5. Adding Flash to the Exposure
So far, our exposure endeavors in this book have been mostly outdoors. Our modern lives take place mostly indoors, though—so even though we should strive to be embraced by the great outdoors and take some nice photos while in it, being able to produce a faithful portrayal of indoor life is a necessary skill. Happily, the principles used in making exposures indoors are much the same as those used outdoors. However, indoor lights are generally much dimmer than the powerful light of the sun. Even during the night, when indoor spaces are lit with artificial light, our rooms never approach the luminance level of sunlight outdoors. This means that our major concern will be setting the ISO and maintaining an operable aperture and shutter speed range. In this chapter, I’ll walk you through some common indoor photography opportunities—and techniques for rising to the challenge!
“Happily, the principles used in making exposures indoors are much the same as those used outdoors.”
Image 5-1. A window light portrait of an Indian bride.
Image 5-2. An editorial image made with on-camera flash and window light.
“When the lighting is insufficient, it’s time to consider using a tripod or to augment the existing light by adding flash.”
For indoor photography with ambient light, we are usually looking at using high ISO settings (800 or up), wide apertures like F2.8 (which means lenses with maximum apertures of F5.6 to F4 are not good choices), and the fastest possible coordinating shutter speeds. When the lighting is insufficient for these settings to be workable—if you’re being forced to use ISOs that create too much noise or such slow shutter speeds that even small subject movements are a blur—it’s time to consider using a tripod (if feasible) or to augment the existing light by adding flash.
Images 5-1, 5-2, and 5-3 are three examples of images taken indoors—under very different circumstances.
Image 5-1 is a candid image of an Indian bride getting dressed for the ceremony. In order to turn off my flash and use just the window light (often the best light for natural portrayals of indoor scenes), I had to turn up my ISO setting to 1600. You can see result of this trade-off; the noise in the background is quite pronounced.
Image 5-2 was an editorial shot for a food critic. Although this is not a polished still life done in the studio (something beyond the scope of this book), it was not made in a hurry or under the threat of these delicious sausages being devoured, either. With extra time, more effort can be spent on ensuring the best light. Here, an on-camera flash was used to enhance the window light. This technique will be discussed later in this chapter.
Image 5-3. A commissioned image made by firing flash through a window to augment the natural light.
Image 5-3 was a commissioned image. The lighting was carefully arranged by placing a remotely fired flash outside the window to enhance the rather weak sunlight in the dark clubhouse. With that re-enforcement from the flash, the light was bright enough for me to keep the ISO at 200, maintaining the highest possible image quality.
In the first chapter of this book, the concept of exposure was explained using a water and bucket analogy. Let’s revisit that as we consider the concept of flash exposure. Image 5-4 compares the two types of light source: continuous light and flash. The continuous light (such as sunlight or light from a bulb) is like rain that keeps falling into the bucket. The light from the flash, on the other hand, is more like a short, controlled burst of water from the faucet. When an exposure is made by mixing flash and continuous light, we often call the latter the “ambient” light, in order to differentiate it from the flash.
Continuous Light
Light, from sources like the sun or a light bulb, that falls on the subject in an uninterrupted stream.
Ambient Light
The light existing in the scene (as opposed to light added by the photographer).
Flash
High-intensity light that is emitted in an instantaneous burst. Also the name for a device capable of producing such a burst of light.
Image 5-4. Returning to our water analogy, we now need to consider two types of streams: continuous (ambient light) and instantaneous (flash).
Image 5-5. The flash capacitor is analogous to a water reservoir.
Image 5-6. The metering system controls the flow so the bucket is perfectly filled from the two sources.
A flash’s burst of light is supplied by its capacitor. You can think of this like the tank of water that supplies the faucet. Just as the faucet’s output is limited by the size of the tank that supplies the water, the flash’s output is limited by the size of the capacitor, as in image 5-5. When the faucet is turned on, the water in the tank quickly pours into the bucket.
