Understanding light should be at the top of your photographic-priorities list. Light is photography. Without it we are doomed. Good photographic light isn’t limited to the sun, yet I, like so many other shooters, held on to that belief for years. Only recently did I begin to embrace another light source, a light source I affectionately call my “miniature sun”: electronic flash. No matter what Mother Nature may have in store, I can still head out the door, confident that I carry with me “a little ray of sunshine.” Sometimes, that little ray has made all the difference.
Light is all around us and comes from many sources, visually sculpting our world by creating highlight, shadow, and shape. Some light we can control, and some we can’t. For example, if light is too bright, too harsh, too dark, or too soft, we may be able to simply turn a lamp off or on. In other cases, we have no control. Existing light sources, whether artificial or natural, are called available light or, more commonly, ambient light. These are, unlike flash, constant light sources and are usually always on. The sun is ambient light and is “always on” during daylight hours. A streetlight is ambient light and is always on during evening hours. Lights in a grocery store or office are ambient light and are almost always on.
Light from an electronic flash, on the other hand, is supplementary light. Unlike most ambient light sources, electronic-flash light is 100 percent under your control. You have the choice of illuminating a large area or a small one—and from just about any angle you can imagine. What’s truly exciting about using flash is that you get to control its contrast, brightness, and volume—all while maintaining complete control over the ambient light and its resulting exposure. With one or more electronic flashes in your camera bag, you’ll begin to feel that you’ve discovered your own planet where numerous suns are rising and setting at the same time, under your command, casting light in places of your choosing!
But before you can master flash, you have to understand how the camera “sees” light. Perhaps you’re familiar with seascapes by painters who capture a very bright, colorful sun setting in the background while maintaining vivid and bright details of the sand, surf, and nearby rocks in the extreme foreground. Ever try to take a photograph of an ocean scene like that? If so, you were probably disappointed. The human eye is an amazing machine that can see a broad range of light and dark simultaneously. Compared to what your digital sensor (or film) can actually see and record, our eyes can see much, much more. A typical digital sensor records an estimated 7-stop range of light and dark. Our eyes see an estimated 16-stop range. A painter who sees a seaside scene in the entire 16-stop range paints a landscape in which all 16 stops are represented, from light to dark. Your camera only records a fraction of that.
The solution? Your portable miniature sun. By adding supplemental light in the right parts of the foreground, your camera’s sensor can “see” those portions of the scene that were previously too dark. With your electronic flash, you, too, can achieve an image on par with a fine painting.
Photographers and painters have been manipulating light for centuries, and when used well, a flash can be perhaps the best manipulator of all. Depending on how you shape it or color it, flash can add drama, mystery, and surprise to an otherwise mundane scene, even turning day into night or night into day.
Posing in front of the Chicago landmark Wolfy’s on Peterson Avenue, my friend Jon Demopolous helped me demonstrate how to combine electronic flash with a storytelling aperture (one that renders great depth of field from front to back and contains distinct foreground, middle-ground, and background elements as different parts of the story).
Similar to a sunset seascape, the range of light in this scene was greater than what the camera was able to capture. If I exposed for the sign and sky, Jon would be underexposed and show no detail; if I exposed for Jon, the background and that great sign would record way overexposed. That’s where flash comes in.
With my camera and lens mounted on tripod, I set an aperture of f/22, because I wanted maximum depth of field. I then adjusted my shutter speed until my camera’s light meter indicated that 1/4 sec. was the correct exposure for the sky and neon sign (with ISO 200). The resulting available-light exposure above is spot-on for that area. Jon, on the other hand, is much too dark. So I dialed in f/22 on the back of my flash, and the flash distance scale (see this page) indicated a flash-to-subject distance of 4.4 feet for a correct flash exposure.
With my flash tethered to a remote flash trigger (see this page) so that it was ready to fire remotely, I held the flash up high with my left hand, just to the left of Jon, and pointed down at about a 30-degree angle. I then tripped the camera’s shutter release with my right hand while Jon began to bite down on his hotdog. Voilà! The background is properly exposed, while the foreground is also illuminated. The satisfaction of eating a Wolfy’s Dog has been recorded.
Both photos: 12–24mm lens, f/22 for 1/4 sec. Bottom: with Speedlight SB-900
All light is mysterious, but flash is especially mysterious, and I’m convinced it’s partly because the flash burst is so fast that if you so much as blink, you can’t say for sure that the flash even fired. Even if you did see it fire, you can’t say where the light went or how far it traveled.
