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Dragging the Shutter Part One: The Step-by-Step Guide to Balancing Flash and Ambient Light

Dragging the shutter is a technique that balances the exposure of strobe and ambient light sources in one photo. For example: taking a photo of someone outside at night by using a portable flash and also capturing the city lights in the background.

Balancing Strobes and Ambient Light

Strobes put out a powerful amount of light so, in order to balance with the ambient light, we need to use a long shutter speed, hence “dragging the shutter.” There is quite a lot to know about this technique because it involves color balance as well as exposure balance. Some cameras have a function called “slow-sync” that works with a dedicated flash on TTL and automatically reads and figures the proper shutter speed. But with a little bit of understanding, you can effectively use this technique with any camera and flash in manual, not just the dedicated flashes on TTL.

The photo that is used for this article is of my husband, Lee Varis, in a gallery. The ambient on him was terrible, but the spot lights on the photos were interesting and dramatic. It was necessary to light him both for balancing exposure and also for flattering light.

The first part of this technique is having an understanding of how flashes work. Since flashes put out a good amount of power in a very short time, our exposure technique is different from ambient. The flash duration can be 1/1000 of a second or shorter, so different shutter speeds will not change the exposure of the flash. We control the exposure only with the aperture. Most small format cameras have focal plane shutters, which consist of two curtains, one travels across the sensor, opening it to light, and the other follows, covering it up, kind of like a scanner. The flash is triggered when the first curtain reaches the other side of the sensor.

Understanding Sync Speed

What is important to know is your camera’s “sync speed.” Your sync speed is the fastest shutter speed you can use that when the first curtain reaches the other side, and the flash goes off, the second curtain has not started to cover the sensor. When so flash fires, the entire sensor is open and exposed all at once. If you use a shutter speed faster than your sync speed, you will get a black line on the edge of the frame since the second curtain has started to cover the sensor. Most cameras’ sync speed is around 1/200; some are faster, some are a bit slower.

First, you need to find out what the ambient light exposure is, so with the flash turned off, take a manual meter reading in your camera, select an f-stop, then turn the shutter speed dial till the meter reading is correct. Here’s what the scene looked like with just ambient light.

Ambient light only
ISO 200 f 5.6 ss 1/20 White balance on daylight

There are two problems with this image: the lights in the background are tungsten, so the color is orange, and the subject is almost a silhouette.

To calculate the flash exposure,  pick the same f-stop as you did in the ambient exposure. Set the flash on TTL and the flash will put out the correct amount of light to achieve that aperture. If you use the sync speed of your camera as the shutter speed, the background will go dark. And you will get this result.

Flash only
ISO 200 f 5.6 SS 1/180 white balance on daylight

Lighting with Multiple Sources

By looking at these step by step photos, you can see that different parts of the sensor are illuminated by two different light sources. The flash is only lighting the subject, not the background, and the background lights are not hitting the front of the subject, so there is no lighting crossover.

You must also be critically aware of how much ambient light is falling on your subject.  If there is the same amount of light falling on your subject as in the background and you then add the flash on the subject, the subject will be overexposed.

The next step is combining the two sources, so the flash is on and set to TTL, the camera settings are f 5.6, and the SS is 1/20. Now we get this image.

Flash and Ambient
ISO 200 f 5.6 SS 1/20 White balance on daylight

Getting Rid of Color Casts

Okay! Great progress! However, we still have the color problem to solve. Flash is the same color as daylight, so it’s bluish, and tungsten is orange light. If you have two different (or more) colors of light sources, you must do something to bring all the light sources to the same color temperature. You can either put a blue gel, called a CTB (color temperature blue) on all the spotlights, or put an orange gel, called a CTO (color temperature orange) on the strobe light making it tungsten. Guess which is easier! Correct, one CTO on the flash.

There are still a couple of steps left. First the gel blocks light, so you need to increase the power of the flash by one stop if the flash is on manual. If you are on TTL, the flash will do it automatically. Then change the white balance on the camera to Tungsten (the light bulb) and everything will be correct!

The top photo is shot at ISO 200 f 5.6 ss 1/20, white balance on tungsten

In Part Two, we’ll cover some creative techniques to use with dragging the shutter so you can achieve some amazing images!

Be sure to check out my upcoming photo workshops and travel tours:

http://www.bobbilane.com/Photo-Education/Workshops/1

Gear Used:

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