camera, sensors and film

8/4/2019 Camera, Sensors and Film

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Camera, Sensors and Film



Camera Principles

� The word photography means drawing (or

writing) with light.

±

Every time you take a photograph you are reallyallowing light from the subject to draw its own

picture on the sensor or film.



Light-sensitive films and sensors

� 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 mixedwith gelatine and the resulting light sensitive

emulsion is coated onto a plastic backing.



� Scientists discovered that it is not necessary to

wait until the silver halides darken in thecamera.

± 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 therecorded image is strong enough to be visible.

± Darkroom processing



� 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 silverhalides.

� After development, the image on the paper is anegative of the negative, i.e. the paper appears

white where the original subject was light, blackwhere it was dark and (assuming you are usingmonochrome materials) a suitable gray tonewhere it was in between. We have a positiveprint.



� The advantage of using negative and positive

stages is that many prints can be run off onecamera exposure. And by putting the negative

in an enlarger (which is rather like a slide

projector), enlarged prints can be made. So

you dont have to have a big camera to make

big photographs.



� The illustration on the

right shows in basic

form, the optical andchemical steps in

making a black and

white photograph.



� Most pictures of course are shot in color, butthe same principles apply.

� Color films are coated with several emulsionlayers, 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.) aswell as in tone.

� When such a negative is enlarged onto multi-

coated color paper the paper responds in asimilar way to give a positive print with colorsbrought back to their original subject hues.



The change to digital

� M

ore recently, photographyhas undergone a massivechange in the way that werecord images.

� Film cameras, though stillreadily available, are beingoutsold by their digital equivalents.

� With these cameras, the film is replaced as thelight-sensitive part of the photographic processwith an electronic sensor or, more accurately, agrid of sensors.



� Instead of the light in a scene being recorded

by silver halide grains, it is captured with smallelectronic sensors.

� Each of the individual sensors provides a small

portion of the full description of the scene

that makes up the digital file.

� After photographing the file is stored on a

memory card held within the camera.



� Unlike with film, there are no chemicalsteps involved in using your digital files

to make prints. ± The camera is connected to your computer

and all the digital photographs stored onyour cameras memory card are transferredinto the memory of the computer. This

process is called downloading. ± Once on the computer, the pictures can be

displayed on screen, enhanced and editedusing a software program called an imageeditor, such as Adobe Photoshop or

Photoshop Elements. ± After all the picture changes have been

made, the image is then printed using adesktop color printer or taken to a photo-laboratory for printing.



The camera

� 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.

� Film and digital cameras vary a great deal in detail,

but they all possess the basic features shown in

Figure 8.1 in some form.



Basic Features on all cameras

� 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 oradvancing the film; and an indicator to show

how many pictures you have taken.



The Lens

� 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 differentdistances.



� Cheapest cameras have the lens focus free, meaning itis 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 thisfocusing control moves the lens slightly further fromthe film the nearer your subject distance setting.

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

� In all cases though, anything nearer than the closest

subject setting the camera allows will not appearsharp, unless you switch to macro mode, fit an extraclose-up lens or extension ring.



The Shutter� The shutter prevents light from the lens reaching the

sensor/film until you press the release button, so itallows you to decide exactly when the picture will betaken

� On the most basic cameras it may function at one

speed only, typically opening for about 1/125 second� Shutters on more advanced cameras offer a range of

ten or so speed settings, from several whole secondsdown to 1/1000 second or less.

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



Having a choice allows you to freeze or blur moving subjects,

and also compensate for dim or bright lighting



The Aperture� The aperture (also known as the diaphragm or stop) is

a circular hole positioned within or just behind thelens. ± adjustable in size like the iris of the eye

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

± The shutter therefore controls the time the image isallowed to act on the film

± the aperture controls the brightness of the image.

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

� has a very important effect on whether parts of scenescloser and further away than the subject on which thelens is focused also appear sharp.



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.



� The smaller the aperture, the greater theforeground to background sharpness or

depth of field



� Very basic cameras have one fixed aperture, ortwo to three settings simply marked in weathersymbols clouds for dull light conditions andsun for bright lights. Most advanced or singlelens reflex cameras offer half a dozen aperturesettings, which are given f-numbers.

� Each change of f-number lets in half or doublethe light;

� Automatic cameras have an aperture setting

selected by the camera mechanism in responseto the brightness of your subject lighting, andoften display no settings at all.



The Viewfinder

� The viewfinder allowsyou to aim the cameraand preview how muchof your subject will be

included in the picture.� Some cameras (non-

reflex types) have adirect viewfinder,which you can

recognize by its ownseparate windowabove 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 bythe lens.

� This is the same for

both digital and film SLR

cameras

camera, sensors and film

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