If you’ve tried to understand how your flash works at all, you have perhaps come across a number of articles both in books and on the Web that talk about the Inverse Square Law. What is this law? Well, the light output from your flash is a physical quantity, and the Inverse Square Law deals with the falloff of light in relation to the distance from flash to subject. And it’s a good concept to get a handle on before we move on to the specifics of making flash exposures.
Think about it this way: You’re at an indoor concert, and you hold your camera up to photograph the musicians onstage. When you check the shot, you see that the people 3 feet in front of you have “skin burn” from the flash because they were too close to it. The people ten rows ahead are perfectly illuminated, and the musicians onstage are dark because the flash never reached them. Despite the flash’s best attempt to illuminate the subject (in this case, the musicians), the light fell off dramatically before reaching it.
Simply put, the Inverse Square Law states that as the flash-to-subject distance doubles, the light reaching the subject is only 25 percent of the original light that left the flash. So, for example, what this means for photographers is that if you move your subject from 5 feet to 10 feet away from the light source, you’ll need four times the amount of light to get the same exposure at 10 feet that you got at 5. It’s also good to be aware that the closer something is to the light source, the faster the light falls off; the farther it is from the light source, the slower the light falls off. So falloff will be more pronounced up close.
The falloff explained by the Inverse Square Law accounts for much of the disappointment many of us have felt when shooting flash exposures. But this fundamental principle is logical when you think about it. When you’re out in your front yard setting off firecrackers, you don’t expect everyone within six blocks to hear the same exact loud bang, do you? Of course not, because you know all sounds eventually fade to silence. The light output of your flash works the same way. Just like the bang of a firecracker, it falls off quickly.
This diagram illustrates the Inverse Square Law. Let’s say your subject is 5 feet from the camera and your perfect exposure is f/16. When the subject is moved to 10 feet away, or double the distance from the flash, only 25 percent of the light from your flash will still reach the subject. Because of this, the new f/stop would be f/8, or 2 stops more than at 5 feet, to let in more light.
Chloë and I headed out the door for some “festive city” photos, and as these shots clearly show, there’s a definite difference in each of the flash exposures—and that’s courtesy of the Inverse Square Law. The first example (top) combines a perfect flash exposure with a perfect ambient-light exposure. The ambient-light exposure was f/8 for 1/15 sec., and based on f/8, the correct flash-to-subject distance was 15 feet, which was where Chloë stood (15 feet from me).
But note how she becomes progressively darker (center and bottom). This happens for one reason and one reason only: The distance between the flash and Chloë has increased. Center, the flash distance between Chloë and the flash was 21 feet; and bottom, the flash was at 26 feet. The exposure used was the same in all three shots, but the flash-to-subject distance increased, and therefore light falloff accounts for Chloë being too dark.
All photos: f/8 for 1/15 sec., Speedlight SB-900. Top: subject at 15 feet. Center: subject at 21 feet. Bottom: subject at 26 feet
If there’s one area in photography that seems to have created its own set of rules, it is the use of electronic flash. For many photographers, the rules suggest that electronic flash requires a completely different approach to image-making. Nothing could be further from the truth, and I’m here to say that over the course of these pages the mystery of electronic flash will be exposed!
If you’re familiar with my book Understanding Exposure, then you will fully appreciate this one simple fact: Everything you learned in that book about what I call the photographic triangle still applies when using electronic flash. If you have any interest in fully exploring and exploiting the sheer joy of this tool, then it’s absolutely vital that you keep the photographic triangle in the forefront of your mind when you pull out your flash.
This was an easy exposure in the overcast light. As I’ve done for years now, I simply asked myself if there were any depth of field concerns in this scene, and the answer was no, since everything is basically at the same focused distance. So I chose the critical aperture of f/11 for maximum sharpness and simply adjusted my shutter speed until the camera’s light meter indicated 1/15 sec. would get a correct exposure.
But I also wanted to emphasize the texture and form in the scene, so I called on my Nikon Speedlight SB-900 portable flash. Keep in mind that my choice of which aperture to use for a correct exposure—even when using my flash—is still based on what concerns if any I might have regarding the depth of field. And again, with or without flash, this scene and it’s focal distance is not going to change one iota, so it was easy to decide to keep that aperture of f/11.