Just as in the ambient-light-only examples we considered back in chapter 1, the goal is still to fill the bucket up perfectly (symbolizing a correct exposure)—without the water overflowing (overexposure) or falling short of the rim (underexposure). With the faucet added to this operation, however, the filling calculations become more complicated. Fortunately, the camera has another TTL metering system for the flash. This is used to detect how full the bucket is and when to shut the faucet off, as seen in image 5-6. With flash, this process takes an astonishingly short time. Nikon’s SB-900 flash, for example, is rated at 1/880 second at full power; that is how long it takes to dump the whole capacitor. At any setting below full power (meaning only part of the capacitor has to be dumped) it would take even less time. Think about this from the water model perspective: clearly, releasing the whole tank takes longer than releasing some fraction of it.
“Did you notice that the burst of flash is substantially faster than any of the shutter speeds we’ve used in this book?”
Did you notice that the burst of flash is substantially faster than any of the shutter speeds we’ve used in this book? This tells us something very important: that the shutter speed isn’t a factor in controlling the flash exposure. The only thing the shutter needs to do is be fully open when the flash fires.
Modern DSLRs use what’s called a focal plane shutter. As noted back in chapter 1, on today’s digital cameras, this is sealed in the camera and obscured from view. In the film days, however, the everyday task of loading the film made the shutter visible. Since today’s shutters have similar structure, let’s look at image 5-7—the shutter of my first camera, a Nikon FE. It consists of two moving parts, called curtains.
“The fastest shutter speed at which the whole sensor is simultaneously revealed is called the maximum flash-sync speed.”
When the shutter release is clicked, the first curtain opens up to start the exposure. When the set shutter time has arrived, the second curtain rushes to close it up and stop the exposure. It takes time for the first curtain to open and the second curtain to close. Therefore, when the shutter speed is set above certain threshold, the second curtain can’t wait for the first curtain to fully open before it starts closing—if it did, it would be too late (the exposure would be too long). This is illustrated in image 5-8.
This becomes especially significant when the shutter needs to coordinate with the extremely quick burst of flash. The burst needs to take place when the whole sensor is exposed, as in the middle stage of the “slow shutter” scenario in image 5-8. The fastest shutter speed at which the whole sensor is simultaneously revealed (the fastest shutter speed at which you can use flash) is called the maximum flash-sync speed. The exact value of this setting varies from camera to camera but is generally in the 1/200 to 1/320 second range on current models.
Image 5-7. The shutter on a Nikon FE.
Image 5-8. At the maximum flash sync speed (slow shutter), the shutter curtains are both completely clear of the sensor.
Image 5-9. Shutter speed of 1/320 second on a camera with a maximum flash sync speed of 1/250 second.
Image 5-10. Shutter speed of 1/500 second on a camera with a maximum flash sync speed of 1/250 second.
You can use flash at that shutter speed or any slower shutter speed (which will allow more ambient light to factor into the image but will not impact the flash exposure), but you must avoid shooting at shutter speeds faster than your camera’s maximum flash-sync speed. If you do, the second curtain will block part of the sensor as the flash fires and that part of your frame will be not be exposed. Images 5-9 and 5-10 show examples of such mishaps. These two shots, taken on a Nikon D3 with a maximum flash-sync speed of 1/250 second, were erroneously exposed at 1/320 second and 1/500 second respectively. The dark areas you see at the edges of the frame are the shadows of the second curtain, on its way to close the shutter. The blurriness of its front edge is actually motion blur caused by the moving curtain when the flash fired. Yes, it moves quickly indeed.
I am glad to report that it is impossible to make errors like this when the flash is fully controlled by the camera. This reassurance occurs when you are using a compatible flash model (by your camera’s manufacturer or a third party) that can communicate with your camera. When these models are used, the camera senses the presence of the flash and automatically limits the highest shutter speed to the maximum flash-sync speed.
Does this mechanism rule out mistakes and solve all our problems? Not really. (Did I mention trade-offs several times in this book already? In the world of photography, one solution almost always causes another problem. Sometimes it seems like we can never win!) Limiting the highest shutter speed to the maximum flash-sync speed can be a risky business. The risk is that the ambient light might overexpose the shot.
