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The Camera




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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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




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) .



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




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



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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