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The Camera

 

intro

f-number

depth of field

 

 

 

Introduction - the single lens reflex camera (SLR)

 

 

simple slr camera construction

 

 

 

lens - lenses are compound in nature and of sophisticated design, with most offering the facility to zoom in on an image.

 

shutter - this controls the flip mirror, which rotates about its top edge into the camera body.

 

viewfinder - the image can be viewed directly through the lens system via the flip mirror and the penta-prism.

 

diaphragm/iris - is a variable aperture. By rotation, the light entering the lens increases/decreases. Hence the f-number of the lens can be altered.

 

ccd (charge coupled device) - a silicon chip consisting of an array of capacitor-like elements that store charge when light falls on them. The amount of electric charge stored in each element is proportional to the light intensity at that point.

 

 

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f-number

 

 

the camera - F number

 

 

As might be expected, the total amount of light (L) falling on a sensor/film is proportional to the area of the aperture (A), which in turn is proportional to diameter squared ( d 2).

 

camera equation #0

 

The area A of a circular aperture of radius r is given by:

 

         camera equation #00

 

hence,

 

camera equation #1

 

It can be shown that the area of an image( Ai) is proportional to the square of the focal length (f).

 

camera equation #2

 

The light per unit area of image is given by the the total amount of light in the image (L) divided by the area of the image ( Ai) .

 

Hence,

camera equation #3

 

So the amount of light in the image relates to both the aperture diameter and the focal length.

 

For a bright image the aperture must be large and the focal length small. Note the telephoto lenses used by photographers at sports events. Object lenses are wide with a short tapering barrel.

 

Further, it can be shown that exposure time ET is inversely proportional to the light per unit area of image( L/Ai).

 

camera equation #5

 

hence,

 

camera equation #6

 

camera equation #7

 

The f-number (relative aperture) is defined as the lens focal length (f) divided by the aperture (d).

camera equation #4

 

therefore,

camera equation #8

 

f-number settings (blue) on a camera have discrete values.

 

 

sequence of f-numbers on a camera

 

 

The square of each number (red) in the series is approx. double that of the square of the number preceding it.

 

Since exposure time ( ET ) is directly proportional to the square of the f-nu>

 

 

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Depth of Field (DOF)

 

This is the distance between objects in the foreground and the background that appear in focus.

 

Depth of field depends on:

 

 

camera-to-subject distance : the further a subject is away, the sharper the image.

 

lens focal length : a short focal length lens gives a shallow DOF and vice versa.

 

f-number : a high f-number gives a deeper DOF.

 

circle of confusion CoC : a point is imaged as a spot rather than a point as a result of light being brought to focus infront and behind the prime image.

 

 

 

circle of confusion explained

 

 

 

The DOF can be defined in terms of the CoC as:

 

the region where the CoC is less than the resolution of the human eye

 

(or the medium being used).

 

In the top three diagrams you can see how a region (in red), further away from from the lens, is out of focus. This is because light rays do not meet at a point on the screen.

 

As the lens is stopped (aperture lowered), the blurred area becomes smaller and smaller.

 

As a result of the limitations of the eye/media, a point is reached when a blurred spot is indistinguishable from a focussed point.

 

The scene then has regions infront and behind that appear in focus. A depth of field is perceived.

 

 

 

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