The light sensitivity of your camera is measured in ISO units, 100 to 400 is a typical
number. Setting a higher number makes your camera more sensitive to light. The scale
is linear so a setting of 400 ISO makes the camera twice as sensitive to light than
200 ISO. You used to find this number on the box of photographic film and, for those
who remember that, the number has exactly the same meaning. If you can remember even
further back to ASA numbers, they meant the same thing too.
Before the light reaches the inside of your camera, it has to pass through a lens.
This does two things. It bends the light from a wide area in front of the camera
to the tiny space inside. It also focuses the light so the picture looks sharp. A
lens always has to focus at a specific distance but it will also be in focus in front
and behind this point. Just how far in front and behind depends on various camera
settings and is called the depth-of-field. A large depth of field means that both
near and far objects in your picture are in focus. A narrow depth of field means
that only your subject is in focus and everything else in the image is soft.
The amount of bending is governed by the focal length of the lens. This is a measurement
in millimetres and a short focal length bends the light more than a long one. In
practice, this changes the angle or field of view of the camera. A zoom lens is one
that allows you to change the focal length of your lens and with it, the angle of
view. You can zoom out to a wide angle (short focal length) or zoom in to a narrow
angle (long focal length). The zoom ratio or zoom range is a number that simply tells
you how big the difference is between the wide and narrow end of your zoom lens.
Inside the lens is an iris or aperture. This is simply a hole that can be made bigger
or smaller to let in more or less light. The size of the hole is given in f-numbers.
The only complication being that a smaller f-no. Means a larger hole (therefore more
light getting through the lens).
Once through the lens, the light reaches a shutter, which stops it going any further.
At least that’s what used to happen with film cameras. With digital cameras, only
Dslr types have this arrangement. All other digital cameras emulate the effect of
a shutter electronically. This doesn’t really matter because both types of shutter
perform exactly the same function in a camera with one exception - shutter lag.
Electronic shutters have a slight delay between the button being pressed and the
picture actually being taken. The length of the delay can go from very short to surprisingly
long, depending on your camera. And depending on the type of photography you do,
this will either go unnoticed or be very annoying.
The function of a shutter is to open just long enough for the correct amount of light
to be recorded, this is the shutter speed and will normally be only a fraction of
a second. Together with the ISO number and aperture, this is how you or your camera
set the exposure. When your camera is set to auto-exposure then every time you take
a picture, it will take a light reading and set the ISO, aperture and shutter speed
to give you the correct exposure. Your camera will always adjust at least one of
these parameters for you unless you set it to fully-manual. That is the only setting
that allows you independently adjust all these three camera exposure controls.
In olden times, the aperture was adjusted by a ring around the lens. The ring mechanism
had notches that would allow it to stop at fixed positions. Each stop would let twice
as much light in as the next smaller stop. By moving one stop up or down you either
halved or doubled the amount of light reaching the film, depending on which direction
you went. Although this only applied to apertures, the concept of the stop applies
to everything to do with light and exposure in photography. A stop is a measurement
of the difference between two states in terms of the ultimate effect that it will
have on the exposure of your photograph.
Your photograph can either be too dark (under-exposed), too light (over-exposed)
or just right (correctly exposed). The actual amount of over or under exposure can
be expressed as plus or minus a number of stops. If your scene were lit by a single
light bulb, then adding a second one of the same type next to is would double the
amount of light - an increase of one stop. You (or your auto exposure system) could
compensate for this by adjusting either the ISO, aperture or shutter speed - by one
stop. Halving the ISO number or length of time the shutter is open or increasing
the f-no by one stop would have this effect.
This focused and measured amount of light lands onto an image sensor in your camera,
This is where things start to get digital. The image sensor sits in the same place
and performs the same function as film would, but being digital, it records the image
as a series of digits (numbers). It does this in two ways. First of all, the light
sensitive surface of the image sensor is covered in millions of individual elements
in a rectangular array. These picture elements are called pixels and a million of
them is called a megapixel (Mp). The number of pixels defines the size of your digital
image and, as there are usually millions, image size is normally given in megapixels.
Each individual element records the amount and colour of the light landing on it
and converts it into a number. When you display a digital image, the software you
use to do this reads these numbers and converts them back into their original level
and colour. That’s basically how digital photography works. If you can imagine painting
by numbers where all the shapes you have to fill in are square and extremely small,
you will have got the idea. If you zoom in to any digital image far enough you can
see these individual pixels.
Once the picture is recorded, it is then stored as a computer or digital image file.
All digital files have a format so that other programs can recognise what they are
and display them properly. The three most commonly used formats in digital cameras
(in reverse order) are TIFF, RAW and JPEG. Tiff files are the least common because,
although they are the highest quality, they are also very very big. Raw files are
only an option on top end cameras and they can only be read by specialised software.
However, this usually comes with the camera. They are actually better and smaller
than Tiff files but, the reason most people are put off using them is that they are
still much bigger than the other image file format, which is JPEG. This is by far
the most popular format. Most digital cameras only store pictures in this format.
If your camera manual doesn’t make any mention of file formats at all, that means
your pictures are stored as JPEG's.
It is these individual JPEG (or other) digital image files that you transfer, view,
edit, print or do anything else you wish to them.
There are more jargon terms that crop up from time to time . The ones mentioned here
are the basic ones that you will read or hear over and over again as you explore
deeper into the world of digital photography.