So with the aperture still at f/11, the distance scale on my flash told me that I could use a flash-to-subject distance of 3 feet at 1/8 power (see this page for more on this). Since I didn’t want the ambient light to affect the exposure, I further adjusted my shutter speed from the normally correct ambient exposure of 1/15 sec. to a much faster speed of 1/125. This 3-stop increase (from 1/15 sec. to 1/125 sec.) assured me that the ambient light would record as a 3-stop underexposure. A 3-stop underexposure will often result in the “killing” of the ambient light, and at least in this case, killing the ambient light was critical so that I could allow only the “sunlight” from my strobe to function as the light source. (Note: You won’t always want to kill the ambient light, as you’ll see later on.) So, with my flash held off to the left at a distance of about 3 feet from the faucet and textured surface, I fired away. Voilà! That little ray of sunshine—and the resulting shadows—emphasized all the texture in the subject.
Both photos: 105mm lens, ISO 200. Top: f/11 for 1/15 sec. Bottom: f/11 for 1/125 sec., Speedlight SB-900
While conducting a workshop in Lyon, France, I asked one of my students, Dennis, if he would pose for a simple portrait. Because there were no depth-of-field concerns, I was able to use the critical aperture of f/8. (Critical apertures—f/8, f/9, f/10, and f/11—render the greatest sharpness.) So, with the aperture set to f/8 on my lens and dialed in on the back of the flash, I directed another student to hold the flash and stand at the correct flash-to-subject distance, which was 11 feet according to the scale on the back of the flash. Before firing the flash, I used my camera’s light meter to determine that the available-light exposure for this scene in open shade was f/8 at 1/30 sec.
My goal was a warm, “sunlit” portrait, so I first had to figure out how to overcome the available light. With an ambient exposure of 1/30 sec. at f/8, I simply sped up the shutter speed to 1/250 sec., a 3-stop underexposure that effectively negated the ambient light. Without this faster shutter speed, I would have gotten an unwanted mix of overcast light and flash, resulting in a washed-out portrait. An amber gel added to the front of the flash completed the effect, making the light of one small flash look like the light of the setting sun.
Nikon D300S, 105mm lens, f/8 for 1/250 sec., Speedlight SB-900
Let’s review this idea briefly. Every single available light exposure is a combination of three factors: aperture (f-stop), shutter speed, and ISO, which make up the photographic triangle. How do these three elements work together to create an exposure? If I’m photographing on a sunny day with ISO 200, I can generate a correct exposure with an aperture of f/16 and a shutter speed of 1/200 sec. If I change my ISO to 800, that same exposure at f/16 now requires 1/800 sec. to be correct. The higher ISO makes the camera’s digital sensor more sensitive to light, and since a shorter shutter speed lets in less light, 1/800 sec. compensates for the increased light sensitivity. When you think of ISO numbers as light-gathering receptors, it becomes clear that the more light-gathering receptors you have (i.e., the higher the ISO number), the quicker you can record an exposure: f/16 for 1/800 sec. is a faster exposure than f/16 for 1/200 sec.
In my own work, I spend 98 percent of my time with the ISO set at 200, whether I’m using flash or available light. In combination with light, it is the constant ISO 200 that dictates which combinations of aperture and shutter speeds I can use for a given shot. Suffice it to say, of the three factors that make up the photographic triangle, ISO is the one element I’m quick to set (to 200) and then forget about!
On the other hand, you’ll always want to be on the lookout for opportunities to use various apertures and/or shutter speeds, since one or the other or both will render the most creative exposures. In fact, every picture-taking opportunity presents six possible combinations of aperture and shutter speed that will all render a correct exposure for a given scene. For example, if a scene can be exposed at f/4 for 1/2000 sec., then that same scene can also be correctly exposed at f/5.6 for 1/1000 sec., f/8 for 1/500 sec., f/11 for 1/250 sec., f/16 for 1/125 sec., and f/22 for 1/60 sec. With each increase in f-stop (i.e., each decrease in aperture opening size), the amount of light on the camera’s sensor is reduced. You are compensating for this reduced light by lengthening your exposure; the longer (or slower) the shutter speed, the more light you’re letting hit your camera’s sensor. In terms of quantitative value, each of these six exposures is correct. However, a full understanding of exposure also means that we want not only a correct exposure but the most creatively correct exposure. So which one of these six exposures is the most creative exposure? That depends on what you want to convey with your imagery.
