I've been re-reading Russ Lowell's incredible book on lighting called, Matters of Light and Depth. It reminded me of a subject we photographers never seem to talk about when we discuss lighting, and that is the role played by the distance from the light source to the subject being lit. We love to talk about stuff that's physical; like which softbox is best, or what kind of flash to buy, or which beauty dish will allow us to talk effectively to beautiful models, but the basic stuff seems to slip around the edges of the conversation.
One of my favorite ways to light portraits is the way I've lit the portrait of Renae, just above. I used a very large light source and I moved it in closer than one might imagine. I wanted to take advantage of the inverse square law. It seems that light falls off in some sort of math-y way that basically means every time you move a light away from the subject the light hitting the subject falls off by a factor of four with every doubling of distance. Double the distance? Cut the light energy by about 75%. Oh hell, you're all very smart, here's the formula:
But what it means to me is something less math-y. The idea means that when I put a light closer to my subject the difference in light intensity from one side of her face to the other is much greater than if the light is further away. This engenders light fall off. Light fall off is a tragedy if you are trying to evenly light a copy shot of an illuminated manuscript but a blessing if you are looking for some dramatic modeling of a human face. I want the light to fall off dramatically. It's a nice effect. One I love to use.
If I use an enormous light from far away I get soft light that doesn't fall off very quickly. If I take that same light source and move it in very close I get the benefits of very, very soft light wrapped around the benefit of the faster fall off which adds back the impression of contrast.
The parameter of distance in lighting is really, really helpful. If you need to light a group of five people, lined up parallel to the imaging plane of your camera sensor, and you want the light to be relatively even from the person on the left to the person on the right you could use one light and place it back as far as you can from the group. The light might be at a 45 degree angle from the group. If the distance is far enough the light from the lighting instrument will not experience dramatic fall off (think 25 or 50 feet away) and you may be able to use the image, illuminated with just one light source, with just a little burning or dodging (or both) in PhotoShop.
The "general rule of thumb" for positioning a modifier (which is the de facto light source in your equation; not the light instrument itself) for a portrait of a single person is to position it 1.5 times the distance that is equal to the diameter of the source (or modifier). So a four foot diameter umbrella would satisfy the formula by being placed six feet from the subject.
I'm pretty bad at following the rules and generally find that I like my modifiers (light sources) to be much closer. How much closer? How about just out of the frame? When I shoot with my 6x6 foot scrim I rarely have the light radiating surface more than about 4 feet from my portrait subject.
The point really isn't that there is a right or wrong way to distance your light source but the acknowledgement that the aesthetic/physics driven change can be used as an artistic tool to get a look that you want.
I also use the opposite effect but mostly when I light interiors with sunlight. I love taking big, shiny boards (think almost mirror-like surfaces) on big fat light stands and bouncing sunlight from the surface of the boards (set up outside windows 25 or 50 feet from the house) and ricocheting the light coming from 93 million miles away into the windows. The fall off from one side of a room to the other is almost negligible. It's a much different feel and effect than you'll get from even the best implemented electronic flash units, used in the room. And it works.
Next time you light it might be fun and effective to stop for a few moments and consider the optimum distance for your lights, commensurate with the effect you are working toward.
Not as sexy as a discussion of the new Otus 150mm f1.1.2 lens but then, what is?
Such a fun thing to think about in the new year.