how cameras work 6 by nafees

7/25/2019 How Cameras Work 6 by nafees

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How Cameras Work

A fully manual single-lens-reex camera. See more pictures of cool camera

stu.

Photography is undoubtedly one of the most important inventions in history -- it has

truly transformed how people conceive of the world. Now we can "see" all sorts of

things that are actually many miles -- and years -- away from us. Photography lets

us capture moments in time and preserve them for years to come.

The basic technology that makes all of this possible is fairly simple. A still lm

camera is made of three basic elements! an optical element the lens#$ a chemical

element the lm# and a mechanical element the camera body itself#. As we%ll see$

the only trick to photography is calibrating and combining these elements in such a

way that they record a crisp$ recogni&able image.

There are many di'erent ways of bringing everything together. (n this article$ we%ll

look at a manual single-lens-reex)*+# camera. This is a camera where the

photographer sees e,actly the same image that is e,posed to the lm and can

adust everything by turning dials and clicking buttons. )ince it doesn%t need any

electricity to take a picture$ a manual )*+ camera provides an e,cellent illustration

of the fundamental processes of photography.

The optical component of the camera is the lens. At its simplest$ a lensis ust a

curved piece of glass or plastic. (ts ob is to take the beams of light bouncing o' of
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an obect and redirect them so they come together to form a real image-- an

image that looks ust like the scene in front of the lens.

/ut how can a piece of glass do this0 The process is actually very simple.

As lighttravels from one medium to another$ it changes speed. *ight travels more

uickly through air than it does through glass$ so a lens slows it down.

2hen light waves enter a piece of glass at an angle$ one part of the wave will reach

the glass before another and so will start slowing down rst. This is something like

pushing a shopping cart from pavement to grass$ at an angle. The right wheel hits

the grass rst and so slows down while the left wheel is still on the pavement.

/ecause the left wheel is brie3y moving more uickly than the right wheel$ the

shopping cart turns to the right as it moves onto the grass.

The e'ect on light is the same -- as it enters the glass at an angle$ it bendsin one

direction. (t bends again when it e,its the glass because parts of the light wave

enter the air and speed up before other parts of the wave. (n a

standard conerging$ or conexlens$ one or both sides of the glass curves out.
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This means rays of light passing through will bend toward the center of the lens on

entry. (n a double conex lens$ such as a magnifying glass$ the light will bend

when it e,its as well as when it enters.

This e'ectively reverses the path of light from an obect. A light source -- say a

candle -- emits light in all directions. The rays of light all start at the same point --

the candle%s 3ame -- and then are constantly diverging. A converging lens takes

those rays and redirects them so they are all converging back to one point. At the

point where the rays converge$ you get a real image of the candle. (n the ne,tcouple of sections$ we%ll look at some of the variables that determine how this real

image is formed


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Cameras! "ocus

2e%ve seen that a real imageis formed by light moving through a conve, lens. The

nature of this real image varies depending on how the light travels through the lens.

This light path depends on two maor factors!

The angle of the light beam%s entry into the lens

The structure of the lens

The angle of lig#t entrychanges when you move the obect closer or farther

away from the lens. 6ou can see this in the diagram below. The light beams from the

pencil point enter the lens at a sharper angle when the pencil is closer to the lens

and a more obtuse angle when the pencil is farther away. /ut overall$ the lens only

bends the light beam to a certain total degree$ no matter how it enters.

7onseuently$ light beams that enter at a sharper angle will e,it at a more obtuse

angle$ and vice versa. The total "bending angle" at any particular point on the lens

remains constant.

As you can see$ light beams from a closer point converge farther away from the lens

than light beams from a point that%s farther away. (n other words$ the real image of

a closer obect forms farther away from the lens than the real image from a more

distant obect.


