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Understanding Exposure

Understanding Exposure

by Andy Stansfield

An invaluable guide to taking better photographs using today's sophisticated digital SLR and compact digital cameras

Aimed at both the novice and more experienced amateur photographer, jargon-free text explains the theory behind digital photography, how light metering affects exposure, and light's relationship to color, color temperature and white


An invaluable guide to taking better photographs using today's sophisticated digital SLR and compact digital cameras

Aimed at both the novice and more experienced amateur photographer, jargon-free text explains the theory behind digital photography, how light metering affects exposure, and light's relationship to color, color temperature and white balance, focal points, and the expression of mood. Aperture, depth of field, and shutter speed are also thoroughly covered, along with chapters on ISO speeds, dynamic range, use of filters, and making in-camera adjustments. Post processing techniques round off this invaluable guide to getting the best results from your photography.

Product Details

Ammonite Press
Publication date:
Expanded Guides - Techniques Series
Product dimensions:
5.70(w) x 6.90(h) x 0.60(d)

Read an Excerpt

Understanding Exposure

The Expanded Guide

By Andy Stansfield, Chris Gatcum

AE Publications Ltd

Copyright © 2010 AE Publications Ltd
All rights reserved.
ISBN: 978-1-906672-99-7




Definition: The camera and lens settings that determine the amount and qualities of light falling on the medium used to record the image. Traditionally shutter speed, aperture, and ISO with film, but the digital age expands this list to include in-camera adjustments that determine how the exposure data is interpreted: color space, white balance, dynamic range, sharpness, noise, filter effects, and more.

Perhaps the first issue to clarify in this book is the difference between the terms "exposure" and "metering." Exposure is a term that describes the actual settings used to capture an image, whereas metering provides only a suggested group of settings that you are free to ignore, override, or adjust as you see fit. It is vital to remember that in-camera metering systems, which many of us rely on quite extensively, may have grown extremely sophisticated, but they are not absolutely perfect for every situation.

The second important point relates to which file format you use to record your images, provided your camera offers both RAW and JPEG options. If you want to print images directly from your memory card, for example, you will need to record images using the JPEG format and you will need to have exposed them with a fair degree of accuracy. However, if you are comfortable with adjusting images on your computer before printing (generally referred to as post-processing), you have a little more leeway with the exposure. This is especially true if you are recording your images as RAW files, as these allow you to adjust almost all of the original camera settings during post-processing, including the exposure.

The three main settings at the heart of exposure are aperture, shutter speed, and ISO. Virtually every camera offers at least one fully automatic mode in which the camera sets all three of these for you. Most will also provide a number of fully or largely automatic modes often referred to as "scene modes" that will also determine the aperture, shutter speed, and ISO automatically, but based on the subject matter of the image or the type of circumstances in which the photograph will be captured. Before we examine their relationship in greater detail, let's first look at these three settings individually.


The word aperture means an opening through which something passes, and in this context we are talking about an opening within a camera's lens through which light passes (the mechanics of which are described more fully in Chapter 3).

The light waves project an image onto the film or sensor, with the size of the aperture controlling the amount of light that is allowed through. This obviously has an impact on the brightness of the resulting image.

If you peer into any lens from a camera that takes interchangeable lenses you will clearly see this opening, formed in the center of a number of overlapping blades. The shape of this can be (but usually isn't) a perfect circle.

When we talk about the aperture used for a particular photograph, we are really talking about the size of the opening and, in order to be precise, we refer to it numerically, as a fraction of the focal length. The number used is preceded by a lower-case letter f, sometimes italicized and followed by a slash; for example, f/8. The actual range of numbers used will be explained shortly.

Shutter speed

The shutter speed refers to the amount of time that the light waves passing through the aperture are allowed to fall on the recording medium, be it film or the sensor in a digital camera. There are several types of shutter mechanism (descriptions of which would cloud the issue at this point), so at this stage it is only necessary to appreciate that the shutter offers another mechanism for controlling light, in this case the duration that light waves are allowed to form an image on the recording medium. This is always expressed as whole minutes, whole seconds, or a fraction of a second (1/500 sec., for example).

ISO setting

The ISO setting refers to the sensitivity to light of the recording medium and is expressed numerically; ISO 200, for example. One disadvantage of film is that a constant ISO has to be applied to every frame on the roll of film, whereas with digital cameras the ISO setting can be changed for individual photographs. The acronym ISO stands for International Organization for Standardization, the body responsible for setting regulations that all manufacturers must adhere to.


A stop is the term used by photographers to describe the doubling or halving of the light received by the recording medium. This means that the term can be used in conjunction with any of the three main settings concerning light: aperture, shutter speed, or ISO. Usually we refer to 1 stop faster, meaning a halving of the light received by the recording medium, or 1 stop slower, meaning twice as much light being received. (It is important to understand that the terms faster and slower in this context do not relate only to shutter speed.)


