The Basics of Photography

In simple terms exposure is how much light we allow to fall onto the sensor or film in our camera.
The correct exposure will allow just enough light to capture the scene with a full range of tones & colours
withought too much shadow (under exposure) or too much highlight (over exposure).
We control exposure (how much light enters the camera) in two basic ways:

By controlling aperture - or how big a hole the light comes through
By controlling shutter speed - or how long the hole is open for

Aperture

The aperture is the hole which light enters through when the shutter is open. The bigger the hole, the more light enters.
Aperture is measured in "F-Stops" with each full stop allowing twice as much light to enter as the aperture gets bigger.
The actual F-Stop number refer to the ratio between the focal length of the lens and the diamater of the hole.

The usual progression of full F-Stops - representing halving of the amount of light at each stage is as follows...
F1.4
F1.8
F 2.8
F4
F5.6
F8
F11
F16
F22
Many cameras will offer F-Stops at intervals smaller than this - typically at a half or a third of a stop intervals.



Shutter Speed

If Aperture is "how much" then shutter speed is "how long for."
In a similar way that common aperture values represent a halving of the size of the hole - common shutter speeds represent
a halving of the amount of light allowed in. Typical shutter speeds, in 1 stop intervals, are as follows...
1"
1/2"
1/4"
1/8"
1/15"
1/30"
1/60"
1/125"
1/250"
1/500"
1/1000"
1/2000"

Combining Aperture & Shutter speed to give a balanced Exposure

There are many combinations of aperture & shutter speed that will allow the same amount of light (same exposure.)
How much will normally be calculated by the light meter inside your camera. For example - your camera may say that the correct exposure is 1/250" at F8.
As far as exposure goes the same amount of light entering the camera would be achieved if we drop the shutter speed to 1/125" and increase the aperture
to F5.6. What we have done is keep the shutter open for half as long - but made the hole (aperture) twice as big.

Why would we wish to use different combinations of aperture & shutter speed?
The answer is that different shutter speeds allow us to control how sharp the subject is - and different apertures
allow us to control Depth of Field - how much of the scene in front of and behind the focus point is sharp

Aperture and Depth of Field

Depth of field is the amount of the photograph that is in focus.
This can vary widely from a few millimetres to the whole photograph depending on the aperture selected and the focal length used.

The bigger the aperture (smaller the F-Stop number) the narrower the depth of field.
The smaller the aperture (bigger the F-Stop number) the wider the depth of field.

Because F-Stop is related to focal length - a longer focal length e.g. using a 300mm zoom, will mean a
narrower depth of field at any given aperture than if, for example, using an 18mm wide angle lens.

Shutter speed and photo sharpness

The longer the shutter is open - the more succeptable the photograph will be to camera shake & an out of focus image.
A good guide to how fast the shutter speed needs to be to obtain a sharp image of a static subject when handholding (not using a tripod)
is 1/F - i.e 1 divided by the focal length being used. For example, when using a 200mm lens a shutter speed of 1/200" is recommended.
This is just a guide and will vary depending on how solid your handholding technique is.

As you can see there are two main reasons why we need to consider various combinations of aperture & shutter speed that will give the same correct exposure.

For example:

If shooting a portrait where we wish the background to be blurred we select a wide apeture to minimise depth of field.
This is when using the "A" or Aperture Priority mode on a camera is ideal.
You select the aperture and the camera chooses a corresponding shutter speed to give a balanced exposure.

If shooting at a football match we would be more likely to select a fast shutter speed to help freeze the action.
This is when using the "S" or Shutter Priority mode (Tv on some cameras) on a camera is ideal.
You select the shutter speed and the camera chooses a corresponding aperture to give a balanced exposure.




ISO Value

ISO sensitivity is a term used to describe how sensitive to light a photographic negative is. It is also sometimes refered to as ASA.
Digital cameras use "ISO equivalent" as they do not use film - but can vary how sensitive to light their sensors are.
A higher ISO equivalent value means that the sensor is set to be more sensitive to light - the equivalent of using a "faster" film.
This means that at a given aperture the correct exposure will have a faster shutter speed.
Sounds good - but the trade-off is in reduced image quality due to digital "noise" - the equivalent to "grain" with film.

As a general rule you should only raise the ISO if you cannot get the shutter speed high enough at the aperture you wish to work
at - or conversly you cannot get a small enough aperture at the shutter speed you wish to work at. This is one major advantage of digital over film. The only way to change the ISO value of film is to load a different film.
I used to rewind films mid-roll to change to a fasteror slower film and then reload them using a special widget.
This meant making a note of how many frames had been exposed and then firing off those shots with the lens cap on to get back to the same spot on the film.
Not an exact science by any means!

With digital you can set the ISO equivalent value differently for each shot you take.
In fact some cameras have an Auto ISO setting that varoes the ISO shot to shot to give what the camera think is the optimum balance
of light sensitivity, aperture and shutter speed.


White Balance

White balance is how we control "colour temperature" with a digital camera. With film this was done by using coloured filters to either
warm up (move towards red) or cool down (move towards blue) the photograph.

With digital this can be done either in camera, or if shooting in RAW format, on the computer.

The basic idea is to render white objects as white - rather than having the colour cast associated with certain light sources.
For example most household light bulbs will give a yellow cast to your photos unless you take steps to correct it.

There are several ways to control white balance, including:
- Auto White Balance. The camera takes a guess! This can be very effective on the right camera.

- Preset White Balance. Most cameras have presets for setting such as tungsten, flourescent, sunny, cloudy, flash light sources etc.

- Manual white balance. This is where you point your camera at either a white object or a neutral grey card and store the resulting light
reading in the camera as a custom white balance setting. This must be done for each lighting situation but is very effective.

- RAW. Shooting in RAW format, if your camera has the facility, enables you to select white balance on the computer.

This gives maximun control to get that white point correct.

It is also possible to get creative by deliberatly using the wrong white balance setting. This can produce some very artistic effects.

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