A Short Course Book
Curtin's Guide to Digital Cameras
And Other Photographic Equipment


Click to explore the inverse square law.
The power of a flash is indicated by its guide number. Click here for an Excel work sheet you can use to explore these numbers.
Since flash falls off with distance, objects near the flash will be lighter in a picture than objects farther away. You can use this to advantage; for example, at night you can isolate a subject against a dark background.
Click to explore the flash sync speed.
Leaf shutters, common on point and shoot cameras, allow faster flash sync speeds because they don't use curtains.
Click to open the Excel worksheet used to explore flash guide numbers.
A flash hot shoe cord lets you hold the flash farther from the lens axis to reduce red-eye.
Click to explore red-eye.
Click to explore fill flash.
Slow sync flash outdoors at sunset captured gulls in mid flight with interesting effects.
Click here to explore first and second curtain sync.
Click to explore flash exposure compensation.
Click to explore high speed sync.
Click to explore stroboscopic flash.
Stroboscopic flash repeatedly froze the head of a dipping bird toy.
A hot shoe or a flash sync connection allows you to attach an external flash unit. Courtesy of Olympus.
Click to explore how a flash head can pivot up and down and rotate for bounce flash.
Many digital cameras have a built-in flash that is so convenient and easy to use that you are usually unaware it even fires. With your camera on automatic, it's always ready when your autoexposure system decides it's needed. But this on-camera flash lighting has certain characteristics that can make a difference in the way your pictures look. For example, the pictures will have a "flat" lighting typical of flash-on-camera shooting. Alternative approaches, such as using an external flash to bounce light off walls or ceilings, or even just turning the flash off may produce more interesting results.

Flash Power and Range

One thing to check is a flash's maximum range. The intensity of the flash when it reaches a subject depends on the flash's power and on how far the light has to travel. The further the subject is from the flash, the less light will reach it and so the less light will be reflected from the subject back toward the camera.When the flash fires, the beam of light expands as it moves father from the camera so its intensity falls off with distance. As a result, subjects nearer the flash will be illuminated with a more intense light than subjects farther away. The rate at which the light falls off is described by the inverse square law. The law states that if the distance between the flash and subject is doubled, only one quarter the amount of light will reach the subject because the same amount of light is spread over a larger area. Conversely, when the distance is halved, four times as much light falls on a given area.

The light from a flash falls off with distance. When you double the distance, you get onequarter as much light. This relationship is called the inverse square law.

When subjects in an image are located at different distances from the camera, the exposure will only be correct for those at one distance—normally those closest to the camera or in the area metered by the autoexposure system. Subjects located farther from the flash will be increasingly darker the farther they are from the flash.

To calculate the maximum range of a flash, you can use its guide number—a measure of its light output. The higher the guide number, the greater the intensity and range of the flash. Guide numbers are determined experimentally, usually by the manufacturer, and listed in their specifications. A flash unit and subject are set up and exposures are made at a variety of f/stops. When the best exposure is determined, the guide number is calculated from the distance and the f/stop used as follows:
More powerful flash units with higher guide numbers have a greater range, have faster recycle times, and make bounce flash more effective. For example,one camera's built-in flash has a guide number of 43 (in feet, with ISO set to 100). Its optional accessory flash has a guide number of 180. When using an aperture setting of f/3.5, the range of the built in flash is about 12 feet and that of the external flash is over 50 feet.

A flash has the power to light the entire scene, but light falls off the farther it is from the camera.

In addition to being an indicator of a flash's power and range, you can use a flash's guide numbers to calculate aperture setting and subject distance when using the camera and flash on manual mode as when you are using a flash not designed to work with the camera.

  • To calculate the f/stop needed, divide the guide number by the distance to the subject.

  • To calculate the maximum flash range, divide the guide number by the f/stop you plan on using.

