitted into a dark chamber by an opening in one end, which allows light from the scene within the angle of coverage of the lens to enter the camera. The light going through the opening will form an inverted image on the opposite side of the unit where a film or other receptor will record the image. The amount of the light is controlled by the size of the opening in the camera or lens and the length of time the opening is left uncovered.

Since its invention, there have been great improvements to the camera’s features but in reality little has changed in its fundamental role and function. In contemporary cameras, the light is focused using sophisticated lenses made from high-quality optical glass and captured using superb film emulsions or sophisticated electronic CCD or CMOS sensors. Using extremely accurate electronic shuttering systems, contemporary cameras have almost unlimited freedom to capture and record images. At no time in photographic history has the camera been more versatile. It is easiest to understand the operation of a camera by tracing the light’s path from an object to the recording at the film plane.

Focus and Viewing

A simple camera with a fixed focal length lens will often have a separate viewfinder that approximates what will be recorded by the camera. This type of viewfinder originated in the first box cameras. It continued to be used for many years and is now common in single-use cameras.

With cameras that have lenses capable of critically focusing at different distances, a camera needs a method to observe if the lens is focused at the correct place. This can be achieved with single lens reflex (SLR) type cameras. Autofocus technology found in many modern cameras all but eliminates this requirement for the photographer.

If a camera has a back that can be removed then often a focusing screen made of ground glass can be inserted where the projected image would be visible. This is the case for large format cameras. After critically focusing using a dark cloth to control ambient light on the focusing screen, the film, which is kept in a light-tight film holder or a large format digital back, is inserted into the plane of focus and the exposure is made.

A twin lens reflex (TLR) camera uses a viewing system that incorporates two identical lenses mounted in two separate paths in the camera. One lens is located above the other and both are controlled using a common focusing movement. One lens is projected to the viewfinder and is used for composing and focusing while the other has a shutter and the camera’s diaphragm and controls the camera’s exposure. A TLR uses a film size that is smaller than a view camera but larger than roll film and has been defined as medium format. The light path to the viewfinder uses the lens, which is located on the top, and uses a 45 degree mirror so that the photographer can peer down into the camera and see what the scene includes, while the bottom lens with the shutter and aperture leads directly to the imaging plane. This type of viewfinder may have a very small error field of view that is described as parallax error.

SLR cameras basically use the idea of the TLR with the exception that it has only one light pathway that is active at any one time. There is a mirror in this camera type, which is located in the light path and is movable. Just prior to exposure, the mirror is lifted out of the imaging path by the camera’s mechanics allowing light to reach the imaging system and exposing the film or digital sensor. When the exposure is made, there is no image traveling to the viewfinder.

A modification to the SLR type viewfinder is a rangefinder focusing system. This system splits the image into two images superimposed into the camera’s viewfinder. When the lens is focused the images move. When the lens is focused at the correct location, the images are observed to be on top of one another.

Other camera viewfinders might be found in subminiature cameras originally designed for spying, stereo cameras, underwater cameras, and other specialized camera types that also exist to support special applications of photography. These viewfinders might have a framing area or a plastic structure that is attached to the front of the camera defining the field of view. In some of these applications it may be challenging if not impossible to look through the system.

The simplest of camera types is by far the point-and-shoot design. This type of camera became popular when technology made fully automated film handling and exposure easy. Point-and-shoot cameras evolved quickly and now have completed integrated imaging sensors and command the largest market share of all cameras sold. This type of camera was the first mass-produced digital camera type. Although some cameras use very small viewfinders, the availability and use of electronic viewfinders on the back of these cameras allows for realtime viewing and relatively “instant” playback. This feature for many makes the optical viewfinder almost obsolete. Now it is no longer common to see a camera pressed up against a photographer’s face (using an optical viewfinder), but rather at arm’s length where the electronic viewfinder is most easily seen except in bright light.


Light enters the lens and passes through an adjustable opening called a diaphragm. This determines how much light will enter into the camera through the lens. A camera also has a shutter, which is often placed between the lens elements and behind the lens assembly, or at the back of the camera directly in front of the film or imaging sensor. The shutter is controlled by a timing mechanism that can be mechanical, electronic, or electromechanical (a combination of both), which opens the shutter for a specific duration of time. The shutter and the lens diaphragm together control the amount of light that causes the exposure of the film or the light-sensitive receptors in the CCD or CMOS chip. Once equipment became more prevalent, hand-held light meters became common and were the only way to determine the amount of light falling on the subject. Cameras did not have built in meters to guide proper exposure determination. Now many cameras have built-in meters and sophisticated fully automated exposure systems.

Image Handling

At this moment in the evolution of photography and its equipment, there is still a wide range of film-based photographic products available. Film still is available in common sizes for each of the different camera types: sheet or cut film used in large format view cameras, roll film for medium format cameras, and sprocketed 35mm roll film used for this format camera. Self-processing film for “instant” cameras and film backs can also be found. Film, of course, requires no extraneous light exposure and must be kept in a dark container before and after exposure. The camera itself is the dark container and controls the lack of ambient exposure. Similarly digital cameras enclose the sensor so only the light traveling to the sensor from the scene is recorded.

Cameras with digital sensors produce electronic data, which may be ready for direct use or further digital processing. Because the amount of data they produce will vary based on the size of the sensor and its related characteristics, the media required to store this data comes in a variety of sizes and data capacities. Rather than film, products such as compact flash, memory sticks, SD (secure digital) card, and other types of removable storage products are available. Some cameras also offer one or more compartments to hold removable storage media. Less sophisticated cameras require that these data be moved from the camera using a physical hard wire while direct wireless transmission of image data during a shoot is possible and gives the greatest mobility and flexibility to more professional users.

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