Exposure.....Part 2 - Shutter Speed

Today's DSLR cameras use a two curtain focal plane shutter.

At left is an image of a two curtain focal plane shutter part way through the sequence of making an exposure. The image sensor is immediately behind the curtains.
This shutter was set to 1/500th of a second. Both curtains move from the top to the bottom. The front curtain is released first and in this image is near the bottom of it's range of travel below the black horizontal gap low in the image. The rear curtain follows close behind due to the the 1/500th of a second shutter speed.
The gap between the two curtains travels down and is spaced to expose each part of the image sensor for the required 1/500th of a second.
Once the exposure has been made both curtains get reset to the top of the frame so they are ready for the next exposure.
If the shutter had been set to 1/60th of a second the front curtain would travel all the way down before the

Exposure...Part 1 - Aperture

While you're fresh we'll tackle the math stuff first. If you skip it I can guarantee you'll be lost further on. In fact you'll probably have to go over this first part several times. Believe me, you won't be the first, nor the last to do so but it's part of the essential knowledge you need to have for making creative exposure adjustments.


Inside the lens is a mechanism called a diaphragm that adjusts the size of the lens opening (aperture), larger or smaller. Controlling that lens opening is the first step in controlling the exposure. The photograph on the left shows a 50 mm lens aperture set to f/2.8 (#1) and then adjusted to an aperture of f/16 (#2).

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: