When making a photograph, achieving the best results requires a deliberate selection of techniques and the use of precisely the right tools in precisely the right way. Electronic flash, frequently a tool of choice, can also do more harm than good if not applied properly. For successful images for which I’ve used flash, I’ve gone through a series of mental questions that have led me to the decision to use it and then how to set the camera and flash unit to their proper settings. A thorough understanding of the flash is critical to the understanding of when and more importantly when not to use flash and how to use it to achieve the appropriate results. In addition, the photographer also needs a knowledge of how light behaves and how the addition of the light from the flash will interact with the natural light in the scene. Knowing the basics can fundamentally improve the quality of your pictures made using flash and fortunately it’s not that difficult to understand.
What is Electronic Flash
In its basic form an electronic flash unit is comprised of a flash tube, a high voltage power supply, a capacitor that holds a high voltage charge, and an electronic component called a thyristor that discharges the capacitor through the flash tube when triggered by the camera, creating a bright pulse of light.
More sophisticate electronic flash units have circuitry to automatically control exposure. Automatic Exposure uses a sensor on the flash unit to control exposure and Through The Lens (TTL) exposure uses a sensor in the camera body to measure flash output as seen by the camera using its lens. TTL Flash units designed for use with film cameras use a sensor that senses light as reflected off the film in the camera as the picture is being made while units designed for digital camera rely on a “pre-flash” to measure the required exposure on a sensor in the camera body prior to the camera releasing the shutter.
Controlling Exposure with an Electronic Flash
The amount of illumination emitted from the unit is determined by the voltage of the power supply and the type of flash tube used. The amount of illumination is the same every time the unit is fired. The exposure, whether on film or on a digital sensor, is varied by changing the duration of the flash or by changing the aperture of the lens, not by changing the amount of illumination emitted by the flash. For a given aperture, the duration of the flash is the determining factor. This duration is extremely short, occurring much faster than the cameras shutter therefore the shutter speed of the camera has no effect on the parts of the image exposed by the flash.
A manual flash or an automatic or TTL flash used in the manual mode, set the flash duration by fractional power settings. Typical settings are 1/2, 1/4, 1/8, 1/16, and 1/32 power. For a flash that is 1/1000 second at full power, 1/2 power would be 1/2000 second, 1/4 power would be 1/4000 second, etc. When a flash is in automatic or TTL mode, the flash automatically adjusts its duration to match the scene conditions.
The range of the flash is determined by the illumination times the duration of the flash for a given lens aperture. The range is easily calculated by the rated guide number of the flash. The guide number is the lens aperture times the distance, or the aperture is equal to the guide number divided by the distance. For example, my SB-800’s have a rated guide number of 184 at full power when zoomed to a 105 mm setting. One caution however is that the manufacturers are often overly optimistic with the Guide Number. I’ve measured my SB-800 at a guide number of 160 rather than the rated 184. For an accurate exposure at full power with it set to 105 mm, I need to set the aperture to f16. The flash has a convenient scale when set to manual the calculates this for me (although I have to take into account the inaccuracy of the guide number). It is wise to measure this yourself by photographing a grey card in a darkly lit room and checking where the aperture needs to be set for a mid-toned gray image at 10 feet. For example if you arrive at a mid-toned image at f11 with the flash 10 feet from the subject, the guide number is 110.
Note with a guide number of 160 if you increase the aperture of the lens by one stop to f11, you’ll get the proper exposure by increasing the distance to 14.5 feet or if you increase the aperture to f8 you’ll get the proper exposure by going to 20 feet. This relationship is called the inverse square law, but all you have to remember is that the guide number divided by the f stop is equal to the distance for the proper exposure.
When you decrease the power of the flash by 1/2 you decrease the distance by a factor of .707, so if your guide number is 160 at full power, it is 113 at 1/2 power and 80 at 1/4 power, etc. Again this is due to the inverse square law.
Fortunately, when operated in the manual mode, this information is calculated for you on every flash I’ve worked with over the past 10 or 15 years.When working in the automatic or the TTL mode, the maximum range is the same as the range at full power calculated by the measured guide number.
The effect of zooming the flash out is to increase the area covered by the flash to accommodate a wider lens. Doing so reduces the effective guide number of the flash. At the 50 mm setting my SB-800 measures a guide number of 120.
In this digital age, if you’re shooting with a digital camera, calculating the distance using the guide number need only be an approximate thing. As you shoot you’ll want to check the histogram and make sure you have the correct setting rather than relying on the guide number exclusively for the exposure setting.
Balancing Flash and Ambient Light
Using a flash with a guide number of 160 with your camera set to iso 100 and a subject 10 feet away from your flash, you need to set your aperture to f16 to properly expose the subject. If the subject is in the shade and the background is sunlight on a bright sunny day and say 30 feet away, the sunny 16 rule says to properly expose the background at an aperture of f16, the shutter speed of your camera needs to be set to 1/100th a second. The subject is sufficiently far away at 30 feet to effectively by only lit by the sun. You can vary the exposure for the background relative to the subject by varying the shutter speed leaving the aperture set at f16. At 1/200th a second the background will be one stop darker and at 1/50th one stop lighter and since the flash determines the exposure time of the subject, making that change will have no effect of the tone of the subject.