    When making these calculations, there are a few things to be aware of:
  • Guide numbers are usually given for a setting of ISO 100. If you increase the ISO setting, the camera needs less light for a good exposure so the range of the flash increases. Doubling the ISO setting, say from 100 to 200 increases the guide number by a factor of 1.4x; quadrupling the ISO, say from 100 to 400, doubles the guide number.
  • It is the flash-to-subject distance that you use, not camera-to-subject. These two distances are the same when using on-camera flash but not when using flash on a cable or a slave flash. When using bounce flash, the distance is the longer path which the light travels.
  • Guide numbers are usually given for both feet and meters so be sure you use the right one in your calculations. The differences can be substantial. For example, a guide number of 12, when using meters as a unit of measure, is the same as a guide number of 39 when using feet. Conversions between a guide number for feet and a guide number for meters are as follows:
    • From meters to feet: GN(ft) = GN(m) x 3.28
    • From feet to meters: GN(m) = GN(ft) x 0.328
The Excel worksheet "Flash Guide Numbers" lets you explore how a flash's guide number is determined and how that number affects the aperture you can use and the flash to subject distance.

Calculating a Guide Number Section

1. f/stop used is where you enter the f/stop used to take the photo.

2. Flash to subject distance is where you enter the subject's distance in feet or meters.

3. ISO is set to 100, the setting most often used to calculate the guide number you find in flash specifications.

4. Guide number in feet formula multiplies the f/stop used times the Flash to subject distance to calculate the guide number at an ISO of 100. Since the guide number increases by 1.44x each time you increase the ISO one stop, you can determined the adjusted guide number by looking up the ISO you want to use in the lookup table. Once you find it, multiply the guide number at 100 by the factor to the right on the table. You can use the original, or any of the adjusted ISOs to explore the next two sections of the worksheet.

5. Guide number in meters multiples the Guide number in feet by 0.328 to convert feet to meters.

Calculating an F/Stop Section

1. Guide number is where you enter the flash's guide number. You can get this from the manual that came with your camera or flash.

2. Flash to subject distance is how far the flash is from the subject in feet or meters.

3. f/stop formula divides the Guide number by the Flash to subject distance to calculate the aperture you would use at ISO 100. Each time you double the ISO you can stop this down one stop.

Calculating the Maximum Range Section

1. Guide number is where you enter the flash's guide number. You can get this from the manual that came with your camera or flash.

2. f/stop is where you enter the camera's f/stop.

3. Distance formula divides the Guide number by the f/stop to calculate the flash range at ISO 100. Each time you double the ISO, the distance increasses by 1.44x.


Open the worksheet by clicking the Excel button to the left and enter numbers in the green cells to explore the questions that follow.

1. If you get the best exposure at f/8 while photographing from 8 feet, what is the flash's guide number?

2. If you set the aperture to f/4, the flash to subject distance is 10 feet, and the ISO is 100, what is the guide number? What is it if you change the distance to 20 feet?

3. If your flash has a guide number of 60 and you are 12 feet from the subject, what f/stop should you use at ISO 100? At ISO 800?

4. If your flash has a guide number of 90, how far can a subject be from the flash when the aperture is ste to f/4?

Flash Sync and Shutter Speeds

When you take a picture, the shutter opens and closes to let light strike the image sensor. When it does so, the shutter is fully open for a very short time. If the shutter speed is too fast, the burst of light from the flash won't fully expose all parts of the image sensor and part of the scene won't be captured in the image. The fastest shutter speed that can be used is called the flash synchronization speed and is usually between 1/125-1/500 second. One advantage of a higher flash sync speed is that you can use fill flash out of doors with a larger aperture to better freeze action or get shallower depth of field-perhaps to throw the background out of focus in a portrait. On an SLR the flash sync speed is determined by the timing of the shutter's two curtains—a front and rear curtain, sometimes called first and second curtains.

A focal plane shutter opens a curtain to begin an exposure and closes a second curtain to end it. At fast shutter speeds (top) the second curtain starts to end the exposure before the first curtain has fully opened so the two curtains form a slit traveling across the image sensor. Flash would only expose the area uncovered by the slit between the two rapidly moving curtains. At the flash sync speed and slower (bottom) the second curtain doesn't start to close until the first one is fully open.