That’s exactly what happened in image 5-11. To reduce the contrast, I wanted to use my flash to add fill light on the violinist who was standing under the shadow of a beautiful tree. At the maximum flash-sync speed, this admitted way too much brilliant sunlight. To compensate, I reduced the aperture by 3 stops. That successfully kept the ambient light in check, as seen in image 5-12—but at this aperture setting, the flash didn’t have enough power to illuminate the subject adequately, causing her to be too dark. Ultimately, a postproduction fix in Camera Raw saved the day, resulting in image 5-13. (We’ll look at Camera Raw briefly in chapter 6. For a more comprehensive study on that topic, please read my book Unleashing the RAW POWER of Adobe Camera Raw (Amherst Media, 2011).
“Does this mechanism rule out mistakes and solve all our problems? Not really.”
Image 5-11. At the maximum flash sync speed for the flash, the aperture admitted way too much ambient light.
Image 5-12. Reducing the aperture cut the ambient light, but didn’t leave the flash with enough power to light the subject adequately.
Image 5-13. A postproduction fix in Adobe Camera saved the day.
So, we’ve established how flash can give us the needed light to conduct photography indoors—and how the TTL metering system can help balance the flash/ambient exposure. Still, it is hardly an ideal light source. It’s a small source, so the light from it is very harsh. And when mounted on the camera’s hotshoe, it fires straight at the subject. This makes the lighting very flat; it doesn’t produce the nice shadowing seen with more directional light sources. Fortunately, there are some approaches we can use to improve the look of the flash in our images, while still maintaining the advantages flash provides in terms of exposure.
To make the light from flash softer, and to allow it to better blend with the ambient light, we have to figure out a way to enlarge the light from the small flash head. (Remember back in chapter 4 how we covered that larger light sources producing softer light?) Unfortunately, the diffuser caps and other accessories the vendors claim to be the best invention to beautify flash really don’t make much of a difference. If you take a look back at image 4-10, you’ll see that the size difference needed to transform a hard light source into a soft light source is pretty big; you have to make the light source a lot larger to have an appreciable effect. Changing the flash head from 2 inches wide to 6 inches wide with a $50 accessory . . . well, it’s just $50 wasted.
You’re much better off spending that $50 on a better flash to begin with—one that can be twisted in a full range up and down and side to side. This allows you to take advantage of one of the most powerful flash-softening techniques around: bounce flash. When using bounce flash, you direct your flash away from the subjects (so none of that harsh direct light hits them) and toward a nearby wall or other surface. As the flash spreads out, it hits a broad part of that surface, continuing to spread out and get bigger as it reflect off that surface and onto your subject. Effectively, the large bounce surface replaces the tiny flash as the main light source on your subject—and the result is big, soft lighting.
Images 5-14 and 5-15 illustrate how the bounce flash technique is implemented. In image 5-14, the flash head was turned 90 degrees upward to bounce the light off the ceiling. In image 5-15, the flash head was turned to the right 90 degrees and up 45 degrees to bounce the light off the wall. In both cases, the size of the light is enlarged dozens, if not hundreds, of times. Now we are in business!
Image 5-16 illustrates how the flash emits its light. It is important to know that the light does not stay focused in a sharp beam like a spotlight. Instead, it scatters out from the flash across the full 180 degree view it faces—with most of its strength near the center line. When the flash is to be bounced, it is important that the direct light from the flash does not partially fall on the subject, creating uneven lighting. Keeping this in mind, the twist of the flash ahead away from the subject should always be at least 90 degrees.
“Effectively, the large bounce surface replaces the tiny flash as the main light source on your subject—and the result is big, soft lighting.”
Image 5-14. Bouncing the flash off the ceiling.
Image 5-15. Bouncing the flash off the ceiling and wall.
Image 5-16. Light is emitted in all directions, not as a constrained beam.
Image 5-17. Flash head turned 90 degrees to the right.
Image 5-18. Twisting the flash head to the left, beyond the 90 degree point.
Image 5-19. Tilting the flash up to 90 degrees.