As I discuss in Understanding Exposure, the apertures of f/16 and f/22 create what I call storytelling images, because the great depth of field they capture from front to back in a scene enables to the photographer to tell an entire story. Apertures of f/2.8, f/4, and f/5.6 create what I call singular-theme or isolation images, because the shallow depths of field they render isolate subjects against softly focused surroundings. And finally, apertures of f/8 and f/11 are ideal for what I call “Who cares?” compositions, meaning depth of field is not a major concern, so “Who cares about depth of field?”
Similarly, different shutter speeds also yield different effects. Fast shutter speeds (such as 1/1000 sec., 1/500 sec., and 1/250 sec.) freeze action, and slow shutter speeds (1/4 sec., 1/2 sec., 1 second, and slower) imply motion by allowing panning or simply by blurring the action. When you’re about to make an exposure, you should ask yourself, Is the scene before me more about creative use of aperture or shutter speed? Once you decide, you then control the creative exposure by first selecting either the right aperture for the depth of field you wish to record or the right shutter speed for the motion effect you wish to capture (and then next selecting the remaining leg of the triangle, whether it be shutter speed if you’ve first selected the aperture or aperture if you’ve first selected the shutter speed).
There really is an unlimited world of creative exposure opportunities. And now for even more good news: The choice to tell a story or isolate, to freeze action or imply motion, is still yours—even, as you’ll soon see, when using your flash!
The sweat was pouring out of me as I pursued the many dragonflies at the Zilker Botanical Garden on a hot August morning in Austin, Texas. The object of my desire was a small damselfly that had taken refuge on a blade of grass in open shade, yet my point of view was such that the background was in full sun. No doubt about it, this shot would require a large lens opening so that I could isolate that lone damselfly from an otherwise busy background. Plus, I needed flash. Lighting up the bug with flash would avoid producing a dark, silhouetted shape and would show the damselfly’s colors.
With my camera mounted on tripod, I set the aperture to f/5.6 to get the very shallow depth of field that would render the background as out-of-focus tones. The addition of a small extension tube also allowed me to focus much closer, and when combined with the focal length choice of 300mm, that little damselfly filled up my frame nicely. But as you can see in the top image, a simple ambient-light exposure silhouetted the subject.
Calling on my portable electronic flash, I quickly set my aperture to f/5.6. The distance scale on the back of my flash told me that if used at full power the flash needed to be 27 feet from the subject. Yikes! I couldn’t have placed that flash 27 feet away from the damselfly if I wanted to; about 4 feet behind me was nothing but a jungle. No problem. I just powered down the flash. As I did this, the flash indicated that at 1/16 power I could shoot at f/5.6 at 4 feet from the damselfly.
I get into powering down the flash in more detail on this page. For now, just know that you have the very useful option of reducing your flash power when you can’t change your flash-to-subject distance—so that you don’t overpower your subject with a superbright flash. Once I’d made the adjustment, I pressed the shutter release and bingo, bango, bongo! The little damselfly lit up like a neon sign, and I maintained a correct exposure of the brighter ambient background.
Both photos: Nikon D300S, Nikkor tele-zoom lens at 300mm, extension tube, f/5.6 for 1/250 sec. Bottom: with Speedlight SB-900
Before we solidify exactly where electronic flash fits into the photographic triangle, it’s important that you, too, think of your flash as the “miniature sun” I’ve been referring to. The sole purpose of your miniature sun is to add light to those areas of your composition where the light (or lack thereof) doesn’t meet your preference.
Assume for a moment that you’re a magician. You wave your magic wand over a scene, and after a few abracadabras, sunlight appears in areas that were once in open shade. Or light (whether outdoors or indoors) begins to illuminate the front of a backlit subject. Or you create low-angled sunlight, like the glow from early morning sunlight, to create frontlight, even though it’s an overcast day. To perform these tricks, you must first understand the workings of your magic wand—i.e., your electronic flash. If your flash is going to work its magic time and time again, it can only do so if the following two conditions are met: (1) the right aperture is set on your camera, and (2) this correct aperture is determined by the distance between your subject and the flash.
Your portable electronic flash should have a distance scale on the back that allows you to determine the correct aperture for your exposure based on the distance between the flash and the subject and on whether you’re using the flash at full power or are powering it down to a fraction of its full strength. I’ll cover the distance scale in more detail on this page, but this is the tool that will tell you the correct aperture for a manual flash exposure based on your distance to the subject.