Flash Modes

On fully automatic cameras, the flash fires automatically whenever the light is too dim to take a photo by natural light. On more sophisticated cameras, there are various flash modes you can choose from for more creative effects.
  • Auto mode fires the flash whenever there is too little light for a good exposure or when the main subject is backlit.
  • Red-eye reduction mode. You'll often see photos of people with what's called "red eye." The light from a flash has entered through the subject's pupil and reflected off the back of the eye (the retina) and back out to the camera. Since the retina is lined with blood vessels, the reflected light takes on a red color. To reduce red-eye, the camera has a red-eye reduction mode that fires a short preflash lamp to close the subject's iris a moment before the actual flash fires to take the picture. It doesn't always work. To eliminate red eye, you need an external flash that's positioned farther away from the axis of the camera lens. If you have to use only the built-in flash, zoom the lens out to a wider angle, tell the subject to look directly at the camera, get close, increase the overall room lighting, or have the subject face a bright window. You can remove red-eye using software, but it's a lot easier to avoid it to begin with.
On most cameras you can turn red-eye mode off. In many situations it's not needed and it does introduce a very brief delay between pressing the shutter button and capturing the picture because the red-eye reduction light needs time to flash.
  • Fill flash (forced) mode, often called Flash On or Forced Flash, fires the flash fires even if there is enough available light to take the picture without flash. It is used when you want to fill in shadows when the subject is back or side lit. In these situations shadow areas can be so dark in the image that they show little or no detail. When the subject is backlit or against a bright background, it can be underexposed. Fill flash is also a good way to get accurate color balance under unusual lighting. Another reason to use fill flash outdoors is to add catch lights to eyes—hot spots that make the eyes sparkle.
  • Flash off mode is used when the light is low enough to trigger the flash but you'd rather use a long exposure to capture it in natural light. On some cameras you can also turn the built-in flash on or off when an external flash is attached.
  • Night scene mode uses flash to correctly exposes the foreground subject while using a slow shutter speed to lighten the background.
  • Slow sync fires a short burst of flash during a longer exposure to freeze objects while still allowing them to blur.
In very dim light, flash pictures show a well exposed foreground subject against a black background. The slow synchro mode is designed to minimize this problem by leaving the shutter open longer than usual to lighten the background. This way the shutter speed determines the exposure of the background while the flash determines the exposure of the foreground subject. In many cases, the slow shutter speed used in this mode allows blur from rapidly moving objects or camera shake to appear as blur in the images. To avoid blur, use a tripod and photograph static subjects. Or use this effect creatively. A short flash burst combined with a long shutter speed gives interesting effects. The flash freezes nearby objects sharply, and the dim ambient light blurs moving objects and moving lights appear as streaks. Some digital SLRs let you choose if the flash fires either when the shutter first fully opens or just before it's about to close.
  • Front/first curtain sync (the usual mode) means the flash fires when the shutter's front curtain first fully opens to expose the image sensor.
  • Rear/second curtain sync means the flash fires just before the shutter's rear curtain starts to close to end the exposure.
Front curtain sync (top) fires the flash at the beginning of the exposure, then records ambient light. As a result, light streaks from the moving subject appear in front of it.

Rear curtain sync (bottom) fires the flash at the end of the exposure, after the ambient light has been recorded so the streaks trail behind the subject.

Flash Exposure Compensation

Flash can under or overexpose a subject so you might want to look for a camera that has flash exposure compensation so you can manually adjust the flash output, the brightness of the flash illuminating the subject, without changing the aperture or shutter speed. This is an ideal way to balance flash and natural light when using fill flash and to correctly expose scenes or subjects that are darker or lighter than normal. The exposure compensation function often lets you vary flash exposures plus or minus 2 stops in one-third stop increments.
You can use flash exposure compensation in conjunction with regular exposure compensation. Doing so lets you use regular exposure compensation to lighten or darken the background that's illuminated by natural light, and use flash exposure compensation to lighten or darken the subject illuminated by the flash. This is a powerful combination of exposure controls that let's you capture images just the way you want them.

Flash Exposure Bracketing (FEB)

Flash exposure bracketing (FEB) takes a series of three consecutive pictures exposed at slightly different settings above or below the exposure recommended by the autoexposure system. The flash output changes with each image while the background exposure level remains the same.

