Full Steam Ahead ......... ---- by Angie Sharp
Anything Nautical ... . ... .. . . by Sheila Brown
Doubletake .. -. . . . . . . . . . . by Nigel Hargreaves
Visionary Experiment --------- by Marie Stuart
The Boat Trip --------............ by Anonymous Dude
A Basic Guide .... . . . . . ...... by George Inglis
Slop Bog visit = = = = - --- - by Mike Legg
2 Boys & a Red Arrow- --- -- by-Mike Legg
Canada Rail Trip -------------- by Ruth Dix
Photo's for fun & sharing------- Submitted by Members
A Basic
Guide to the Mechanics of Photography
Part 1 – Exposure
Intro
This article is intended to be the first part in a small series describing
simply (hopefully) the basic mechanics of photography. In the first article,
I’d like to explain a little about ‘Exposure’ what it means
and what affects exposure.
What is
meant by Exposure?
Exposure is a term that is used to describe the amount of light that reaches
a recording medium, light sensitive film, light sensitive paper or an electronic
light sensor.
To obtain the correct exposure of a photograph there are three elements involved, the aperture of the lens, the shutter speed and the film speed (electronic sensitivity). We will look more closely at each of these elements in turn.
Each of these elements adjust the level of light in increments, called stops. ‘Going up’ one stop doubles the amount of light reaching the recording media and ‘Stopping down’ by one stop halves the amount of light.
When the correct exposure is selected, there should be discernable
detail in the whole image. Strong light or very low light could make this
very difficult, so development techniques, or programs like Photoshop (for
digital) can be used to improve the exposure.
Aperture
The aperture is the ratio between the focal length of the lens to the diameter
of the lens open to the light e.g. a 50mm lens allowing a beam of light of
diameter 12.5mm would be f:4 . The ‘f:’ is used to designate this
ratio. The following diagram shows the main aperture settings, the difference
between each one is classed as 1 stop. Moving a stop, say from f:4 to f:2.8,
allows twice the amount of light into the camera. Moving the other way halves
the amount of light entering the camera. Modern 35mm cameras often have aperture
settings in half stops as well.
The aperture setting controls the depth of field on a photograph.
(see later articles)
Changing the aperture is achieved by a series of leaves that cover more of
the lens as the aperture narrows.
Shutter
Speed
The next element that controls the exposure is the shutter speed. The following
series show the main settings on a camera. Each value represents 1 stop, a
'' following the number denotes whole seconds, numbers on their own are fractions
of a second, e.g. 30 = 1/30th sec, 500 = 1/500th sec.
Typical shutter speeds:
2”, 1”, 2, 4, 8, 15, 30, 60, 125, 250, 500, 1000……
Many cameras have shutter speeds outside these values and may include half-stop speeds. There is usually another setting, ‘B’ (bulb), this keeps the shutter open while the shutter release button is depressed.
Modern cameras use a focal plane shutter, this involves two blinds that travel across in front of the film, the width of the gap between the two blinds determines the length of time the film is exposed. This type of shutter can provide speeds up to 1/4000th sec.
Film speed
Film speed is another characteristic that defines the amount of light that
creates the photograph. The International Standards Organisation (ISO) have
defined a scale that determines the speed at which film reacts to light. The
American Standards Association (ASA) and the German standards authority (DIN)
also produce a scale but these measures are not commonly used nowadays.
• Slow film speeds used in very good lighting conditions,
giving greater definition to colours and sharper images
o ISO 50 – used for studio photography
o ISO 100 – general photography
o ISO 125 – black & white film
• Medium speed film used for general photography.
o ISO 200
o ISO 400 – tends to be used if using longer focal lengths
• Fast film speeds, more light sensitive and used in low light conditions
when flash use is not appropriate. From ISO 400 upwards the photographs produced
tend to be progressively grainy (or noisy with digital).
o ISO 800
o ISO 1600
o ISO 3200
Graininess was associated with the size of light sensitive
crystals, faster films used larger, faster reacting crystals than slower films.
Digital Sensor light sensitivity uses the same scale, usually ISO 100 –1600
or even greater. However, with digital sensors, the light signal is electronically
amplified, generating increasing amounts of noise due to the increasing sensitivity
of the sensor.
I hope this article is useful, feedback would be appreciated, also possible topics for future articles.
George Inglis