Exposure

Exposure
Exposure' we mean the amount of light that falls onto the CCD. In modern cameras the exposure is usually set to automatic by default and, most of the time, it can be left there and will produce beautiful pictures. There are times though, when the lighting conditions are difficult or we want to produce a particular effect and it would be nice to understand what is going on 'under the hood'.

The problem with all types of recording media is that they cannot record the entire range of contrast (black to white) that the eye can see. Especially when you take into account that the eye is constantly adjusting to cope with high contrast. On a sunny day if you look into the shadows of a scene then into the bright areas, the iris in your eye will quickly adjust so you can see detail in both.

Auto Exposure
The camera manufacturers have come up with all sorts of ingenious metering systems to try to help, there are now multi mode metering systems, which give you a choice of 'centre weighting', 'spot metering' or 'multi spot metering' on many of the better cameras, but none can guarantee to give you what you want every time.

The exposure is the amount of light received by the film or sensor and is determined by how wide you open the lens diaphragm (aperture) and by how long you keep the film or sensor exposed (shutterspeed). The effect an exposure has depends on the sensitivity of the film or sensor.

The exposure generated by an aperture, shutterspeed, and sensitivity combination can be represented by its exposure value "EV". Zero EV is defined by the combination of an aperture of f/1 and a shutterspeed of 1s at ISO 100 (1). Each time you halve the amount of light collected by the sensor (e.g. by doubling shutterspeed or by halving the aperture), the EV will increase by 1. For instance, 6 EV represents half the amount of light as 5 EV. High EVs will be used in bright conditions which require a low amount of light to be collected by the film or sensor to avoid overexposure.

A camera uses two things to control the amount of incoming light (exposure):
A Shutter:
In a camera, the shutter blocks all light from exposing the film UNTIL you press the button.Then it quickly opens and closes, giving the film a brief flash of light. You can control the length of time the shutter remains open by setting the SHUTTER SPEED.


A half second exposure is ONE STOP darker than a one second exposure.
A 1/125 exposure is TWO STOPS brighter than a 1/500 exposure.
A 1/1000 exposure is THREE STOPS darker than a 1/125 exposure.

An Aperture:
When light passes through a camera's lens, it must pass through an opening called an "Aperture". it's a hole that lets in more light when it's wide open and less when it's small. You can control the aperture by setting the "Aperture Opening", also known as an F-Stop.


F-Stops :
Control how much light is passed through the lens. Every step in that table represents a ONE STOP change in light. The aperture on a camera controls light. It does so by closing up to restrict light, and opening up to let it through. Lower F-Stop numbers indicate MORE LIGHT.