Exposure.....An Introduction

Most people put their camera in Auto mode and are content with the results. Some people would like to have better looking images. One of the ways to accomplish that is by selecting some or all of the exposure parameters that the camera has available.

The exposure of an image is controlled by three parameters.

• The size of the lens opening.
• The length of time the shutter is open.
• The sensitivity the image sensor is set to, known as ISO

In the first two installments we saw that light is first captured by the lens, controlled by the lens opening or aperture, then the shutter controls how long the light falls on the image sensor.

The Basics of the Lens

A DSLR camera lens is not very different from a refracting telescope or even a microscope.We need the lens to gather light and focus it while transmitting the light to the image sensor.

Here is an example of a simple lens:
It gathers light to a focus. Sort of. There are some inherent problems with a simple lens.Lets first talk about Chromatic aberration.


Because the lens is thicker in the middle than at the edges all colors of light do not focus in the same place.

There are 2 other undesirable properties of a simple lens:

How Digital SLR Cameras Work (basic version)

First lets get SLR defined: it stands for Single Lens Reflex.

For the photographer it means when you look through the viewfinder you are actually looking through the lens you have mounted to your camera body. Who hasn't looked into the viewfinder and discovered pure black....because the lens cap was still on. The main advantage of a SLR camera is you see pretty much what the image sensor will see when you trip the shutter.

Here's how the image gets from the lens up to the viewfinder: