Digital SLR camera

Sunday, 15 October 2006

The single lens reflex (SLR) camera is a type of highly developed and popular camera which uses a hinged mirror to reflect the image formed by the lens onto a viewing screen until exposure. This enables a photographer to see the image exactly as it will be formed on film. A single lens reflex camera generally consists of an optical system comprised of an objective lens, a pivoted movable mirror to reflect an inverted image upward to a viewing screen, and optics (a pentaprism) to erect the inverted image and direct it to an eyepiece. A typical SLR camera also includes hardware to pivot the mirror out of the path of the image so the image can pass through the camera to a focal plane shutter and fall upon a light-sensitive surface at the back of the camera, and thereafter to reposition the mirror back to its original position for subsequent camera operations. Single lens reflex (SLR) cameras have several advantages as compared to other types of conventional cameras, the most important advantage being the ability to view the object precisely as it will be imaged on the film, regardless of the lens focal length or the object distance. Compared to a rangefinder type camera, SLR cameras are that parallax misalignment is eliminated, and focusing and composing are easier. With the advance of digitization of cameras in recent years, digital cameras have become widely used instead of cameras using silver-halide films. Among such digital cameras, an SLR type digital camera is advantageous since the image of the object formed by the photographing lens of the camera is observed through the finder, no parallax is generated between the image captured by an image capturing element such as a CCD (charge coupled device), and the image observed through the finder.

A digital SLR camera combines the advantage of an image processing function, which is unique to digital cameras, with the advantage of the optical system being free from parallax error, which is unique to SLR cameras. A single lens reflex type digital camera is provided with a viewfinder device to which a subject image passing through a taking lens is guided by a quick return mirror, an image capturing device such as a CCD that is provided at a stage to the rear of the quick return mirror to capture the subject image and output image data, an image processing circuit that performs image processing, a compression circuit that compresses data having undergone image processing by adopting a method such as JPEG to store them in a storage medium such as a flash memory and a monitor that displays data that has undergone the image processing. A digital single lens reflex camera has a reflector in a photographic light path to reflect object-carrying light transmitted through a photographic optical system toward a finder optical system, an image pickup device is provided at the imaging plane of the photographic optical system. The reflector placed in the photographic light path can be a quick return mirror, a quick return half mirror, or a stationary half mirror. The light from an object is incident on a photographing lens system including a plurality of lens groups. The light passed through the photographing lens system is incident on a beam splitter, which splits the incident light into light directed toward a finder optical system, and light directed to an image sensor such as a CCD (charge coupled device). The photographing lens system and the finder optical system as well as the beam splitter are integrally formed as a lens unit. One of the most significant differences between a digital still single lens reflex camera and a single lens reflex camera is that the image pickup surface of the image pickup device is considerably smaller than a picture surface of the silver-halide film camera.

A digital SLR camera executes a focus detecting operation by guiding rays of light to a phase difference AF sensor via a quick return mirror, sub mirror and so forth placed on the optical axis of the imaging lens. The lens mount of a digital SLR camera is often made common to that of an SLR camera. Due to this arrangement, many types of photographic lenses for SLR cameras can be utilized as an exchangeable photographic lens for digital SLR cameras. An SLR digital camera is provided with a beam splitter which splits light passed through the photographing lens into light directed to the image capturing element and light directed to the finder optical system. The beam splitter includes a beam splitting surface which may be formed with a multi-layer film or coating made of dielectric material. SLR cameras are generally provided with a viewfinder optical system through which the object image formed on a focusing glass positioned above the mirror box is viewed as an erect image at the eye of the user. In most cases, the viewfinder optical system is of a type which includes a pentagonal roof prism and an eyepiece positioned behind the exit surface of the pentagonal roof prism. The SLR digital camera is provided above the mirror box with a focusing glass and a pentagonal roof prism which are arranged in this order from the side of the mirror box. Digital cameras are provided with an optical finder to avoid these disadvantages. Digital cameras provided with an optical finder include the separate type wherein light is directed to a finder without mediation of the taking lens, and the single lens reflex (SLR) type wherein light passing through the taking lens is reflected and directed to the finder. Digital cameras are provided with a display such as a liquid crystal display, and the display is used for displaying shot and recorded images for the purpose of reproduction and for displaying the image that is being shot.

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