APERTURE.(f stop)
The aperture of a lens is the 'hole' through which the light passes.
The size of the aperture can be varied to control the amount of light that passes
through the lens.
Apertures are measured in f numbers known as 'f stops', (e.g. f,3.5, f,4.6, f,8, f,11,
f,16 etc) each 'stop' being 1/2 the area of the previous one. (Note the smaller the
'hole' the larger the f No.).
The f No is derived from the focal length divided by the diameter of the aperture,
(e.g. 55mm focal length/5mm diameter = f,11), this means that lenses of any focal
length will pass the same amount of light for a given f No. thus making exposure
calculation simpler.
DEPTH OF FIELD.
Another use for the aperture control is that it allows the photographer to select which
parts of the subject are in focus.
Only a certain part of the image will be in focus (depth of field), areas in front and
behind the focal point will be less sharp the further they are from the focal point.
FOCAL LENGTH.
The focal length of a camera lens is the distance from the film plane of the camera
to the optical center of the lens when the lens is focused on infinity.
FOCAL PLANE.
The focal plane is the plane at the point where the image is focussed and is where the film
plane is positioned
FORMAT.
Format refers to the negative/film size. Various formats are in use:
medium format-6x6cm, 35mm-35x24mm, 120-21/4x31/4, APS is
a relatively new format.
110, 127, Kodak disc film are examples of some obsolete formats.
LENSES.
What are lenses?
Lenses are discs of transparent glass or plastic with concave or convex surfaces
which can bend (refract) light.
There are two basic lens types:
CONVEX (POSITIVE) LENSES.
Converge (focus) the light rays and can form an image on a surface.
CONCAVE (NEGATIVE) LENSES.
Diverge (spread out) the light rays and will not form an image.
COMPLEX LENSES.
Complex lenses are needed because simple lenses will not form sharp images.
Lens manufacturers combine various simple lenses to create
complex lenses that allow the image to be accurately focussed.
NORMAL LENS.
The normal lens, so called because it provides a natural angle of view
similar to that of the eye, and is approximately equal to the diagonal of the negative.
With the 35mm camera it is around 50mm focal length.
TELEPHOTO LENS.
Telephoto (long focus) lenses havea restricted field of view and magnify parts of the
subject in a manner similar to a telescope.
WIDE ANGLE LENS.
A wide angle lens covers a wider field of view a normal lens and therefore records
much more of the subject. All parts of the image will be smaller on the negative.
ZOOM LENS.
A zoom lens is a complex lens in which the focal length can be varieed over a
preset range.
Common zooms are 28-80mm, 70-300mm & 28-300mm.
Zoom lenses of 1000mm and more are available.
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