The easiest analogy to draw when it comes to the relationship between the aperture and shutter speed is that of cooking a meal in the oven: You can opt for two hours at 150° or one hour at 300° and the outcome will be largely the same. Think of these as being the shutter speed and aperture. Continuing this analogy, you have a third factor that can also affect the outcome — the height of the oven shelf. This equates to the ISO speed. OK, so my cooking skills fall a little short of any photographic expertise I may possess, but you get the idea: the key to exposure is getting the correct balance between these three variables.

A simple way of witnessing the relationship between the shutter speed and aperture is to set your camera to Program mode. If you shift the exposure, you will see that the camera scrolls through a variety of different combinations, each of which will give the same overall result at the selected ISO speed.

Camera modes

The technological rollercoaster that has hurtled through camera development at breakneck speed throughout the last decade should have made life easier. It did, for a while, but even inexpensive compact digital cameras now offer a huge range of settings options. There is much to explore, to learn, and to experiment with these days, but most of the technology can be broken down into several topics that are outlined here and explained in greater detail later in this guide.

Primary and secondary exposure

The terms primary and secondary exposure are not standard terminology, but they have been used here to separate the adjustments made in-camera at the time of shooting from post-capture corrections.

Primary exposure settings are the ones that you will use in-camera when you capture an image. They incorporate the main settings of ISO, aperture, shutter speed, exposure compensation, exposure bracketing, Highlight Tone Priority (Canon), and Active D-Lighting (Nikon) or Auto Lighting Optimizer (Canon), or similar settings from other manufacturers. They can also include anything else that affects the exposure, the dynamic range, or the color balance.

It is worth noting that after dialing in any adjustment to the dynamic range through settings that affect the balance between highlight and shadow areas (such as Active D-Lighting or Auto Lighting Optimizer), the aperture and shutter speed combination will sometimes need further adjustment before capturing an image.

Secondary exposure correction describes any exposure corrections made to an image after it has been captured. Essentially, secondary exposure corrections can take place either in-camera (a facility that is slowly becoming more widespread) or on the computer after downloading your captured images. At the present time, the vast majority of photographers are still limited to making secondary exposure corrections via a computer.

Post-processing is a term commonly used to describe post-capture adjustments, which may incorporate exposure adjustment in addition to other changes to an image that do not fall into the category of exposure; the post-capture application of digital filters, for example. The most important factor determining which secondary exposure corrections are available is whether you shoot JPEG or RAW files, with RAW files providing a greater range of options.

Full Auto

Full Auto — or Intelligent Auto on Panasonic cameras — is probably the shooting mode that is most commonly used by beginners, and it is likely to be one of several automatic modes on your camera. The term Fu//Auto has been used to differentiate this mode from other partially automatic modes such as Program, Aperture Priority, Shutter Priority, or any one of the various scene modes.

Set to Full Auto (commonly marked as A), the camera will make all your decisions for you with regard to the aperture, shutter speed, and ISO. It is also likely to set the file quality (to JPEG) and a range of other options — almost everything except focus. It is usually the case that the user cannot override many, if any, of these settings, but Full Auto is still capable of delivering perfect images provided there aren't any awkward lighting challenges: the main thing you are giving up is control, not quality. Well-lit, evenly toned scenes should turn out fine. When they don't, it is time to start learning again.


Program mode offers a little more control than Full Auto and is often the next step on the learning curve for beginners. In this mode, the camera's meter determines the exposure and suggests a combination of aperture and shutter speed. This pairing can be changed using what is sometimes referred to as program shift, but the overall exposure will remain the same. For example, if the camera suggests an exposure of 1/500 sec. at f/8 this can be changed to any equivalent exposure such as 1/1000 sec. at f/5.6, 1/250 sec. at f/11, and so on.

This allows the user to select a suitable aperture or shutter speed, while still making sure that the overall exposure renders a satisfactory result without the confusion of changing shooting modes. This kind of thinking is the precursor to experimenting with Aperture Priority or Shutter Priority.

A couple of years ago, Canon introduced a new shooting mode in the form of Creative Auto, which is similar to Program but with a low-tech approach to the user interface. A new slider control is incorporated into the menu screen (shown above), which allows the user to make conscious decisions about how blurred or sharp the background needs to be. The slider is actually controlling the aperture (and therefore the depth of field), but without reference to any of the technical terminology. The shutter speed is set automatically.

Aperture Priority

When you want to have complete control over the aperture, this is the mode to use. For example, you may want to use a wide aperture for a very restricted depth of field, blurring the background behind the main subject. Alternatively, you may want to use a small aperture to provide a greater depth of field, so the foreground, middle distance, and far distance are all in focus. Having selected the aperture, the camera will determine an appropriate shutter speed according to the meter reading. The difference between this and the more automated modes is that the user retains control over a wider range of settings.