Flash Exposure (FE) Lock

Flash exposure lock (FE Lock) works much like AE Lock. When you use this feature, a preflash is fired and the exposure system reads the flash exposure so you have time to recompose the scene or make exposure or focus adjustments without losing your flash exposure information. FE lock is extremely useful when the main subject is off-center. It can also eliminate potential exposure errors caused by unwanted reflections from highly reflective surfaces such as windows or mirrors.

High-Speed Sync (FP) Flash

In situations where you want to use a shutter speed that's faster than the camera's flash sync speed you can do so if the camera or your external flash supports what's known as high-speed sync flash (also called FP or focal plane sync). High-speed sync can capture a fully exposed image because the flash fires repeatedly as the focal plane shutter's "slit" moves across the image sensor during the exposure. The only drawback is that the flash power is reduced so you can't be positioned as far from a subject. The higher the shutter speed you use, the closer you have to be. There are at least three situations where you might find it useful:
  • When you want to freeze a moving subject as when photographing a wildflower in the wind.
  • When using fill flash to lighten shadows, you can use a fast shutter speed to freeze action, or a wide aperture to throw the foreground or background out of focus.
  • When doing a portrait and want catchlights in the subject's eyes.

Stroboscopic Flash

Stroboscopic flash fires the flash a number of times at high speed to capture multiple images of the same subject in the same photograph. You've probably seen examples of this mode in sports photography where it can be used to demonstrate or analyze a swing of a bat or club. This feature is usually set on the flash, not on the camera.

External Flash Units

Built-in flash is convenient to use because every place you and your camera go, it goes. However, these built-in flash units don't have much range and you can't position the flash away from the camera to eliminate red eye. Pictures taken with these units also have a characteristic flat shadowless lighting that minimizes surface textures and volumes. They can't be rotated to bounce flash off a wall or ceiling to soften the lighting.
One thing you may want to look for in a new camera is how you can attach a more powerful flash:
  • A hot shoe secures a flash on top of the camera and provides electrical connections between the flash and camera and also holds the flash in place. You just slip the flash into the shoe and tighten a wheel to lock it in place. There are also hot shoe cords that let you mount or hold the flash away from the lens for more dramatic side lighting and to eliminate red-eye.
  • The PC (Prontor-Compur) terminal is used to connect studio lights and the connector is threaded for a secure connection. This connector also lets you connect a separate flash using a sync cord, basically a small cable. This synch cord makes the same electrical connection that the hot shoe does but lets you take the flash off camera.
External flash units are available from the camera company and third parties. What you want is a dedicated flash. This means it is designed to work with your specific camera model. Only a dedicated flash will be integrated into your camera's autoexposure system and offer additional features to extend your cameras capabilities. Flash units that are not dedicated usually have to be operated on manual mode and not worth the savings.

Slave Flash Units

If your camera doesn't have a hot shoe or other flash connector, and many pocket cameras don't, you can use a slave flash unit. These flashes have a sensor that fires the flash when it senses the burst of flash from the camera's built-in flash unit. Since many digital cameras fire the flash twice for each picture (the first is to set white balance and perhaps focus), these units are designed so they fire when the camera's second flash goes off.
A small slave flash can be attached to a pocket camera to give you an increased flash range.

Ring Flash

There are two important reasons to use flash in close-up photography. With flash, you can use smaller apertures for greater depth of field, and extremely short bursts of light at close distances prevent camera or subject movement from causing blur. You can even bounce the flash off a reflector to illuminate the subject from an angle for a better lighting effect. A special kind of flash is the ring flash. These units fit around the lens and fire a circle of light on the subject. They are ideal for close-up photography such as that used in medical, dental, and nature photography. Because ring flash is so flat (shadowless), the unit can be set to fire just one side of the ring, or one side of the ring can be fired with more intensity than the other so the flash casts shadows that show surface modeling in the subject. Canon makes the Macro Twin Lite, designed for serious close-up photography and other companies make similar units. Two separate flash heads can be swiveled around the lens, can be aimed separately, and even removed from their holder and mounted off-camera.
Canon's Ring Lite (left) and Macro Twin Lite (right) are both designed for close-up photography.

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