Mixing Window Light and Artificial Light

Often, while shooting in interior locations, I find myself needing to mix natural light, coming through windows, with artificial light. Everyone from editorial shooters to photo-journalists to wedding photographers (and more) often find themselves in the same situations.

Photographically, window-light can be a wondrous and beautiful thing! Often though, there's not enough of that window-light filling the model unless she's posed quite near to the light-producing window and in such ways that the natural light fills, uhhh.... wonderfully and naturally. This can be tricky and sometimes limiting. (In terms of posing and blocking the model.) Many photographers, of course, use reflectors to bounce back the light coming through the window, thus providing appropriate fill. But when you're in a hurry, using reflectors can be time consuming and require assistance. For those reasons, i.e., since I'm usually in a hurry and I rarely have an assistant, I turn to my strobes for the easiest and quickest solutions.

First, I need to know what exposure the background--the light outside the window--is reading. To accomplish this, I take an ambient light reading and that becomes my starting point for lighting the model. (Note: Since I'll be using flash-sync, I'll need to shoot using an appropriate shutter speed: That means a shutter speed of less than 200th with my Canon 5D.) Generally, I shoot at low ISOs. This means I need to find an exposure that balances ISO, shutter speed, and an aperture that makes sense for the image. If you don't have a light meter and you're serious about photography, shame on you. But, if you don't, you can still determine the outside light's exposure by bracketing with a few shots, reading the histograms, and adjusting as needed. I'm not going to write about that technique, i.e., finding your exposure without the help of a light meter, because... well, because I always use a light meter.

Once I know what I need to know about the light outside the window, I know I'm going to be artificially lighting the model at least to that exposure. Otherwise, she's going to silhouette to some degree against the background. Yeah, there are times when silhouetting is the photographer's intent but my clients usually want to see the model in all her glory; naked glory and otherwise.

I set my mainlight in position and, by taking a few meter readings, I adjust the strobe's output until I match that of the natural-light reading I took at the windows. Since I almost always want to "pop" the model off the BG, I'll usually set my mainlight a bit hotter than the BG's reading. In other words, I flash more light on the model to capture her with a slight over-exposure. Of course, that's not the only thing I'll be doing to "pop" the model off the BG, but it's one of the techniques I generally use.

Okay, let's say I'm going to expose my images to the reading I obtained off the natural light coming through the window. To get some subtle "pop," I adjust my mainlight to slightly over-expose. (With aperture, that is.) There are times, of course, when I want to under-expose the BG for dramatic and/or aesthetic effect. When that's the goal, I expose with more artificial light than the BG is providing. In other words, I use strobes to overcome daylight.

Adjusting the recorded ambient in your exposure, BTW, is easily accomplished by changing the shutter speed. The aperture (in your exposure) remains the same since the strobe is firing bursts of light at shorter durations than the shutter remains open. Changing the shutter speed, therefore, has no real effect on the level of exposure provided by the strobe. Man, I hope I'm not wandering all over the place here. I tend to ramble. I don't think I could ever be effective as a technical writer. (Note to Self: Cross technical writing off list of potential, future, career-changes.)

The easiest way to understand this, I suppose, is by remembering that, when shooting with strobes, aperture is dictated by the strobe's output but shutter speed, which is not really controlled by the strobe's output unless you exceed maximum sync speed, can be adjusted to limit or increase the amount of ambient captured on the sensor. Does that make sense?

Where was I? Oh yeah. Once I've determined my exposure by reading both the natural and the artificial light and adjusting to taste. I make some decisions about highlights to further "pop" my model from the BG and the environment. (My clients love it when the model's "pop," especially since they're not so big on moody, shadowy, dark images... and the clients, of course, are always right.) Anyway, since I'm often slightly overexposing the model (due to my clients' preferences) from the BG, the natural light isn't going to provide much in the way of back-lit highlights. No problem. I'll simply use another strobe and set it in such a way that I get those highlights and, hopefully, get them in ways that don't look like they're too-obviously and too-magically coming from some too-mysterious of an unknown source. Often, the strobe providing the highlights doesn't need to be set much, if any, hotter than the mainlight as the physical laws of angle-of-reflectance will insure the highlights "read."

In the image at the top--not a particularly great image but one that adequately serves the subject of this post--I posed Sascha in the middle of the room. Duh, Jim. Like they can't see that for themselves. (Note: In the spirit of stating the obvious and my self-critique aside, Sascha looks great in almost any image.) That room, BTW, was the only place in the house available to me to shoot in. (Someday, they'll set me free to shoot anywhere I choose!) I set my mainlight, modified with a 3'x3' scrim, slightly "hotter" than the reading I took through the windows. It was mid-day on a hazy, overcast, smoggy(?) day. Since there was neither blue sky nor puffy white clouds available, I figured I'd expose merely to get some read on the landscape outside the window. I didn't want to completely blow out the windows and I sure as heck didn't want Sascha silhouetted against them. I set a strobe modified with a small, shoot-thru umbrella, camera-left and behind her, to provide some highlights: Easily-digestible highlights that don't require too much suspension of disbelief for viewers to accept.

Sascha captured with a Canon 5D and a 28-135 IS USM Canon lens zoomed out to 56mm. ISO 100, f/5.6 @ 160th, color temp set to "flash" default. MUA Melissa. If I was paying more attention to detail, I would've noticed that shopping bag on the floor in the BG. Oh well. My bad.