Image 5-20. Swiveling the flash 180 degrees to the left and then adjusting the tilt for backward bounce flash.
Image 5-21. Flash positioned as if on a camera taking a landscape-orientation shot.
Image 5-22. Flash positioned as if on a camera taking a portrait-orientation shot.
“When turned to portrait orientation, the same angle now points the flash up and to the left.”
Bearing this in mind, images 5-17 through 5-20 illustrate different ways to twist the flash head of a Nikon SB-800 (most flash heads twist in the same way). Image 5-17 shows the flash head turned 90 degrees to the right; it reaches a limit there. Image 5-18 illustrates how the twist to the left can go beyond 90 degrees—all the way to 180 degrees. Image 5-19 shows how twisting the flash upward is similar to twisting it to the right; it stops at 90 degrees. However, if the 180 degree turn in image 5-18 is performed first, then the head can cover a full range of angles behind the camera, as illustrated in image 5-19.
Images 5-21 and 5-22 show a useful angle to stay in as a default setting: 90 degrees to the right and 45 degrees up. With this, a landscape orientation image points the flash head up and to the right—good for the wall and the ceiling bounce. When turned to portrait orientation, the same angle now points the flash up and to the left, delivering the same type of bounce light from the left of the camera.
Images 5-23 to 5-25 (next page) show how much using this bounce technique improves the look of the lighting with on-camera flash. Image 5-23 was shot with the flash head pointed right at the subject. Image 5-24 was shot in the same spot with the flash head turned 90 degrees to the right and up 45 degrees (as shown in Image 5-21). Compare these two images. There are several problems with the direct-flash image. Under her back and behind her thighs there are sharp shadows created by the harsh light from the small flash head. Her skin looks rough and even her garments don’t look good—as if they are overworn. All these problems are solved by a single twist of flash head. Image 5-25 was shot with a different flash head angle; this time it pointed backward (as was shown in image 5-20). With this angle, the light comes from the same direction as the camera, so it produced equal brightness on the left and right sides of the subject. This is evident on her arms.
Image 5-23. Flash directed at the subject.
Image 5-24. Flash bounced up and to the right.
Image 5-25. The flash bounced off the wall and ceiling directly behind the camera.
For this exercise, you’ll be shooting indoors with bounce flash. Attach the flash to your camera’s hotshoe and set it to TTL mode. If you have a Nikon, shoot in the aperture priority (A) mode. For a Canon, use the manual (M) mode and keep the metering in the negative side. Set the aperture at the maximum and use an ISO setting of 400 or higher.
Experiment with taking a few shots. As you do, you might notice that the light from the flash isn’t as strong as you might have expected. When the light is bounced, it is scattered, absorbed, and blocked. It’s totally possible that the light reaching your subject is less than 10 percent of the light you’d get if the flash were pointed directly at the subject. This calls for an increased flash output setting—at least 4 stops above what you might use when pointing the flash directly at the subject.
While shooting, get a sense of how fast the flash recharges, how often the flash fails (in other words, it does not provide enough flash and your image is underexposed), and how often the AA batteries need to be replaced. Check the impact of varying the ISO (from 400 up to 1600) and the aperture (F2.8 to F5.6). (Note: As mentioned earlier in the chapter, an F2.8 lens is just about essential when using bounce flash. If you don’t have such a lens, something else has to give: use a higher ISO or invest in more AA batteries for frequent replacement). Also, explore the power/ exposure differences achieved by changing the distance between the flash and the bounce surface—as well as the distance between the bounce surface and the subject.
To vary the aesthetics, twist the flash head to different angles, using the guidance provided earlier in this chapter. You’ll find that some bounce angles produce nicer results than others—and that the best angle will vary from location to location. For example, a vaulted ceiling with dark green paint does not make for a good bounce surface at all; too much light will be absorbed and what does return to the subject will pick up a green color cast. Conversely, a mirror is also not a good bounce surface. At best, it provides the effect of relocating the flash to a off-camera position; it does not scatter and enlarge the size of the light (see image 5-26). Feel free to try though; it is perfectly okay to make mistakes!