I can’t stress enough that a correct flash exposure is 100 percent determined by the aperture. Your flash doesn’t care what shutter speed you use. This news may come as a surprise. Many of you have been led to believe that a correct shutter speed, the sync speed, is vitally important when using flash, but that’s a myth, as we’ll see as we progress. The sync speed is simply the fastest shutter speed you can use with your flash. Lowering the shutter speed allows you to manipulate the ambient light in your overall composition, and manipulating this ambient light is what often creates much of the drama in exposures that include both flash and natural light.
As the principles of the photographic triangle tell us, both aperture and shutter speed work in combination to create a perfect exposure when shooting in natural light. Hopefully, you’re now in the habit of asking yourself what kind of picture you wish to take (storytelling, isolation, or “Who cares?”). Your answer to that question will determine which aperture to use. Now, this may come as a surprise, but when you add flash, I do not want you to change a thing in your approach to image-making. Why? Because it’s the flash-to-subject distance that determines which aperture you can use. And you can always move your portable electronic flash closer or farther away (or you can power it down). And because your flash is, indeed, portable, you can almost always move it to the distance required by a particular aperture, so you can still use storytelling, isolation, or “Who cares?” apertures to make your ideal composition.
Let there be light! The amount of the light output from your flash that reaches your digital sensor is 100 percent dependent on combining the right aperture with the right flash-to-subject distance. A bit of grass against a rusty 55-gallon drum illustrates the importance of this simple fact. As you can see top left, there was no sunlight. I captured the rusty barrel and tall grass on this overcast day with f/11 for 1/30 sec. and only ambient light.
The image at top right shows the addition of some “sunlight” with flash. As you just read a moment ago, when you want to kill the ambient light, you should strive for no less than a 3-stop under- exposure of the ambient light, so with my aperture still set to f/11 but my shutter speed now set to 1/250 sec., all that remained was to fire up the flash and dial in the aperture on the flash unit to f/11. When I did this, the flash distance scale (see this page) stated that the flash-to-subject distance must be 9 feet to record a correct flash exposure. Turned out I was about 9 feet away from the subject, so I fired off the shot in what now looked like low-angled, natural sidelight, complete with the telltale signs of sun and shadow. I held the flash in my hand, off to the left a bit, and fired it wirelessly with Nikon’s Commander mode feature. (Canon also has a similar feature on some cameras.)
And just to prove how integral the proper aperture is with flash, note how much darker the exposures became (bottom, left) as a result of my changing the aperture from f/11 to f/16 and (bottom, right) as a result of going from f/16 to f/22. The smaller apertures greatly reduce the amount of flash light that hits the camera sensor. A great example of how aperture and flash-to-subject distance control the flash exposure—always!
All photos: 70–300mm lens at 135mm, Speedlight SB-900. Top, left: f/11 for 1/30 sec. Top, right: f/11 for 1/250 sec. Bottom, left: f/16 for 1/250 sec. Bottom, right: f/22 for 1/250 sec.
Flash photography follows the same principles as manual-exposure photography in natural light. The only difference is the addition of supplemental light from a portable “miniature sun.” With that in mind, these are the only two rules you need to remember when shooting with electronic flash:
1. The aperture—and only the aperture—controls the amount of electronic flash light allowed to expose the picture. All flash exposures are 100 percent dependent on the right aperture being selected and the right flash-to-subject distance that corresponds to that aperture choice. And with the ability to adjust the power of the flash at any moment (see this page), your aperture choice is virtually guaranteed no matter how close your flash is to your subject!
2. The shutter speed controls the amount of time that any ambient light is allowed to expose that same picture for which you’re also using a flash.
Unless you’re shooting in a completely darkened room, there’s always ambient light present, and it’s your creative input that decides how much or how little of this ambient light contributes to the light from electronic flash in your exposure. The reason for this is that the flash fires quickly and then that light source is gone. At that point, the shutter speed is either ending the exposure (if it’s fast) or extending the exposure (if it’s long) by keeping the shutter open to let ambient light continue to hit the digital sensor. (This is discussed more on this page.)
I’m often asked, Does it matter what shutter speed I use, as long as my shutter speed is in sync with my flash? My answer is always the same question: How much of the available light do you wish to record in your overall exposure? While it’s aperture that controls the light exposure emitted by the flash, the shutter speed controls the ambient-light exposure in your composition.
