The In-Camera Light Meter

Since we rely so heavily on the in-camera light meter it's very helpful to have a good understanding of how it works and how you can configure it so it provides you the most reliable information.

Reflected Light and Algorithms, oh my!

Your in-camera meter measures how much light is reflecting off the scene it is metering. If everything reflected the same amount of light we would be home free. Obviously, a nice polished piece of chrome on a motorcycle will reflect more light than the skin of a pretty girl sitting on the bike.

Since the amount of light being reflected from different parts of the scene varies, in-camera meters measure the light intensity in different parts of image and uses an algorithm to determine the most suitable exposure for the final picture. We get to tell the meter which algorithm to use. Well, the camera manufacturer gives us the chance to select from several metering modes that each represent a different algorithm.

Using The Exposure Adjustment Triad

Exposure is the total amount of light allowed to reach the image sensor during the process of taking a photograph. We've seen there are 3 things that we can adjusted to control that exposure: the lens aperture, the shutter speed and the ISO sensitivity.

The DSLR Reflective Light Meter

Your DSLR has a built-in reflective light meter. It tells you if your current settings will make a good exposure or an over/under exposure.

Exposure...Part 3 - ISO

ISO - Sensor Sensitivity to Light ISO stands for the "International Organisation for Standardization". The ISO was founded in early 1947 and is headquartered in Geneva, Switzerland and is composed of representatives from various of it's 158 members national standards organisations.

One of the standards defined by this organization is photographic film's sensitivity to light. An image sensors 'film speed', is often referred to as its "ISO number."

ISO - and Stops of Exposure

Just like aperture and shutter speed ISO is set up as multiples of 2. ISO 100 allows twice as much light as

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: