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Tutorials Page 2

N.E.P.W. ONLINE TUTORIAL SERIES
No.2. LENSES

LENSES and their uses in Photography.
A Basic Introduction.

LENSES, made from glass or plastic or sometimes a combination of both materials, are used in photography to record an image in a camera.

Basically a lens transmits an image of a scene or subject to the photosensitive surface of the film, or the CCD in a digital camera.


LENSES.
What are lenses? 
Lenses are discs of transparent glass or plastic with concave or convex surfaces
which can bend (refract) light.

SIMPLE LENSES
There are two basic lens types:
CONVEX (POSITIVE) lenses and CONCAVE (NEGATIVE) lenses.

THE CONVEX LENS

Convex lenses are thicker in the centre of the lens and progressively thinner towards the periphery.

Convex lenses converge (focus) the light rays and can form an image on a surface.
Light rays 'B' entering the lens 'L' from the left, are refracted towards the axis 'A' of the lens. The rays are focussed at the focal plane 'F', which is the point where the film plane 'P' of the camera is positioned.

THE CONCAVE LENS

Concave lenses are thinner in the centre of the lens and progressively thicker towards the periphery.

Concave lenses diverge (spread out) the light rays and will not form an image.
With a concave lens the light rays 'B' entering the lens 'L' from the left, are refracted away from the lens axis 'A' to points 'B1'. They do not come to a focal point and therefore do not produce an image at the film plane 'P', nevertheless they are useful in complex lens design to correct some of the inherent faults in simple lenses.

FOCAL LENGTH.

The focal length of a camera lens is the distance 'D' from the film plane 'P' of the camera to the center of the lens 'C' when the lens is focused on infinity.
The focal length of a lens determines the image size that is projected onto the film plane. Short focal length lenses have a wide angle of view so that objects appear smaller on the negative, while long focal lengths have a narrow angle of view and magnify the image on the film. The size of objects in an image, for a given focal length, is the same irrespective of the film format.

FOCAL PLANE.


The focal plane 'P' is at the point where the image is focussed 'F' and is where the film plane is positioned.

DEPTH OF FIELD.

Depth of field (DoF) is the distance through which the subject may extend and still form an acceptably sharp image, in front and behind the point of focus.
If the camera is focussed on a subject 'C' the zone of acceptable sharpness will extend to a distance 'B', in front of, and to 'D' behind the subject.
Parts of the subject in the shaded zones 'A' & 'E' will be out of focus. Approximately 1/3 of the DoF zone will be in front of the point of focus, and 2/3 behind, (in close up or macro work, assume 1/2 and 1/2 ratio). The depth of field can be increased by stopping down to a smaller aperture.



A typical DOF scale on a 50mm lens

DEPTH OF FOCUS.


Depth of focus is the distance through which the film plane can be moved while still recording an acceptably sharp image.

HYPERFOCAL DISTANCE.

The hyperfocal distance is the distance from the camera to the nearest point in focus when the lens is set at infinity.
This can be useful if you need to take pictures quickly with no time to focus the camera, such as at a sports event or where the subject is constantly moving. For these subjects the lens can be preset to its widest possible DoF for a given aperture, as follows:-.
First set the smallest aperture for the exposure that gives a reasonable shutter speed.
Then set the focus to infinity.
Next look the DoF scale on the lens for the aperture in use, and note the nearest point of sharp focus.
This is the hyperfocal distance.
Now refocus the lens to this distance, the zone of sharpness will then extend from half the hyperfocal distance to infinity.

As an example:-
With a 50mm lens, set the aperture to f11.
Now set the focussing scale to infinity.
Examine the DoF scale, it will indicate a DoF from about 15ft[4.5m] (the hyperfocal distance) to infinity.
Now reset the focus to15ft.
The DoF will now extend from 7.5ft[2.3m] to infinity.(1/2 the hyperfocal distance to infinity)
Therefore any part of subject within that range will be of acceptable sharpness.

COMPLEX LENSES.

Complex lenses are needed because simple
lenses have inherent faults (aberrations) and
will not form sharp images.
Lens manufacturers combine various simple
lenses to create complex lenses that allow
the image to be accurately focussed.
The cutaway drawing shows a modern lens with
eight elements consisting of a combination of
positive and negative lenses.

CHROMATIC ABERRATION.

Light rays of different wavelengths (colours) are refracted by different amounts. The longer wavelengths (red) are refracted less than the shorter wavelengths (blue & green). This causes the different colours to be focussed at different distances from the film plane creating a colour fringe effect which makes the image appear out of focus.or blurred. By using a combination of positive and negative lenses, manufacturers can eliminate chromatic aberration.
White light 'W' entering the lens is split into it's various colours which are refracted at different angles The blue 'B', rays are refracted more than the green 'G', and red 'R' rays. As all the rays cannot be in focus at any single point the image will appear blurred and exhibit colour fringing.
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