Consider these three examples made on an overcast day in Glacier National Park. With my camera and lens mounted on tripod, I made the first exposure (top, left) of f/11 for 1/45 sec. using only ambient light. After setting my flash for an exposure of f/11, I shot again for 1/45 sec. (top, right). As you can see, the combination of flash and overcast natural light resulted in an overexposure of both the flower and the rock. Sure, there’s a subtle shadow, but it’s an overexposed shadow, because the combination of available light and the light output from the flash was too much.
The third time (bottom), a much more pleasing image resulted when I increased my shutter speed to control the ambient-light exposure. If the correct available-light exposure is f/11 for 1/45 sec., how much of that available light will I see if I set my shutter speed for 1/250 sec.? Again remember, you need as close to a 3-stop reduction as you can get to kill the ambient light. In this case, there’s a bit more than a 2 1/2-stop underexposure (1/45 sec. to 1/250 sec.). Since there’s a 2 1/2-stop difference between that and the correct available-light exposure (f/11 for 1/45 sec.), all of the illumination will be from the output of the flash. The result is the now-correct exposure in which a shadow is apparent against a perfectly flash-exposed rock. Using Nikon’s Commander mode, I held the flash with my left hand above the rock in a twelve o’clock position, which accounts for the shadow seen here in the six o’clock position.
All photos: Micro-Nikkor 105mm lens. Top, left: f/11 for 1/45 sec. Top, right: f/11 for 1/45 sec., Speedlight SB-900. Bottom: f/11 for 1/250 sec., Speedlight SB-900
There is a direct relationship between f-stops and correct flash exposure, while your choice in shutter speed is more indirectly related to correct flash exposure. That is, since a flash’s output duration is instant (milliseconds), the shutter speed doesn’t control the exposure of the flash itself but what happens after the flash fires. Think about it. If your shutter speed is 1/60 sec. and the flash duration is 1/5000 sec., then the shutter remains open for what comparatively is an eternity after the instant burst of light from the flash. This is where shutter speed has its influence, as the images on this page show. This is true whether you’re using a shutter speed of 1 second or 1/125 sec. The flash’s output duration is relatively the same—it’s an instant burst of light that occurs quickly within the time the shutter is open.
(For you photo scientists out there, you’re right: The actual duration of the flash can vary widely. If the flash is at full power, the duration of the flash “spike” might be around 1/1000 sec. Change to 1/4 power, and it will be around 1/2500 sec. And if you go all the way to 1/128 power, the flash spike duration is around 1/40000 sec. Each of these flash duration speeds still fits within the time the shutter is open. Even for superfast shutter speeds like 1/8000 sec., most modern flash units have a high-speed sync mode that keeps the flash spike within the shutter opening time, and there’s more on sync speeds.)
During one workshop I put together this “timeline” of four different shutter speeds and their relationship to correct flash exposure. As you look at these shots, it’s easy to see a clear distinction between each one: The brightness of the sky and the subject (the ambient exposure) decreases from the first composition to the last, yet the foreground flash exposure of my hand’s shadow and the steps remains the same in all four.
How is this possible? As we’ve learned up to this point, a correct flash exposure is 100 percent under the control of the right aperture choice. The correct aperture choice is always dictated (1) by any depth of field concerns you may have, (2) by the flash-to-subject distance (determined by you), and (3) by adjusting the power settings on your flash. A correct ambient exposure, when combined with the use of flash, is 100 percent under the control of the shutter speed—and these four examples really make that point.
With my camera on tripod, I first determined that this scene required a great deal of depth of field, so I chose f/22, and before I even pulled out my flash, I pointed my lens to the sky and adjusted my shutter speed until 2 seconds indicated a correct exposure. I then quickly arranged my composition, and although the steps were really faint and hard to see, I didn’t worry since I was about to light them up with my flash. With the flash at full-power (1/1) and at an aperture setting of f/22, I got a flash-to-subject distance of 9 feet. One of my students held the flash about 1 foot behind my raised arm, and I quickly made the four exposures you see here, changing the shutter speed 1 full stop for each one. All were shot at f/22, which is why the flash exposure is correct and constant, but note the change in the ambient light of the building and sky: After the first exposure of f/22 for 2 seconds, I shot the next at f/22 for 1 second, then the next at f/22 for 1/2 sec., and finally the last at f/22 for 1/4 sec. It’s pretty clear that the role of shutter speed in flash photography is meant to do one thing: control the exposure of any and all ambient light. You get to decide how much or how little ambient light gets exposed—assuming, of course, that you’re shooting in manual exposure mode or engaging your autoexposure overrides when shooting in Aperture or Shutter Priority mode.
