- Related Topics:
- history of photography
An older type of flash is an oxygen-filled glass envelope containing a specific amount of aluminum or zirconium wire and means for igniting the wire in the bulb. The wire burns away with a brilliant flash lasting typically about 1/100 to 1/50 second. Each flashbulb can, however, yield only one flash. Current flashbulb systems use four to 10 tiny bulbs, each in its own reflector, arranged in cube or bar carriers that plug into cameras designed for them. The individual flashes are fired in turn by a battery and circuit in the camera through mechanically generated current pulses or other means. In view of the greater convenience of electronic flash, flashbulbs in their various forms are largely obsolescent.
Firing and synchronization
Flash units are usually fired with a switch in the camera shutter to synchronize the flash with the shutter opening. A contact in the camera’s flash shoe (hot shoe) or a flash lead connects the unit with this shutter switch. The shutter contact usually closes the instant the shutter is opened. A focal plane shutter must fully uncover the film (generally at a shutter speed of 1/60 second or slower) for flash synchronization. With flashbulbs the shutter must also stay open while the flash reaches its peak brightness—about 1/50 second.
System cameras
From the development of the 35-mm miniature camera in the 1930s evolved the concept of the system camera that could be adapted to numerous jobs with a range of interchangeable components and specialized accessories. Today, most moderately advanced 35-mm miniatures take interchangeable lenses, close-up and photomicrographic attachments, filters, flash units, and other accessories. The most elaborate camera systems also include such accessories as alternative finder systems; interchangeable reflex screens, film backs, and magazines; and remote-control and motor-drive systems. Modular professional roll-film and view cameras are built up from a selection of alternative camera bodies, film backs, bellows units, lenses, and shutters. This is the nearest approach to the universal camera, assembled as required to deal with practically every type of photography.
Characteristics and parameters of lenses
The lens forming an image in the camera is a converging lens, the simplest form of which is a single biconvex (lentil-shaped) element. In theory such a lens makes a light beam of parallel rays converge to a point (the focus) behind the lens. The distance of this focus from the lens itself is the focal length, which depends on the curvature of the lens surfaces and the optical properties of the lens glass. An object at a very long distance (optically regarded as at “infinity”) in front of the lens forms an inverted image in a plane (the focal plane) going through the focus. Light rays from nearer objects form an image in a plane behind the focal plane. The nearer the object, the farther behind the lens the corresponding image plane is located—which is why a lens has to be focused to get sharp images of objects at different distances.
Focal length and image scale
The image scale, or scale of reproduction, is the ratio of the image size to the object size; it is often quoted as a magnification. When the image is smaller than the object, the magnification of the object is less than 1.0. If the image is 1/20 the size of the object, for example, the magnification may be expressed either as 0.05 or as 1:20. For an object at a given distance, the scale of the image depends on the focal length of the lens (). A normal camera lens usually has a focal length approximately equal to the diagonal of the picture format covered. A lens of longer focal length gives a larger scale image but necessarily covers less of the scene in front of the camera. Conversely, a lens of shorter focal length yields an image on a smaller scale but—provided the angle of coverage is sufficient (see below)—takes in more of the scene. Many cameras, therefore, can be fitted with interchangeable lenses of different focal lengths to allow varying the image scale and field covered. The focal length of a lens in millimetres (sometimes in inches) is generally engraved on the lens mount.
Aperture
The aperture, or f-number, is the ratio of the focal length to the diameter of an incident light beam as it reaches the lens. For instance, if the focal length is 50 millimetres and the diameter of the incident light beam is 25 millimetres, the f-number is 2. This incident-beam diameter is often roughly the lens-diaphragm diameter, but it may be appreciably larger or smaller. The maximum aperture (f-number at the largest diaphragm opening) is also marked on the lens, usually in the form f:2, f/2, or 1:2.
Angle of coverage
A lens must cover the area of a camera’s film format to yield an image adequately sharp and with reasonably even brightness from the centre to the corners of the film. A normal lens should cover an angle of at least 60°. A wide-angle lens covers a greater angle—about 70° to 90° or more for an ultrawide-angle lens. A long-focus lens covers a smaller angle.
The angle of coverage depends on the lens design. Designations like “wide angle” or “narrow angle” are not necessarily synonymous with “short focus” and “long focus,” as the latter terms refer to the focal length of the lens relative to the picture format.