Odd things .... this and that: Photography 101.8

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Photography 101.8 - The Light Meter

via Digital Photography School by Neil Creek on 7/14/09

Photo: Rainer Ebert - CC license

The following post is from Australian photographer Neil Creek who will soon be teaching a class in portrait photography in Melbourne Australia, and is developing his blog as a resource for the passionate photographer.

Welcome to the seventh lesson in Photography 101 - A Basic Course on the Camera. In this series, we cover all the basics of camera design and use. We talk about the 'exposure triangle': shutter speed, aperture and ISO. We talk about focus, depth of field and sharpness, as well as how lenses work, what focal lengths mean and how they put light on the sensor. We also look at the camera itself, how it works, what all the options mean and how they affect your photos.

This week's lesson is The Light Meter.

Here's What We've Covered Previously in this Series:

Lesson 1: Light and the Pinhole Camera
Lesson 2: Lenses and Focus
Lesson 3: Lenses, Light and Magnification
Lesson 4: Exposure and Stops
Lesson 5: Aperture
Lesson 6: Shutter
Lesson 7: ISO

In previous lessons we have talked about the basic theory of how a camera works, including some basic optics, and introduced the idea of exposure and how we control it with the exposure triangle. Now that we have covered each of the points of the exposure triangle, it's time to bring them all together with the tool at the core, the light meter.

What is the Light Meter?

A challenging scene to meter

For as long as people have been taking photos, there has been a need to determine how bright a scene is. Any method of recording light can only work in a relatively narrow band without over or under exposing the image. To find the correct exposure that will record the image without over or under exposing it too much, photographers need to know how bright the scene is. An extremely talented photographer may be able to guess a near-enough exposure, but a light meter is a far more accurate and convenient way to do it.

Light meters in cameras react to how intense the light is as seen from the camera. SLRs measure the light (called metering) through the lens - TTL. They collect light that has actually passed through the camera's lens and measure its intensity. There are problems when the scene has parts that are much brighter or darker than others, for example shadows on a sunny day. This can trick the light meter into measuring the intensity of the light incorrectly, depending on which part of the scene was illuminating the sensor.

Modern SLR cameras use multi-point light meters, meaning that several light meters are actually scattered around the projected scene, each measuring the light intensity at that point. Very sophistocated cameras may have dozens of metering points. How much the measured intensity of the light at each point influences the final meter reading depends on the metering mode selected by the photographer.

For a more detailed look at metering modes, you can read: Introduction to metering modes.

How to Use the Light Meter

As we now know, the correct exposure is created by juggling the three points of the exposure triangle: aperture, shutter and ISO. The light meter is the tool that puts us in the right neighbourhood for how these should be set. If you are shooting on full auto, then when you meter the scene - usually done at the same time as focusing, by half pressing the shutter - the light meter gives its best guess for each of these variables.

If you want to take creative control of the photo, you can manually set each of the three variables yourself. Typically ISO is left at the default, or previous setting, and you take control by choosing aperture priority or shutter priority. On most DSLRs that's done by turning the exposure mode dial. If you set the dial to Av - aperture priority, the photographer chooses what the aperture will be, and the light meter adjusts the shutter speed to mantain the correct exposure. The reverse is true for Tv - shutter priority.

When using these modes, it's useful to refer to the exposure meter display on the camera. The exposure meter (display) shows the result of the measurement taken by the light meter (sensor). It will typically look something like this:

Exposure meter display on LCD

Exposure meter display in viewfinder

Each number represents a stop change in the light, as indicated, with the central mark being the "correct" exposure, as determined by the light meter. Each pip between the numbers represents one third of a stop. The arrow underneath indicates how close the current settings are to the correct exposure. Usually in priority modes, the arrow will stay in the middle as the light meter will be able to set the exposure correctly. However, if for example you set your aperture to 1/400sec in Tv (shutter priority mode) and the light meter indicated that you needed an aperture of f4, but your lens was only capable of f5.8, then the exposure meter will display one stop of underexposure. You will need to compensate for this by setting a longer shutter time, or increasing the ISO.

The juggling act becomes more complicated, and the light meter's assistance more valuable, when you go to full manual control of the exposure. Here the exposure meter simply displays whether the current settings will under or over expose the image, according to the light meter. The photographer can freely change any of the values on the exposure triangle, and see the change to the predicted versus recommended exposure.

Exposure compensation

Even though the light meter in your camera is pretty sophistocated, sometimes it can get it wrong, especially with harsh contrasts, or highly reflective surfaces. Changing metering modes may help this, but a more controlled approach is to use exposure compensation. Imagine you are photographing a person against a large bright sky. The light meter thinks the sky is the most important part, and exposes correctly for that, leaving the person a dark silhouette. By using exposure compensation, you can tell the camera to take the metered exposure and make it brighter by a chosen amount. This will then allow the photographer to correctly expose the person. I'll look at exposure compensation in more detail in a future post.

Examples

To show you how the different exposure modes might work in real world situations, here are some scenarios. The settings given below are what they happened to be for the examples shown. Settings for your own photo will be different.

Scanario 1 - Sports

High speed is needed to freeze action
Use Shutter Priority
Set shutter speed to 1/800sec
The light meter sets the aperture to f10
If under exposed, change ISO to compensate - ISO400

Scanario 2 - Portrait

An artistic narrow depth of field is desired
Use Aperture Priority
Set aperture to f5.6
The light meter sets the shutter to 1/160sec
If under exposed, change ISO to compensate - ISO100

Scenario 3 - Night scenery

Ambient light is too low to accurately meter
Use full Manual
Set aperture to suit scene, erring to wider - f11
Set a long shutter speed to light meter's best guess - 20sec
Set ISO to lowest possible for correct exposure - ISO100
Take a test shot and adjust settings if the light meter got it wrong

Scenario 4 - Off-camera manual flash

On auto, meter the scene and note settings
Set camera to one or two stops under exposed
Set up flashes and tweak power to expose correctly
Tweak the flashes exposure by adjusting aperture
Tweak the ambient light by adjusting shutter speed
Settings for example shot: 1/160sec f8 ISO125, click image for flash details.

Homework

Put the camera in auto mode and half press the shutter. While looking through the viewfinder, pan around a scene and see how the automatically selected camera settings - f ratio and shutter speed - change. This preview will disappear after a few seconds, so half press the shutter again for another look.
Set the camera in shutter priority mode and choose a shutter speed for effect, eg: short for sports, long for motion blur. Shoot different scenes and note how the camera adjusts the aperture to balance the exposure.
Do the same as above for aperture - wide for shallow depth of field, narrow for focus detail at all distances.
Get adventurous and put the camera on full manual. Adjust the camera settings yourself, and watch the arrow below the exposure meter. Tweak the settings to get the arrow in the middle of the meter - half press the shutter while looking at your scene to take a meter reading.
Try to apply what you have learned to make creative photos that take advantage of the different exposure modes.