The reader who has begun to feel that a box camera derives its name from the physical appearance of the device needs to think again. Many early devices like the camera obscura as well as the apparatus used by early inventors such as Daguerre and Fox Talbot consisted of a wooden box with a lens sticking out, but it would be wrong to refer to these instruments as box cameras. The term in fact came into use only after Eastman introduced his simple daylight Kodak. The use of the word was undoubtedly prompted by the external appearance of these early Kodaks and Brownies ; by popular usage, it came to mean a rigid-body, handheld camera designed for a person with little or no knowledge of photography, and having a simple lens and shutter suited to exposures in bright daylight.
Despite its simplicity and the cheap lenses employed, a box camera gives pictures of a high quality and the figure here shows in a simple way how this is achieved.
|A meniscus lens forming an image of a scene|
on a flat surface. Note that the picture projected
by the lens is inverted.
Rays of light coming from the top of an object fall in a bundle on the lens and on passing through it converge towards a point forming an inverted image on the film. Not all rays meet at the common point though, and the worst offenders are those at the periphery of the beam. A metal plate with a hole in it (called a stop) is therefore interposed at the position shown ; this allows only the central part of the beam to pass through resulting in a picture with pin-sharp definition.
In practice, the stop often takes the form of a rotating plate with 2 or more circular apertures punched in it as shown below. The control lever which positions each opening before the lens is made to slide along a scale on the lens barrel graduated in f-numbers.
|Rotating stops were commonly|
used in box cameras
A variable lens aperture is said to offer more scope to the user. This is because the manufacturer usually designs his box so that with a medium speed film in the camera, the smallest lens opening is just enough to give an optimally exposed negative of a subject in bright sunshine. In less favourable conditions as on a cloudy day, less light reaches the film and the user has to compensate by selecting a larger stop. The variable f-stop thus provides the user with a means by which he can fine-tune the amount of light falling on his film during exposure—the more dull the light, the larger the lens opening he has to select.