This I Learned: 2 Exposures, 1 Picture

This is the first in what will hopefully be an on going series of posts about what I think are critical technical concepts for good photography. Photography, like many of the arts is actually mix of technical competence and creative talent. To be a good photographer one needs to be able to see the image before it's captured. This is where your creative side comes in. Putting all the elements together in your mind's eye and coming up with that one fantastic image. Once you have that you now must call on your technical abilities to create that image with your camera.

This series of posts addresses the technical part of photography. I hope to pass on to others things that I learned which took me to a new level in my photography. Things that made that little light bulb go off and re-kindled my passion for this complex art form.

Contrary to what the title of this post might indicate, it's not about double exposure. It's about flash photography. I always hated the look of flash photography and thought that all those great images on books and magazines must have been done with natural light somehow, because flash just made the picture look like crap. As I started learning more about flash, I realized that my pictures looked like crap because I just didn't know how to use my flash. When I finally understood the concept I will discuss here, it was like someone had opened up a door to whole new world for me.

Hopefully this will open a new door for you too. Read on to learn about the 2 exposures you get when using a flash.

How Your Camera Works

In order to understand this concept we must first understand how the camera works. Your camera works by exposing a sensor or film to a certain amount of light for a certain period of time. When the shutter button is pressed, the lens will "stop down" and make a certain size opening to allow light to hit the sensor. The bigger the opening, the more light hits the sensor. The next thing that happens is the shutter opens up exposing the sensor to this opening in the lens. The shutter will then close after a certain period of time to end the exposure.

The size of the opening in the lens is your aperture and the length of time the shutter is open is your shutter speed. For any given scene, there is a total quantity of light that must hit the sensor for a correct exposure. There are however many different combinations of aperture and shutter speed that will give you the same exposure. If, for example, you pick a large aperture like f/ 2.8 the correct exposure for a given scene might be at 1/250 second shutter speed. If you then choose to change your aperture to a smaller value like f/ 8 for example, you will have to expose the sensor longer to get the same amount of light. You will need to shoot at 1/30 second.

The actual numbers are not important at this time. The concept that needs to be clear is that you can get the same amount of light exposure on the sensor through several combinations of aperture and shutter speed.

Sync Speed

Another key concept in flash photography is your sync speed. The sync speed is a property of a given camera model and it has to do with how your shutter works. The shutter has 2 curtains that block light from reaching the sensor. When the shutter button is pressed, the first curtain opens to expose the sensor. After the specified amount of time, the second curtain closes behind the first. To achieve very high shutter speeds, like 1/4000 second or 1/8000 second, the second curtain starts closing before the first is fully open. This forms a sliver of an opening that travels the length of the sensor. The maximum sync speed of a given camera is the maximum shutter speed where the first curtain fully opens before the second curtain closes. Maximum sync speeds are tipically between 1/200 and 1/500 second.

Flash Duration

The flash duration is the amount of time your flash is on when it fires. A typical flash will charge up its capacitor fully and then discharge it when the flash is fired. The way to get less power out of a flash is to abruptly cut off this discharge when the desired flash power level is reached. Because of this, lower power settings on your flash will have shorter flash duration. Even at full power, flash durations tend to be very short, on the order of 1/1000 second or less. The Nikon SB800 for example has a flash duration of about 1/1000 sec at full power and 1/6000 sec at 1/8 power.

The actual duration of the flash is important for some more advanced flash topics. For the purpose of this discussion, the important thing to note is that the flash duration is about 4 times shorter than the typical sync speed of 1/250 second. What that means is that the shutter speed of the camera cannot be used to limit the flash exposure on the sensor. The flash goes on and off much faster than the shutter opens and closes.

The Two Exposures

The two exposures I was talking about are the one due to a flash and the one due to the ambient light. We just saw that shutter speed does not influence the flash exposure, so the only way to control the amount of light created by a flash is with the aperture setting. 

We now have everything we need to understand this concept of two exposures. Since the shutter speed is irrelevant for the exposure caused by a flash, it can be used to control the exposure of the ambient independently. Here is the technique:

1. Find an aperture and flash power level combination that gives you the correct exposure for your subject.
2. Change your shutter speed to control the exposure level of the background due to ambient light.

It's really quite simple after you have tried it a couple of times.

In Practice

The image at the top of the post was shot using exactly this technique. It was an early morning shoot at the beach just before sunrise. I wanted to place the model in a circle of light. I also wanted her to really stand out, so I wanted the background to be very dark but still recognizable. I picked an aperture that would give me enough depth of field to keep the background in focus. I then set the flash power to expose the model correctly. I used 2 flashes for the shot since the overhead one created dark shadows in her eyes. The second flash was in front of her and pointing to her face. Once the flash exposure was set, I picked a shutter speed that would underexpose the background by some 2.5 stops. That's it. I got the shot that was in my head.

Conclusion

This concept of using a flash to control the exposure of your subject and the background independently is, in my opinion, the most critical concept in flash photography. A good grasp of this concept will open endless doors for you and your photography. It allows you to fully control the image and place emphasis where you want. Make your subject 'pop' by contrasting it against the background. Add drama and mystery to your images. You will be limited only by your creativity.

Although there are many different flashes out there including studio strobes and they all work a little differently, unless you are using continuous light (light that is always on and does not "flash"), this concept works. All flashes have a much shorter duration than your maximum sync shutter speed, and therefore allow you control the flash and the ambient exposure independently.