Shutter Priority

This works just like Aperture Priority, except that the photographer sets the desired shutter speed. This may be because you want a fast shutter speed to freeze movement in a fast- moving subject, or it could be that you want to deliberately blur the motion in an image by using a slow shutter speed, such as when photographing a waterfall. In both situations, the camera chooses the appropriate aperture.


In Manual mode the user has to set both the aperture and the shutter speed independently of one another. If the selected combination does not match the exposure reading suggested by the camera's meter, there will be a mechanism for indicating that overexposure or underexposure is likely. However, it is highly likely that the user has already determined that it is the meter reading that is inappropriate, which is why the camera has been set to Manual mode in the first place: The camera isn't always correct, which is why you are reading this book, right? If you start reading this guide relying on Full Auto and finish it using Manual, then all my efforts will have been worthwhile.

Scene modes

This is the generic term used to describe a wide range of shooting modes that are geared toward specific shooting situations. There is a great deal of overlap between the different modes adopted by different camera manufacturers, but most will provide scene modes that include Portrait, Landscape, Sports, and Close up, for example.

The thinking behind these modes is that an inexperienced user will know what type of shot is desired — a landscape, which would require ample depth of field, for example. So the manufacturer ensures that an adequate aperture is selected automatically when the camera is switched to the Landscape scene mode.

The disadvantage of these modes is that, as with Full Auto, the majority of camera settings are determined automatically, with little scope for adjustment. They are helpful for the beginner, but they also take away any incentive to learn about taking control of all that the camera offers. Scene modes are also likely to record images as JPEGs, removing many post-processing options.

With a greater understanding of both exposure and the camera itself, it soon becomes obvious that just because a scene mode is labeled "Sports," that doesn't mean this mode isn't suitable for other subjects that demand a fast shutter speed.

Picture styles

For want of a better expression, I have used the term picture styles (note the lower case "p") to describe the facilities provided by Nikon (Picture Control), Canon (Picture Styles — note the capitals), and other manufacturers to control groups of parameters that affect the sharpness, contrast, and color characteristics of an image, on which there is more in Chapter 8.

Picture styles can usually be applied to either JPEG or RAW files, although to keep the style with a RAW image you will likely need to be using the manufacturer's own conversion software. If so, they can also be applied retrospectively to RAW files.

Although there are a number of different styles, most manufacturers include the same "basic" package: Standard, Natural, Vivid, Landscape, and Portrait. The differences between them will vary from manufacturer to manufacturer, but in general you should be wary of over-using any "vivid" setting — you may be better off using a less saturated mode in-camera and then increasing the color saturation during post-processing for greater control.

Focal length: its impact on metering and exposure

Your choice of focal length will determine how much of the scene before you is captured in the image. In turn, this means that it may also determine the extent to which light or dark areas dominate the image, depending on the composition. However, it is important to note that while changing the focal length for a scene can affect the meter reading, it may not necessarily change the exposure that is required.

A longer focal length can make the subject more dominant in the frame (as demonstrated in the example below), which can impact on the exposure when your subject is especially light or dark. It is important that the main subject is correctly exposed.

In the images below, the castle looks almost identical in tone in both shots, but it received an exposure that was 2/3 stop darker in the second photograph, taken using a 22mm focal length. This is because an identical exposure would have rendered the foreground too light, drawing the eye from the castle itself. This highlights one of the peculiarities of working with wide focal lengths for landscapes in particular: the more elements there are in an image, the more they need to integrate, in terms of exposure as well as composition.


Just as focal length can influence the metering suggested by your camera, so too can your camera position. In the examples below, the upper image provides a fairly straightforward situation for a camera's meter, but the lower image contains a greater amount of shadow area, which will cause the meter to suggest a greater exposure than is required.

Both images were captured at exactly the same exposure, by taking the exposure reading from the center of the frame and setting it manually for both shots. If you are using a zoom lens, you can zoom into the scene to obtain your exposure reading, then zoom out to recompose the shot, using the reading obtained to set the exposure manually.

Sense of purpose

Unless you know what you were trying to achieve, how can an image be deemed a success as opposed to a happy accident? For those who own a range of camera equipment, the sense of purpose begins before stepping out of the door — by deciding what equipment to carry. This might also include how that equipment should be carried, which will determine how readily accessible it is.


Excerpted from Understanding Exposure by Andy Stansfield, Chris Gatcum. Copyright © 2010 AE Publications Ltd. Excerpted by permission of AE Publications Ltd.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.

Meet the Author

Andy Stansfield is a highly experienced freelance photographer and author whose work has been published around the world in both magazines and books. He has photographed a myriad of diverse subjects from Queen Elizabeth’s 80th birthday celebrations to the wilds of Eastern Europe and Asia.

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