Maximum Flash Sync Speed
One complicating factor is the maximum speed you can set your shutter. The focal plane shutter used in modern SLR cameras, is actually a moving slit that goes across the film / sensor plane. If the shutter is set too fast, light from the flash does not extend across the whole plane of the picture, you just get a slit of exposure. Flash units that are electronically connected to a matching camera typically force the camera to that maximum shutter speed, for example my D200 and D2x has a maximum flash sync shutter speed of 1/250th second. If you vary the above example to a case where the subject is 20 feet from the flash the aperture must be set to f8 for the proper exposure and since the sunny 16 rule states that you need to set the shutter speed to 1/400th a second for a proper exposure, you will end up over exposing the background at the required maximum sync shutter speed of 1/250th of a second. The only solution to this problem is to move the flash closer to the subject or use a more powerful flash. One note is that some of the older CCD based cameras, such as the Nikon D1, D1X, and D70, use a combination of a mechanical and electronic shutter. With those cameras, if you connect the flash unit via a PC connection rather than the nikon cable connection, you can sync the camera at much faster shutter speeds and work around this problem. Unfortunately this means you can only use the flash in manual mode. Another partial work around with Canon and Nikon Cameras is to use their fast synch options. Essentially this sends out a series of shorter duration pulses designed to light the whole frame as the shutter is opened. This indeed allows you to sync at a much higher speed, but at the expense of much lower effective power output, a much lower guide number, meaning you’ll not have enough power to effectively light the subject without decreasing the aperture, which in turn requires a faster shutter speed to properly light the background which in turn reduces the guide number even more. Unfortunately unless you’re only after a larger aperture to decrease the depth of field, with the ability to move the flash closer to the subject, this option is of little value.
Light Characteristics and the use of Flash
For the majority of cases in the natural world, we use flash to augment natural sunlight. There are several critical characteristics of natural light that influence the ability to use flash in a natural looking way. The first is the size of the light source. Direct sunlight is a relatively small light source. Small light sources by their nature produce hard shadows. Scenes taken on a bright sunny day feature very dark distinct shadows. Indirect sunlight, such as light on a cloudy day or light in the shade are example of large light sources. Light from large light sources is characterized by very soft indistinct shadows. Light from an electronic flash with no light modifiers is an example of a small light source and if used solely to light a scene produce very hard distinct shadows.
Another characteristic of light is its color temperature. Due to the ability of our eyes to adapt a scene to adjust the color temperature of a scene, images may appear to be a different color than we remember when we photograph the scene. For example a scene taken in very warm early morning or early evening light, will appear to be too orange when viewed as a photograph. Conversely scenes photographed in open shade where the principal light source is from a blue sky will produce a blue / cyan cast to the image produced. Scenes with brightly sunlit areas and darker shadow areas will have a mixed color temperature with the sunlight areas very warm and the shadow areas very cool. Flash is normally balanced for the nominal daylight color temperature of 5000 to 5500 degrees (a measure of the color temperature of light). Colored gels are available to modify this color temperature of the flash to match the color temperature of the scene you’re photographing.
Another characteristic is the direction of the light. Shadows are cast opposite the direction of the light source. Images with multiple light sources, such as one lit by the sun with a secondary source from an electronic flash, must be carefully composed to insure the shadows are uniformly cast or the image does not look real.
I’ve briefly mentioned the inverse square law. I want to leave the mention brief as it is a bit much to comprehend. But the effects are not. Essentially light from the flash drops off very rapidly with distance. Since the sun is so far from the earth, from a practical point of view it does not vary with distance. The net results is that if you’re photographing a close subject with flash and the back ground is further away, the flash light the subject but has no effect on the background. For example the subject is 10 feet away as in our previous example, with a guide number of 160 the aperture is set to f16 to light the subject, if the background is 40 feet away you would need an aperture of f4 to light the background, or a 4 stop difference, you wouldn’t see the flashes effect on the background at all.
The Better Beamer
Photographers with long lenses have an option to extend the range of their small flashes. The Better Beamer, a Fresnel lens that attaches to the front of a flash unit, focuses the light of the flash down to a beam approximately the width of the coverage of a 300 mm lens on a 35 mm camera. The effect of this is to increase the illumination of the light in the narrower beam. I measured the Guide number of my SB-800 at 320 with the addition of a better beamer, as compared to 160 without. This gives you an additional 2 stops of light to work with. So with the lens aperture set to f4 a proper exposure can be made at 80 feet.
Flash Main and Flash Fill
Flash can be used in two ways. One is for the main source of illumination, typically used when the subject is in the shade and the background is bright. The scene is exposed with the amount of light to properly expose the scene. The background and foreground exposures are balanced so they appear natural. This is called Flash Main. Flash Fill is used to fill in darker shadow areas in the image. Typically the exposure is 1/2 to 2 stops below the ambient exposure and the intent is to reduce the impact of distracting shadows in the image.
