Balancing Strobe and Ambient

Balancing various light sources with strobe or continous ligthing is a technique I use often in my photography. I routinely mix flash and available light, reflectors, mixed color sources and types, to get the look I am after. Sometimes it is just that the ‘found lighting’ is a great start, and I just need to agument that with strobe or reflectors to balance the image. Other times I purposely mix up the sources to get a particular look I am after in the studio. There is unique look and feel that comes from properly mixing these sources.

This image was taken behind the beach house at around 8pm. I had a JTL Mobilight 300 positioned to the left and behind the model with a orange gel taped to the front to give the warm highlights along the left side of her body. This light was set to overexpose the main light by about 1/2 stop over the main light, or +1/2. The orange gel was also selected to match the natural sunset happening behind her.

The main light was another JTL Mobilight 300 in a medium softbox and positioned to the right of the camera at about a 45 degree angle from the model. This light was the basis for the normal exposure value, or +0.

The background was set at a -1 underexposed to darken it, as well as bring up the saturation in they sky which was a natural sunset. I didn’t do any photoshop tricks to the lighting, exposure, or color. I did soften and smooth the skin, added a touch of sharpening, and some minor levels adjustments. The actual exposure was f4 (based on the strobe) at 1/30th (for background), handheld, ISO 100, Canon EOS 1Ds and a 24-70mm 2.8 lens. Here is how it works.

The duration of the flash is somewhere around 1/1000th of a second. So whether you are shooting at 1/60th of a second or 1/250th of a second, it doesn’t matter for anything that is exposed by the flash. As long as the aperture was correct for the amout of light coming from the flash, you will get a properly exposed object illuminated by the flash.

If you were in a completely dark room and the correct exposure was f4 for the flash, you could set the shutter speed for 1/50th of a second or 1/200th of a second. What will really happen is the shutter will open, the flash will fire at 1/1000th of a second (or whatever the appropriate speed is for that flash unit), and the shutter will remain open until the shutter time expires. But since the flash fired for a total of 1/1000th of a second and output the proper amount of light, the shutter remaining open longer doesn’t add anything to the overall exposure. Because of this, as long as the shutter speed is below the maximum shutter sync speed your camera supports, you will get a properly exposed object illuminated by the flash unit.

But now what happens if you aren’t in a completely dark room? Well, it gives you some more creative tools to play with. Since the flash level will remain the same no matter if you are shooting at 1/250th, or 1/15th of a second, you can vary the shutter speed anywhere between the longest exposure time your camera supports, and its maximum sync shutter speed (typically around 1/250th or 1/500th of a second).

When taking the image above, the flash exposure I set required an aperture of f4. When I measured the ambient light to see what shutter speed I would need when using f4, it showed the image required a shutter speed of about 1/15th for a properly exposed image. If I shot the image using 1/250th at f4 which is the higest supported flash sync speed, the background would be underexposed considerably and Olga would be a peoperly exposed person against a drak black backround.

But adjusting the shutter speed to 1/15th at f4, she would be properly exposed by the flash at f4, and the background would be properly exposed at 1/15th at f4. The image would be completely and evenly balanced for both the background and the subject illuminated by the flash. To add a bit more drama, I purposely underexposed the background by about a stop, thus making her stand out much better and giving the background a darker and more dramatic look.

So in summary, once you set the aperture thats appropriate for the flash, you can then freely adjust the shutter speed to lighten or darken the background simply by pretending you don’t have a flash in use, and adjusting it either above or below a normal expsoure for the available lighting that prevails.

The image on the left is another shot of the same basic lighting setup. I used a slightly slower shutter speed to bring more exposure in the background and make it a bit lighter. Remember that the aperture was the primary exposure control on anything hit with flash, while the shutter was the primary control on anything hit with available light.

Another example of mixing available light, strobes, and continous lighting. In this image below, the strobes were balanced with the available lighting, as well as balancing the halogen lighting from the BMW behind her. Thus all three light sources need to be adjusted to register properly during the exposure without over or under exposing and correcting for the color balance as needed. I’ll dive into how to do that in another post in the near future. However, the principals are the same, but there is a definite technique to being able to dial in the lighting quickly and accurately.

~ by sfaust on July 14, 2007.