All photos: 16–35mm zoom lens, f/22, Speedlight SB-900. Top, left: 2 seconds. Top, right: 1 second. Bottom, left: 1/2 sec. Bottom, right: 1/4 sec.
So shutter speed doesn’t actually control the flash exposure itself, but it does control the ambient light. Putting the use of flash aside for a moment, you know that when shooting in natural light, you adjust your shutter speed and/or aperture to arrive at a correct exposure. And if the exposure for this natural light is wrong, your picture is too light or too dark. Let’s say you’re shooting some flash pictures outside around dusk. Despite the fact that you’re using your flash (which is controlled by aperture), you can still have total control of the natural light. You maintain your ability to control the amount of natural light that exposes the scene with your shutter-speed choice. The longer your shutter remains open, the more ambient light you allow in your exposure (after the flash has fired), making it lighter; the shorter your shutter remains open, the less ambient light you allow in, making the exposure darker.
When you use flash, you now have two light sources: the ambient light and the supplemental light from your electronic flash. Simply put, controlling the ambient light is a task assigned to the shutter speed; controlling the flash exposure falls to the aperture. So again, aperture (f-stop) controls the amount of flash light allowed to reach your camera’s digital sensor (or the film); shutter speed controls the length of time the ambient light is allowed to strike the digital sensor (or film).
How does this affect your photographs? Changing your aperture to make your lens opening bigger or smaller and control the brightness of your flash (i.e., the amount of flash light that reaches your sensor) primarily affects the main subject and anything else within the flash range (see for more on flash range). Changing your shutter speed to control the amount of time that ambient light hits your sensor primarily affects the background and any surrounding elements that are outside the flash range. The slower the shutter speed, the brighter these elements will be; the faster the shutter speed, the darker these elements will be.
Now fast-forward to your son’s next birthday party being held in the basement of your home. You take a picture with the flash set on manual and at full power, and let’s say the aperture is set to f/11. You quickly see that the subjects in the picture are too dark, so you open your aperture to f/8. This bigger hole now lets in more light, and voilà, it’s better. Or perhaps the picture you took at f/11 was too bright, so you stop down the lens to f/16 and take another, and again get a much better exposure. At no time did you change your shutter speed. The flash exposure of your subjects was 100 percent controlled by the right choice in aperture.
In the same scenario, if the subjects were well exposed but the background were too light or dark, then you’re probably not getting the right amount of ambient light in your exposure to illuminate the full composition beyond the subjects. To fix this problem, you would adjust your shutter speed up or down to let the ambient light strike the camera’s sensor (or the film) for the appropriate length of time.
Whenever you combine flash with ambient light, you always have the option of manipulating the ambient light (making it darker or lighter) by adjusting your shutter speed. Here, my friend John was sitting in a small, interior dining area (first image). Outside and behind him, the Stars and Stripes had been woven into a cyclone fence. When I checked the distance scale on the back of my portable electronic flash, it indicated an aperture of f/11 to render a correct flash exposure of my subject. This setting was based on the distance between John and the flash (which I had set on a nearby table). With the aperture set at f/11, I adjusted my shutter speed until the camera’s light meter indicated 1/125 sec. as a correct exposure for the bright, frontlit flag. But I decided to not turn on the flash for the first exposure (top). I simply shot the ambient-light exposure. As you can see, this resulted in a perfect exposure of the flag; however without any flash, John was a stark silhouette because the ambient-light levels surrounding him were nowhere near as bright as the light outside shining on the flag.
If I want to render the flag darker, I underexpose it 1 stop by shortening my shutter speed to 1/250 sec. (center). To render the background lighter, I overexpose it 1 stop by lengthening my shutter speed to 1/60 sec. (bottom). As you can see, the background in those shots has changed, yet the flash exposure on John’s face remains correct and constant. Why? Because the shutter speed controls only the ambient light, and the aperture controls the flash. Since I only manipulated the shutter speed and not the aperture, the flash exposure on John’s face remained the same. Simple enough, right? If my hunch is correct, this is one of those “aha!” moments in your journey toward understanding electronic flash.
Top: f/11 for 1/125 sec. Center: f/11 for 1/250 sec., Speedlight SB-900. Bottom: f/11 for 1/60 sec., Speedlight SB-900