Scenario Number One
This image of a snowy egret doing its courtship display was shoot with the sun to the right and the body of the bird is in deep shadows. This image could have been shot with flash to lighten the shadow side and make a more pleasing image (I’ve use photoshop to lighten the bird, but it’s always better to do it in the camera if you can). The image was shot at f4 at 1/640th second at iso 200 using my 600 mm lens, I was 60 feet from the subject. My SB-800 has a measured guide number of 160 at iso 100. If I reduce the camera sensitivity to iso 100 and decrease the aperture to f6.3 and set the camera to the 1/250th maximum synch shutter speed, I would get an identical exposure, and be able to use the flash without using the fp flash mode and hence get its full power. With a guide number of 160 the exposure would be correct at 25 feet if I were attempting to use the flash as the main light. However I would like to simply fill in the shadows of the bird and provide a little more detail but preserve the backlit nature of the image. A one stop difference from the indicated exposure would provide a better highlight to shadow ratio. Unfortunately this would require a camera aperture of approximately f4 and a shutter speed above 1/250th which would reduce the output of the flash. A solution would be to add a better beamer to the flash giving me a guide number of 320 which would provide the proper exposure at 1/2 power. Another approach would be to leave the camera at iso 200 and f4 and use the fp mode. I’ve measured the guide number of the better beamer / fp mode output at 1/640th second at 220, giving a good exposure at full power. The advantage to setting it this way is to reduce the potential for the image to be soft due to the motion of the bird, which is probably the best option. In practice I would have set the flash up, made a guess as to the proper setting and look at the result of a test shot of two in the lcd. I would have probably tried full, 1/2, and 1/4 power based upon past experience. In this case the bright areas of the image have much warmer light than the shadow areas. The white balance for the image was set to 5500 degrees making the highlights appear slightly yellow and the shadows distinctly bluish. The flash has a color temperature of 5500 degrees so the fill would be warmer than the current shadows. I would consider adding a 1/2 or 1/4 CTB (color temperature blue) filter in front of the flash to better match the existing scene.
Scenario Number Two
This photograph of a sparrow was taken in my backyard. The perch in the tree is 40 feet from the camera with my 600 mm lens and a 1/4 teleconverter. I had a flash bracket attached to the lens with a better beamer attached to my SB-800 on the bracket. It was a cloudy day and the background was neutral grey. To make a blue background I set the camera white balance to 3700 and applied a CTO (color temperature orange) filter to the front of the SB-800 rendering the subject it’s normal color and shifting the sky to blue. Exposure was f5.6 at 1/250th achieved in aperture priority by setting -1 ev compensation staying at the maximum flash synch speed of 1/250th. The flash was set at TTL with no compensation acting as the main light. With a Guide Number of 320 with the beamer at iso 100, proper exposure can be achieved at f5.6 up to 56 feet (but since there is a filter in front of the flash this is reduced by some small amount). This is an example of using flash to manipulate the colors in an image. Since the flash only covers the foreground due to the inverse square law and has no effect on the color of the sky, applying the filter to the flash lighting the subject and balancing it for the proper color allows the sky to shift independently.
Scenario Number Three
The first image was shot with the flash in TTL mode with no compensation acting as the main light and the shutter set to 1/30th a second to properly expose the background. A manual focus lens was used and the aperture was not recorded. I decided I also wanted an image with a black background, so leaving the flash and aperture set as before, I adjusted the shutter speed of my D1X to its maximum flash synch shutter speed of 1/500th second and made the second exposure giving me the black background. Note that the exposure on the flower on the foreground has not changed while the background is now totally black.
This image was shot with a 600 mm lens and a 2.0 teleconverter attached. The aperture was set to f8 and the shutter speed was 1/80th second. The iso of my D1 was set to the minimum of 200. The bird was about 90 feet away. The first test shots showed the inside of the tree hollow as completely black providing an unnatural look to the subject. A flash was attached to a flash bracket with a better beamer. At iso 200 the Guide Number of the SB28-DX I was using with the beamer was approximately 400, giving a normal exposure at 50 feet. Full power, which was slightly less than a one stop below a normal exposure, was required to fill in the inside the hollow with light. With the performance of the cameras I use today, I would have set the ISO to 400 rather than 200 to increase the shutter speed to 1/160th to reduce the risk of camera shake.
Setting up flash is time consuming and makes shooting cumbersome. Setting up the flash usually is best accomplished in cases where you’ve carefully planned what you’re shooting rather than taking what comes along.
The time for the flash to recycle is often several seconds, particularly in the situations encountered when photographing birds, You’re certainly not going to shoot in burst mode.
*The flash may scare the subject. *I take particular care to avoid situations where I’m putting the subject at any risk.
For many subjects, you may get peculiar colors in the animals eyes, much like red eye in humans.
Originally posted July 2008