The word "PHOTOGRAPHY" is derived from Greek. "Photo" means "Light" & "Graphy" means "Draw", together it would mean "Drawing" with "Light" or you can say "Painting" with "Light".

What is Exposure?
It's the amount of light reaching the Film or for the case of digital cameras the Charge Coupler Device (CCD). The exposure is controlled basically by 3 factors, namely:

1. Aperture
2. Shutter Speed
3. ISO/ASA speed selection

Aperture
The aperture settings on the lens which controls the amount of light passing through the lens reaching the Film/CCD. To increase or decrease the amount of light passing through the lens, you have to change the size of the aperture -- the lens opening. This is the job of the iris diaphragm, a series of overlapping metal plates that can fold in on each other or expand out.

Essentially, this mechanism works the same way as the iris in your eye -- it opens or closes in a circle, to shrink or expand the diameter of the lens. When the lens is smaller, it captures less light, and when it is larger, it captures more light.

 

The plates in the iris diaphragm fold in on each other to shrink the aperture and expand out to make it wider.

Shutter Speed
The shutter speed which controls the duration of the light passing through.

The length of exposure is determined by the shutter speed. Most SLR cameras use a focal plane shutter. This mechanism is very simple -- it basically consists of two "curtains" between the lens and the film. Before you take a picture, the first curtain is closed, so the film won't be exposed to light. When you take the picture, this curtain slides open. After a certain amount of time, the second curtain slides in from the other side, to stop the exposure.

ISO/ASA
The ISO/ASA Film speed settings which controls the camera's sensitivity to light.

The same exposure can be achieved using different combinations of 1)Aperture, 2)Shutter Speed 3)Film Speeds. However there'll be a difference in the results besides exposure.

Aperture - by varying the Aperture, you can control the Depth-of-field of the image. Depth-of-field determines the amount of sharpness in an image. An image with a shallow depth-of-field by using a large aperture setting (eg. F2.8), will have only the main subject of even part of the main subject in focus. This will directly lead the eyes of the audience to the main area of focus. There are times which you want sharpness in most part of the image (as in a group photo), you need to use a smaller aperture (eg. F11).

Shutter Speed - by varying the shutter speed you can either "freeze" the motion of a moving object using a higher shutter speed (eg. 1/500seconds) or emphasize the speed /motion by recording the object in streaks of blurred lines using a slow shutter speeds (eg. 1/15seconds).

Film speeds - by using different film speeds you control the camera's sensitivity to light. The higher the ISO/ASA the higher the sensitivity and the brighter the resulting image. However on the other hand you lose some degree of sharpness as well as getting a more grainy image.

Common mistakes/problems

a) Shooting against a bright background (eg. windows ), the , the usual results would be the main subject appearing dark. To avoid this, you need to use flash to fill-in the face of the subject. If you do not have a flash then avoid the bright background altogether.

b) Blurred images, there are 3 possible reasons for this. It could be due to

(i) camera shake - movement during the exposure, usually during longer exposure, especially when you are capturing night scenes. You can avoid camera shake by using a tripod.

(ii) focusing the wrong subject - this is very common when shooting 2 persons, the camera tends to focus on the background instead of the 2 persons' faces. This is because most cameras have the focusing point in the middle of the lens. You can avoid this by focusing on either one of the 2 persons by pressing the shutter release halfway to focus and then re-framing the shot to take the shot. Usually there's an light indicator inside the viewfinder to tell you that the subject is in focus.

(iii) the shutter speed is too slow to capture and freeze the subject. You could either select a higher shutter speed or use a flash to freeze the action.