Cameras and Film The basic camera The modern camera has all the basic parts of a pinhole camera, except that a lens replaces the pinhole. A simple lens is a disc of glass that has been shaped and polished. It will bend a set of parallel light rays so that they meet (focus) at a point (the focal point). 90 Parallel Light Rays Convex surface Focal Point Focal length Optical axis The light rays travelling to the lens from a distant object are almost parallel, and an upside down (and left-right inverted) image of the object is produced on the focal plane. The distance between the focal plane and centre of the lens is called the focal length. Compound lenses A compound lens is made up of several lenses of different shapes, sizes and thicknesses. These cancel out each other’s imperfections (aberrations) to form a much sharper image. Nearly all modern lenses are compound lenses and they also have a coating of magnesium flouride to cut down unwanted reflections, or flare.
Basic parts of a modern camera The camera body is a light tight box. The shutter stops light reaching film except when taking a picture. When the shutter button release is pressed fully down, the shutter opens to allow light to pass through the lens to the film. The lens focuses light on to film to form an image. It can be manipulated to adjust the focus. The lens opening is also known as the aperture or f stop . The size/diameter can be changed by moving a set of circular blades called the iris diaphragm . This also changes the strength of light falling on the film. The film transport moves the film and holds it flat on the focal plane. The emulsion faces the lens.
400 ASA/ISO Film Film is photo-sensitive. It is made up of tiny silver crystals that vary in size. The bigger the crystal, the more sensitive it is to light, therefore less light is required for an image to be captured. The emulsion of black of white film is panchromatic , which means that is sensitive to all colours. In colour film there are three separate emulsions sandwiched together. They are sensitive to blue, green and red light, in turn. 100 ASA/ISO The higher the ASA number, the faster and more sensitive to light is the film FILM A FILM B FILM C Small grains give fine detail Slow film Used in very good light Good for copying and billboards Medium Grains Medium film Used in average light General purpose Large grains give coarse detail Fast film Used in poor light Good for action photographs SLOW SPEED FILM 25 – 200 ASA Longer exposures are required to pick up as much information as possible. Needs a tripod or flash in low light situations. FAST SPEED FILM 400- 1800 ASA Can be used at night time without a flash or tripod. Very grainy. When printed large the grain is very obvious. Used by sports photographers, journalists etc.
Aperture The size of the lens opening, the aperture or the f-stop, controls the amount of light that passes through the lens. The lens shown ranges from f/2.8 to f/22. Each setting is one stop from the next and each lets in twice as much light as the next smaller opening, half as much light as the next largest. The greater the f-stop number, the smaller the lens opening and the less light that is let in. Which one lets in the most light?
At slow shutter speeds, like 1/30 any movement blurs the image. At medium speeds, like1/125, there is still some blur, but there is generally sharper focus. Fast shutter speeds, like 1/500 will freeze all movement and make the point of focus sharp. Slow speeds: 4s 2s 1s 1/2 1/4 1/8 1/15 1/30 Medium speeds: 1/60 1/125 Fast speeds: 1/250 1/500 1/1000 1/2000 Shutter Speed
Depth of field Depth of field refers to the amount of space (foreground, middle-ground and background) that is in or out of focus within an image. It is the distance between the nearest and farthest objects that appear in focus in a picture. Depth of field can be used creatively to isolate a subject and make it stand out against its background. This is a useful technique for portraits. It is possible to blur unsightly or unwanted backgrounds, or add mood to an image by including a blurred foreground. All of this can be achieved by using a large aperture (e.g. 2.8) which gives a shallow depth of field. This technique is called selective focusing. It is also possible to have a greater depth of field, with the foreground, background and subject all in sharp focus. This is achieved with a small aperture.