Exposure

The exposure is a combination of the length of time and the level of illumination received by the sensor. Exposure time is controlled by shutter speed, and the illumination level by the lens aperture at a given ISO sensitivity.

These are the three main ingredients to expose a photograph: ISO sensitivity, shutter speed and lens aperture. The ISO sensitivity is a numeric indication of the sensitivity to light. A higher ISO sensitivity number indicates a higher sensitivity to light. The shutter speed is the length of time a shutter is open; the total exposure is proportional to the duration of light reaching the sensor. It is expressed in seconds(1", 2") or fractions of second (1/2, 1/80, 1/500, 1/2000) There are three factors that affect the shutter speed: the scene luminance, the ISO sensitivity and the aperture size.

A scene can be exposed in many ways, it all depends on the photographer's goals. There is no such thing as a "correct" exposure.

Under exposed 2 steps	Under exposed 1 step	Exposed correctly	Over exposed 1 step	Over exposed 2 steps
ISO 400 60mm f/14 1/1600	ISO 400 60mm f/13 1/1250	ISO 400 60mm f/10 1/800	ISO 400 60mm f/9 1/640	ISO 400 60mm f/7.1 1/400

Having total control of all the variables in a exposure is what the Manual mode is all about. The camera will expose using averages. Although more advanced cameras have better exposure and metering systems they are limited and difficult scenes won't be handled the way you want it. A dawn is a difficult scene to meter because the sun may be extremely bright and the surroundings extremely dark. Digital cameras are not very good at such contrasty scenes. Controlling each parameter will provide you with the exposure you need but also with all the parameters tailored to your taste.

ISO 100 35mm f/4 1/1250	ISO 80 35mm f/4 1/640	ISO 80 35mm f/4 1/400	ISO 80 35mm f/4 1/160	ISO 80 35mm f/3.5 1/160

Having a total control of the exposure is also needed if your camera tends to under expose or over expose.

Exposure: combining ISO, shutter speed and aperture

The following diagram shows the interaction between ISO, shutter speed and aperture in three different kinds of light for a given scene.

	Good light	Medium light	Low light
	Compact Cameras dSLR Cameras ISO 80-100 ISO 50-100	Compact Cameras dSLR Cameras ISO 80-200 ISO 50-200	Compact Cameras dSLR Cameras ISO 400-800 ISO 1600-3200
	Since there is good light, it is possible to use a reduced aperture (high f number).	The aperture may be medium. The main subject will be sharp but near and far subjects may be blurred.	The aperture may be wide open but even the main subject may be slightly blurred depending on the lens.
	Compact Cameras dSLR Cameras Up to 1/2000sec Up to 1/8000sec With good light the fast speeds will freeze moving subjects.		In the dark moving subjects will look blurred. This may be an intentional effect e.g. to create trails in the lights of cars. In the dark the shutter speeds may be very long. In really low light situations it may be necessary a 1 sec shutter time or even more (the BULB setting allow an indeterminate duration of the shutter speed). In such conditions a tripod is mandatory

Exposure Compensation
When you use the Manual mode you are in command of the exposure. In semi-automatic modes P, Av and Tv you choose only some of the parameters and the camera sets everything else. In such cases you can still have control of the exposure using the Exposure Compensation. In example, you want to shoot everything automatically but the scene has to be slightly under exposed, in such case you should choose the P mode and use the exposure compensation to under expose the scene. The same applies to Tv and Av settings. In fully automatic cameras (such as the Digital Elph line) you can only use the exposure compensation in the so-called "Manual" mode that in reality is pretty much the same as the P mode.

Bracketing

Once you've determined the exposure, it is possible to let the camera shoot three photographs: one under exposed, other with the exposure that you selected and a third over exposed. In this "bracketing mode" you can decide to over expose or under expose up to 2 stops. Finally, you can bracket the exposure in one third of stops increment.

These may be an example of bracketing one stop.

ISO 80 35mm f/4 1/640	ISO 80 35mm f/4 1/400	ISO 80 35mm f/4 1/160