Balancing Strobe and Ambient

Balancing various light sources with strobe or continuous lighting 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 augment 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. The background was basically dark with very little detail, but there was a nice pink light in the sky. I had a JTL Mobilight 300 (product not recommended! Junk IMO) 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. Adding some red with the orange would have brought the strobe light color closer to the background color, but time was running short.

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 exposure in the camera was set at a -1 underexposed to darken it bit since I didn’t want the sky to bright. It also increased the saturation in the 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), hand held, ISO 100, Canon EOS 1Ds and a 24-70mm 2.8 lens. Here is a description of the process to balance them all together.

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 is correct for the amount of light coming from the flash, you will get a properly exposed object illuminated by the flash regardless of what your shutter speed is set at. That is of course, as long as the shutter speed is not overexposing.

Here is why; if you were in a completely dark room and the correct exposure was f4 for the strobe, you could set the shutter speed for 1/50th. 5 seconds, 1/200th of a second, or just about any other setting. 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 you are in a completely dark room, and your f4 aperture at whatever shutter speed you selected will be underexposed.

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), and not effect the exposure of the lighting falling on your subject from the strobe.

In the image above, the flash exposure required was an aperture setting of f4 to properly expose the subject with the light from the strobe. 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. 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.

The creative fun now starts. We know if we shoot at 1/15th at f4, the background lit by the ambient lighting will be properly exposed, and if at f4 the subject will be properly exposed. The aperture will stay locked in at f4 since changing that will change the exposure on the subject. But we can vary the shutter speed and change the background without affecting the light falling on the subject by the flash as long as we don’t overexpose the ambient lighting. So we can play with the shutter speed setting from perhaps a stop or so overexposed, to 5 or stops unexposed. If we open up the shutter speed to 1/8th, 1/4, or so, we will start to see the background lighten up. If we start to go toward underexposure with 1/30th, 1/60th, or 1/125th, we will start to see the background underexposure and go darker. Generally, about 1 stop underexposure works well, making the subject stand out much better and giving the background a darker and more dramatic look.

So in summary, once you set the aperture that’s 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 exposure for the available lighting that prevails.

The image above is another shot of the same basic lighting setup, but used in a different environment. I used a more balanced exposure between the background and the subject. My assistant was holding a 580EX strobe in a medium softbox and following the model around at a set distance to keep the exposure on target. If I wanted a little more kick from the strobe I would ask him to move in a bit closer, or if I wanted more of a fill effect, I would have him more further away.  We were shooting on the street in Times Square in NYC, and didn’t have the luxury of using light stands. My assistant was a perfect mobile light stand with built in intelligence!

Another example of mixing available light, strobes, and continuous 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.

If you want to read another post on this technique, click here-(Sunlight and Ambient How To)

By the way if you haven’t already noticed, all of these images are of the same model. I’ve worked with her frequently over the years on assignments and for testing purposes.