Camers, Sesons and Film

Light-sensitive films and sensors

Camera Principles

Camera, Sensors and Film

Processing gives a negative picture on film.

Subjects that were white appear as black metallic silver, and dark subjects as clear film.

Parts of the subjects that were neither light nor dark are represented as intermediate gray density.

The negative is then printed in the darkroom onto paper coated with a similar emulsion containing silver halides.

Scientists discovered that it is not necessary to wait until the silver halides darken in the camera.

Prior to this you had to wait until the silver halide turned black to get proper light exposure

You can just let the image light act on it for a fraction of a second, keep the film in the dark and then later place it in a solution of chemicals that develops the silver until the recorded image is strong enough to be visible.

Darkroom processing

After development, the image on the paper is ‘a negative of the negative’, i.e. the paper appears white where the original subject was light, black where it was dark and (assuming you are using monochrome materials) a suitable gray tone where it was in between. We have a positive print.

The word photography means drawing

(or writing) with light.

Every time you take a photograph you are really allowing light from the subject to draw its own picture on the sensor or film.

For many years, most cameras used film coated with chemical compounds of silver called silver halides to record the scene.

The silver halides are extremely light sensitive and change from a creamy color to black when exposed to light.

To construct the film, the silver halides are mixed with gelatine and the resulting light sensitive emulsion is coated onto a plastic backing.

The change to digital

When such a negative is enlarged onto multi-coated color paper the paper responds in a similar way to give a positive print with colors brought back to their original subject hues.

The illustration on the right shows in basic form, the optical and chemical steps in making a black and white photograph.

Instead of the light in a scene being recorded by silver halide grains, it is captured with small electronic sensors.

The advantage of using negative and positive stages is that many prints can be run off one camera exposure. And by putting the negative in an enlarger (which is rather like a slide projector), enlarged prints can be made. So you don’t have to have a big camera to make big photographs.

Each of the individual sensors provides a small portion of the full description of the scene that makes up the digital file.

Unlike with film, there are no chemical steps involved in using your digital files to make prints.

The camera is connected to your computer and all the digital photographs stored on your camera’s memory card are transferred into the memory of the computer. This process is called downloading.

Once on the computer, the pictures can be displayed on screen, enhanced and edited using a software program called an image editor, such as Adobe Photoshop, Lightroom or Pixlr.

After all the picture changes have been made, the image is then printed using a desktop color printer or taken to a photo-laboratory for printing.

More recently, photography has undergone a massive

change in the way that we record images.

Film cameras, though still readily available, are being

outsold by their digital equivalents.

With these cameras, the film is replaced as the light-sensitive part of the photographic process with an electronic sensor or, more accurately, a grid of sensors.

Most pictures of course are shot in color, but the same principles apply.

Color films are coated with several emulsion layers, sensitive to blue, green and red.

After appropriate processing, color negative film carries images that are reversed in color (blues appear yellow, greens magenta, etc.) as well as in tone.

After photographing the file is stored on a memory card held within the camera.

The camera

Basic Features on all cameras

The Lens

every camera is basically just a light-tight box with a lens at one end and a light-sensitive surface (e.g. sensor or film) at the other.

Most modern cameras have lenses with an auto-focusing mechanism able to alter focusing to suit the distance of whatever the camera is pointing at in the central area of your picture

In all cases though, anything nearer than the closest subject setting the camera allows will not appear sharp, unless you switch to macro mode, fit an extra close-up lens or extension ring.

The lens is the most important part of the whole camera.

It must be protected from fingerprints and scratches, otherwise images resemble what you see when your eyes are watering.

The spacing of the lens from the sensor/film has to change for subjects at different distances.

Cheapest cameras have the lens ‘focus free’, meaning it is fixed for what the makers regard as the subject distance for average snaps.

Some have a ring or lever with a scale of distances (or symbols for ‘groups’, ‘portraits’, etc.). Operating this focusing control moves the lens slightly further from the film the nearer your subject distance setting.

first and foremost, a lens positioned the correct focusing distance from the film/sensor

A shutter

a lens aperture

a viewfinder

a means of moving to the next picture or advancing the film; and an indicator to show how many pictures you have taken.

Film and digital cameras vary a great deal in detail, but they all possess the basic features.

The Aperture

The Shutter

The smaller the aperture, the greater the foreground to background sharpness or ‘depth of field’

changing to a smaller or larger diameter makes the image dimmer or brighter, so again this is a means of compensating for strong or weak lighting conditions.

Having a choice allows you to ‘freeze’ or ‘blur’ moving subjects, and also compensate for dim or bright lighting

The shutter prevents light from the lens reaching the sensor/film until you press the release button, so it allows you to decide exactly when the picture will be taken

On the most basic cameras it may function at one speed only, typically opening for about 1/125 second

The aperture (also known as the diaphragm or stop) is a circular hole positioned within or just behind the lens.

adjustable in size like the iris of the eye

changing to a smaller or larger diameter makes the image dimmer or brighter, so again this is a means of compensating for strong or weak lighting conditions.

Shutters on more advanced cameras offer a range of ten or so speed settings, from several whole seconds down to 1/1000 second or less.

On fully automatic cameras, such as most compacts, the shutter speed is selected by the camera mechanism itself, according to the brightness of the scene and light sensitivity of your sensor/film.

The Viewfinder

The shutter therefore controls the time the image is allowed to act on the film

the aperture controls the brightness of the image.

Together, they allow you to control the total exposure to light the film receives.

has a very important effect on whether parts of scenes closer and further away than the subject on which the lens is focused also appear sharp.

The viewfinder allows you to aim the camera and preview how much of your subject will be included in the picture.

Some cameras (non-reflex types) have a direct viewfinder, which you can recognize by its own separate window above the lens

Digital cameras which look like compacts often use a combination of a direct viewfinder and a small flat display screen (LCD monitor) on the back.

Direct viewfinders are bright and clear, but less accurate than SLR or LCD monitor systems for composing pictures, particularly close-ups.

SLR (single lens reflex) cameras allow you to look inside the camera itself and view the actual image formed by the lens.

This is the same for both digital and film SLR cameras

Very basic cameras have one fixed aperture, or two to three settings simply marked in weather symbols – ‘clouds’ for dull light conditions and ‘sun’ for bright lights. Most advanced or single lens reflex cameras offer half a dozen aperture settings, which are given ‘f-numbers’.

Each change of f-number lets in half or double the light;

Automatic cameras have an aperture setting selected by the camera mechanism in response to the brightness of your subject lighting, and often display no settings at all.