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6ou can observe this phenomenon with a simple e,periment. *ight a candle in the

dark$ and hold a magnifying glass between it and the wall. 6ou will see an upside

down image of the candle on the wall. (f the real image of the candle does not fall

directly on the wall$ it will appear somewhat blurry. The light beams from a

particular point don%t uite converge at this point. To focus the image$ move the

magnifying glass closer or farther away from the candle.

This is what you%re doing when you turn the lens of a camera to focus it -- you%re

moving it closer or farther away from the lmsurface. As you move the lens$ you

can line up the focused real image of an obect so it falls directly on the lm surface.

6ou now know that at any one point$ a lens bends light beams to a certain total

degree$ no matter the light beam%s angle of entry. This total "bending angle" is

determined by the structure of t#e lens.
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A standard $% mm lens doesn&t signi'cantly s#rink or magnify t#e image.

Camera (enses

(n the last section$ we saw that at any one point$ a lens bends light beams to a

certain total degree$ no matter the light beam%s angle of entry. This total "bending

angle" is determined by the structure of the lens.

A lens with a rounder shape a center that e,tends out farther# will have a moreacute bending angle. /asically$ curving the lens out increases the distance between

di'erent points on the lens. This increases the amount of time that one part of the

light wave is moving faster than another part$ so the light makes a sharper turn.

(ncreasing the bending angle has an obvious e'ect. *ight beams from a particular

point will converge at a point closer to the lens. (n a lens with a 3atter shape$ light

beams will not turn as sharply. 7onseuently$ the light beams will converge farther

away from the lens. To put it another way$ the focused real image forms farther

away from the lens when the lens has a 3atter surface.

(ncreasing the distance between the lens and the real image actually increases thetotal si&e of the real image. (f you think about it$ this makes perfect sense. Think of

a proector! As you move the proector farther away from the screen$ the image

becomes larger. To put it simply$ the light beams keep spreading apart as they

travel toward the screen.

The same basic thing happens in a camera. As the distance between the lens and

the real image increases$ the light beams spread out more$ forming a larger real


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image. /ut the si&e of the lm stays constant. 2hen you attach a very 3at lens$ it

proects a large real image but the lm is only e,posed to the middle part of it.

/asically$ the lens &eroes in on the middle of the frame$ magnifying a small section

of the scene in front of you. A rounder lens produces a smaller real image$ so the

lm surface sees a much wider area of the scene at reduced magnication#.

Professional cameras let you attach di'erent lenses so you can see the scene at

various magnications. The magnication power of a lens is described by its focal

lengt#. (n cameras$ the focal length is dened as the distance between the lens

and the real image of an obect in the far distance the moon for e,ample#. A higher

focal length number indicates a greater image magnication.

;i'erent lenses are suited to di'erent situations. (f you%re taking a picture of a

mountain range$ you might want to use a telep#oto lens$ a lens with an especially

long focal length. This lens lets you &ero in on specic elements in the distance$ so

you can create tighter compositions. (f you%re taking a close-up portrait$ you might

use a wide-angle lens. This lens has a much shorter focal length$ so it shrinks thescene in front of you. The entire face is e,posed to the lm even if the subect is

only a foot away from the camera. A standard 8< mm camera lens doesn%t

signicantly magnify or shrink the image$ making it ideal for shooting obects that

aren%t especially close or far away.

()*S)S +* ,H) ()*S

A camera lens is actually several lenses combined into one unit. A single converging

lens could form a real image on the lm$ but it would be warped by a number

of aberrations.

=ne of the most signicant warping factors is that di'erent colors of light bend

di'erently when moving through a lens. This c#romatic aberrationessentially

produces an image where the colors are not lined up correctly.

7ameras compensate for this using several lenses made of di'erent materials. The

lenses each handle colors di'erently$ and when you combine them in a certain way$

the colors are realigned.

(n a oom lens$ you can move di'erent lens elements back and forth. /y changing

the distance between particular lenses$ you can adust the magnication power --

the focal length -- of the lens as a whole.

Cameras! ecording (ig#t

The chemical component in a traditional camera is'lm. >ssentially$ when you

e,pose lm to a real image$ it makes a chemical record of the pattern of light.


