Exposure principles

This could be the most important thing you have to understand if you are serious about photography. I have been lucky to have a father who explained this to me in simple terms when I was a kid. In those good old times, it was simple, you HAD to understand it because there was no other way, no automatic mode for metering, no autofocus, nothing. 

First of all, back to the principles: photography is about light, optics and a device collecting the light for further processing (i.e. film to be processed chemically or a sensor in a DSLR with information to be processed electronically) to produce pictures...

Light can be represented as a wave or as small energy particles called photons. Photography is about collecting the light, or energy particles on a "receiver" being a film or a sensor in a digital camera. To get the proper result you need the correct amount of light (or energy) on the sensor.

The amount of light arriving on the sensor is influenced by the following elements:

- the shutter speed or better the shutter opening time, the longer the shutter time, the higher the amount of light on the sensor
[The amount of light is proportional to the shutter opening time, or proportional to the inverse of the shutter speed]

- the lens aperture: the bigger the aperture, the more light comes in. Large apertures correspond to small f numbers, for instance f1.8, small apertures correspond to large numbers (f16 for instance)

A correct exposure on a digital sensor corresponds to a quantity of light that will make use of the whole dynamic range of the sensor. The pixels are encoded as numbers, one for each color. If a pixel element is encoded on 8 bits, it means it can take values from 0 (black) to 255 (full intensity).

A picture will be underexposed if all values are smaller than 255 (you don't make use of all the possibilities). You can correct this by allowing more light to reach the sensor or by amplifying the signal (increase the values). A picture is overexposed if too many pixels are above 255, meaning that you would have needed larger values. In that case you need to decrease the amount of light the sensor is receiving.

What are the ways you can influence the amount of light:

- as already said: the amount of light is proportional to the shutter open time. This means that a shutter speed of 1/200sec will give you half the light of a shutter speed of 1/100sec.

- another way is to play with aperture. The amount of light is proportional to the square of the inverse of the f-number (to be explained later). This means for instance that f2 will give you 4 times more light than f4, or f2 half the light of f1.4.

- another parameter you can play with is the ISO value. This is a translation (in the digital world) of the amplification factor applied to the picture. The ISO value is proportional to that amplification factor, i.e., ISO400 gives you twice as much as ISO200. The drawback is some noise in the picture: the higher the ISO value, the more visible the noise pattern becomes in your pictures.

A few links with more information: