Physics_Form III_OPTICAL INSTRUMENTS

 PARTS OF THE LENS CAMERA

                        diaphragm

The diaphragm determine the amount of light that passes through the lens by changing the size of the aperture. The aperture is an opening whose diameter is adjustable.

Most cameras use an iris-type diaphragm, which consist of a number of very thin metal blades(sharp). They are mounted so that the size of the lens opening can be changed by a rotating ring or moving lever.

 

                                                                 shutter

The shutter is the mechanical device that acts as a gate, controlling the duration of time that light is allowed to pass through the lens and fall on the film.

                                                            Viewfinder

The viewfinder defines the area covered by the lens that is in use on the camera.

On a typical camera, a bright line in the viewfinder outlines the area covered by the lens .The outline changes size automatically to correspond with lenses of different focal lengths.

                             Film

The film is a light-sensitive surface of the camera. It is normally rolled to the back of the camera. The ordinary film is made up of a layer of gelatin (protein material) carrying minute suspended silver halide crystals or grains [the emulsion].

 Silver halide is a compound of silver with fluorine, chlorine, bromine, or Iodine. Only the last three are light-sensitive .The most commonly used silver halide crystal or grain [the emulsion].Silver halide is a compound of silver with fluorine, chlorine, bromide, or iodine. Only the last three are sensitive. The most commonly used silver halide is silver bromide with some little amount of silver iodide.

Exposure of the film to light produce an invisible change on silver halide grains yielding a latent image. The image becomes visible on being treated with certain chemicals in process known as              developing. In most camera, the film is wound up on a spool with an interleaving light-tight (light proof) backing paper.  

             Mode of action of the lens camera                                 

The object is placed in such away that is real, inverted image of the object is formed on film.The amount of light entering the film region can be adjust by adjusting the aperture of the lens .The lens system contains two chromatic lenses which are separate

They can minimize chromatic aberration /chromatic disortion and spherochromatic, is a failure of the lens to focus all colours .It caused by dispersion

The light box is painted black inside to avoid unnecessary reflection inside. A shutter is placed between the lens and the film when ,photograph is taken the shutter is open and closes quickly, thereby exposing the film to light for short time to entering the camera

                 A Sharp image of an object is focused on film by adjusting the distance of lens from the film . There is focusing a ring which turns over a scale engraved on the lens.

By means of this ring the distance between the lens and film can be adjusted  by moving the lens forwards and backward.

The film speed is measure of how quickly the film will be exposured when in use

A fast speed film needs a relatively a short exposure time

                          while

 A slow sleep film needs relatively longer exposure time.

Most of cameras have a view finder which help the user to see what the camera is focusing before exposing the film.

a circle of confusion is an optical spot caused by a cone of light rays from a lens not coming to a perfect focus when imaging point source.

The lens aperture is generally expressed as the fraction of the focal length, f of the lens. Many camera have built in exposure meters, having light sensitive surfaces. Current flows into the meters depending on the amount of light falling on it. So the photographer has to adjust a suitable set of aperture and exposure times for correct exposure.When aperture is large the image is not focused in the single plane but gets spread out. The depth of focus indicates the range of the object distance.The lens aperture is generally expressed as the fraction of the focal length, f of the lens. Many camera have built in exposure meters, having light sensitive surfaces. Current flows into the meters depending on the amount of light falling on it. So the photographer has to adjust a suitable set of aperture and exposure times for correct exposure.

When aperture is large the image is not focused in the single plane but gets spread out. The depth of focus indicates the range of the object distance.

The brightness of the image focused on the film depends on focal length and diameter of the lens. Clearly, amount of light that reaches the film increases as the size of the lens increases. That is, as the lens diameter increases, the brightness of image formed on the film increases.

Relationship between focal length and image size

The focal length of lens affects the brightness of the image formed by considering magnification equation for thin lenses.

 

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