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(t does this with a collection of tiny light-sensitive grains$ spread out in a chemical

suspension on a strip of plastic. 2hen e,posed to light$ the grains undergo a

chemical reaction.

=nce the roll is nished$ the lm is developed -- it is e,posed to other chemicals$

which react with the light-sensitive grains. (n black and white lm$ the developerchemicals darken the grains that were e,posed to light. This produces a negative$

where lighter areas appear darker and darker areas appear lighter$ which is then

converted into a positive image in printing.

7olor lm has three di'erent layers of light-sensitive materials$ which respond$ in

turn$ to red$ green and blue. 2hen the lm is developed$ these layers are e,posed

to chemicals that dye the layers of lm. 2hen you overlay the color information

from all three layers$ you get a full-color negative.

@or an in-depth description of this entire process$ check out ow Photographic @ilm

2orks.

)o far$ we%ve looked at the basic idea of photography -- you create a real image with

a converging lens$ and you record the light pattern of this real image on a layer of

light-sensitive material. 7onceptually$ this is all that%s involved in taking a picture.

/ut to capture a clear image$ you have to carefully control how everything comes

together.

=bviously$ if you were to lay a piece of lm on the ground and focus a real image

onto it with a converging lens$ you wouldn%t get any kind of usable picture. =ut in

the open$ every grain in the lm would be completely e,posed to light. And without

any contrasting une,posed areas$ there%s no picture.

To capture an image$ you have to keep the lm in complete darkness until it%s time

to take the picture. Then$ when you want to record an image$ you let some light in.

At its most basic level$ this is all the body of a camera is -- a sealed bo, with

a s#utterthat opens and closes between the lens and lm. (n fact$ the term

camera is shortened from camera obscura$ literally "dark room" in *atin.

@or the picture to come out right$ you have to precisely control how much light hits

the lm. (f you let too much light in$ too many grains will react$ and the picture will

appear washed out. (f you don%t let enough light hit the lm$ too few grains will

react$ and the picture will be too dark. (n the ne,t section$ we%ll look at the di'erent

camera mechanisms that let you adust the e,posure.
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,#e plates in t#e iris diap#ragm fold in on eac# ot#er to s#rink t#e

aperture and expand out to make it wider.

Cameras! ,#e ig#t (ig#t

(n the last section$ we saw that you need to carefully control the lm%s e,posure tolight$ or your picture will come out too dark or too bright. )o how do you adust this

e,posure level0 6ou have to consider two maor factors!

ow much light is passing through the lens

ow long the lm is e,posed

To increase or decrease the amount of light passing through the lens$ you have to

change the si&e of the aperture-- the lens opening. This is the ob of the iris

diap#ragm$ a series of overlapping metal plates that can fold in on each other or

e,pand out. >ssentially$ this mechanism works the same way as the iris in your eye-- it opens or closes in a circle$ to shrink or e,pand the diameter of the lens. 2hen

the lens is smaller$ it captures less light$ and when it is larger$ it captures more light.

The length of e,posure is determined by the s#utter speed. Cost )*+ cameras use

a focal plane s#utter. This mechanism is very simple -- it basically consists of two

"curtains" between the lens and the lm. /efore you take a picture$ the rst curtain

is closed$ so the lm won%t be e,posed to light. 2hen you take the picture$ this

curtain slides open. After a certain amount of time$ the second curtain slides in from

the other side$ to stop the e,posure.

W#en you click t#e camera&s s#utter release/ t#e 'rst curtain slides open/exposing t#e 'lm. After a certain amount of time/ t#e second s#utter

slides closed/ ending t#e exposure. ,#e time delay is controlled by t#e

camera&s s#utter speed knob.

W#en you click t#e camera&s s#utter release/ t#e 'rst curtain slides open/

exposing t#e 'lm. After a certain amount of time/ t#e second s#utter


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how cameras work 6 by nafees

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