Image 5-26. Bouncing flash into a mirrored surface does nothing to soften the light; it only changes its direction.
Another common usage for flash, especially outdoors, is as a fill light. This is a convenient way to brighten the shadows (reduce contrast) without the help of an assistant to position a reflector. In my opinion, the sunlight modified with a diffuser and reflector still beats flash fill—but when there is no extra hand available, flash has to do.
To do this, set the camera and flash as you did in the bounce flash exercise, but turn the ISO down to 200 or 400 and twist the flash head to point at the subject. For comparison here, I made some example shots outdoors. These were shot on a day that was sunny, but not completely clear; there was a thin layer of clouds, so the sunlight was mildly diffused and weakened. This might turn out to be a good day to use flash fill.
Image 5-27 was shot without flash, as a baseline comparison. This image shows highlights on the camera left side, telling us the position of the sun. The shadows are not dark but the image is lacking light on the eyes and lips. It also has a white balance discrepancy between the highlight and the shadow areas; the highlights are pretty much neutral but the more scattered blue rays in the shadow areas are making those tones look a bit too blue.
Image 5-27. Portrait shot with the ambient light only.
Image 5-28. Adjusting the flash exposure compensation.
Image 5-29. Flash exposure compensation at 0.
Image 5-30. Flash exposure compensation at –1.
Next, I turned the flash on. Knowing that I just wanted to fill the shadows—not totally overwhelm the sunlight—I needed to consider how much light I wanted to add with the TTL flash. To do this, I adjusted the exposure compensation settings. Image 5-28 shows how the flash exposure compensation is set on Nikon’s SB-800. When the flash is set in TTL mode and connected to the camera, you simply press the + or – on the selector and the setting will change accordingly, as shown here in the red circle. This increases or decreases the exposure (from that suggested by the TTL metering) in increments as small as ⅓ stop.
“I needed to consider how much light I wanted to add with the TTL flash.”
Exposure Compensation
A setting used to manually override the settings determined by TTL metering, providing more/less light than suggested by the camera.
Image 5-29 was shot with an exposure compensation setting of 0. At this level, the flash overpowered the mild winter sun through the thin clouds. The face looks flat and a harsh shadow is evident; this calls for reduction in the power of the flash. Image 5-30 was shot with the flash compensation set at –1. It’s a modest improvement, but not free from the problems of the previous image. Images 5-31 and 5-32 have –2 and –3 compensation settings, respectively. The best results seem to be among those two shots. Image 5-31 has satisfactory highlights on the eyes, but the fill seems to be slightly too much. In image 5-32, the fill is slightly weaker, but it still manages to bring the white balance in the shadows into better balance with the highlight tones on her skin.
Image 5-31. Flash exposure compensation at –2.
Image 5-32. Flash exposure compensation at –3.
Image 5-33. This overexposure resulted from the camera automatically choking down the shutter speed to the maximum flash sync speed.
Image 5-34. When the amibent light is mild (left), the flash can compete with it. When the ambient light is very strong (right), the flash becomes insignificant.
As you’re adjusting the exposure compensation, pay attention to the maximum flash-sync speed. If the speed reading showing the maximum flash-sync speed is blinking, you have exceeded the limits. Try it—make a mistake. If you have “successfully failed,” your shot should look like image 5-33. Here, the camera choked down to the maximum flash-sync speed. This was not fast enough for the amount of ambient light in the scene, so the image is overexposed.
This problem shows the limitation of flash as a fill light. When the ambient light is too strong, using a flash for fill light is like asking an ant to wrestle an elephant. It is not the photographer’s fault; our flash just isn’t up to battling the almighty forces of Mother Nature. Image 5-34 illustrates the problem. When the flash is stronger than the ambient light, the amounts of light they contribute to the exposure are comparable; this makes the fill light a success. When the ambient light is way stronger than the flash, the fill light from the flash becomes insignificant.
“When the ambient light is too strong, using a flash for fill light is like asking an ant to wrestle with an elephant.”