Starting with the Basics - Exposure triangle

Exposure determines how light or dark an image will appear after it's been captured by your camera. It is nothing more than the amount of photons available for capture by the sensor. If there are too few photons, the image doesn’t register at all (underexposed). Too many photons, and the photo will be overexposed.

Exposure is determined by 3 camera settings: Shutter Speed, Aperture and ISO; this is called the Exposure Triangle.

 

Imagine you are standing out in the rain, with a bucket. The bucket's width will be the Aperture, the amount you leave it in the rain will be the Shutter Speed, and the quantity of the rain you want to collect will be the ISO. You want just the right amount of water in the bucket. Not too much (overexposure) and not too little (underexposure).

 

 

There are a lot of ways to achieve this.

One would be to have a wide bucket, and leave it a short time in the rain. The other would be to have a narrow bucket, but leave it longer in the rain. In both situations the amount of water collected should be the same.

 

Each setting controls exposure differently.
Aperture: determines the width of the opening that admits light to the sensor. The wider the aperture (smaller the f number), the more light that can reach the sensor, making it possible to take pictures in less light.
ISO: this is the degree of sensitivity of the sensor to light.
Shutter Speed: controls the duration of the exposure (how long the shutter is open).

So you can use many combinations of the above settings to achieve a good exposure, but you will have to sacrifice something else (motion blur, noise or depth of field), because these setting also influence other image properties. As you can see in the image above, Aperture not only affects the opening in your lens, but also depth of field. ISO will make the camera sensor more sensitive, but at the expense of more image noise; while Shutter Speed affects motion blur.

Image brightness in photography is measured in stops. A stop is either a halving or a doubling of light. +1 stop is doubling the light while -1 stop halves the light.

Example: If you start with a single light bulb and then add another bulb, the light intensity will increase by one stop. To increase the light by another stop you would need to double the light for a total of 4 bulbs, and so on..

The "formula" to calculate the amount of light among stops is: 2^{number of stops}

1 stop gives 2¹ = 2x more light

2 stops give 2² = 4x more light

3 stops give 2³ = 8x more light

and so on…

Stops are interchangeable. Aperture, shutter speed, and ISO are all divided up into "stops", even though the numbering systems are different.

It sounds a little complicated, but as you read on, everything will make sense. :)