Previously, we looked at how the direction and size of a light source affects the appearance of your photographic subject. Today we’ll look at how the distance between the light source and your subject impacts your photographs and how you can creatively take advantage of this factor. We’ll start off our exploration with the most friendly and accessible method that anyone could ask for: with a mathematical formula!
The Inverse-Square Law
Cribbing from Wikipedia, an inverse-square law is any physical law stating that a specified physical quantity or intensity is inversely proportional to the square of the distance from the source of that physical quantity. Typically any discussion of the inverse-square law is followed by a diagram showing how it works, but since there’s one on the Wikipedia page, I’ll dispense with it. The inverse-square law is not too difficult to understand, but since many people have a phobia of any form of mathematics more complicated than arithmetic, I’ll deliver the good news and say that you don’t really need to memorize the scientific definition.
The main thing you should take away is that the intensity of light diminishes as it travels further away from the light source. You probably already understand this; anyone who has ever used a flashlight or turned on a car’s headlights knows that light loses brightness over distance. However, what the inverse-square law tells us is that light loses brightness very quickly, possibly much faster than you might expect. This can cause some serious complications in photography as well as opening up certain possibilities. To illustrate this point, we’ll look at some specific examples in which the inverse-square law comes into play.
Subject and Background
One of the most common situations in which the distance of light becomes significant is when you have a background in your image and you want to control its brightness relative to the exposure of your figure. We’ll demonstrate how this can be controlled with this set of images featuring long-forgotten Shuraki girl Liu Meifeng. The background is located about 30 centimeters behind the subject and the light source was placed approximately 120 centimeters, 50 centimeters, and 15 centimeters from the figure.
At 120 centimeters, both Liu Meifeng and the background are brightly lit. When the light is brought in to 50 centimeters from the figure, the background gets darker, and when it’s brought in to 15 centimeters, it becomes even darker still. This is because the relative distance between the subject and background is much higher than that between the light source and the subject. Looking at it mathematically, at a distance of 120 centimeters, the intensity of the light hitting Meifeng is 1/1202, or 1/14400; the intensity of the light hitting the background (placed 30 centimeters behind the figure, so it is about 150 centimeters from the light source) is 1/1502, or 1/22500. Dividing the figure’s light intensity by the background’s light intensity, we find that the figure gets about 1.5 times more light than the background; looking at the image, that seems reasonable, as the exposures of the figure and the background look similar. Let’s compare that with the amount of light hitting the figure from a distance of 15 centimeters. Following the same procedure, the light hitting Meifeng is 1/152 and the light hitting the background is 1/452; dividing the former value by the latter, we find that Meifeng is getting 9 times more light than the background, which is why the background becomes so much darker.
If the numbers scare you, you can just remember this simple, though somewhat non-intuitive, rule: if you want your background to be darker, move your light closer to your subject.
One other thing to note from this set of images: notice how the shadows on Meifeng’s body get darker as the light is moved closer to her. This effect is particularly noticeable on her hip and on the back side of her left arm. We’ll talk more about this in a bit.
Two Points on a Subject
The inverse-square law can also rear its head without the background being involved. Case in point:
Here, the key light was placed off to camera left (the figure’s right side) and is shining on the side of her head. However, the light source is placed quite close to the figure and because Sena’s right arm is closer to the light, her elbow becomes the brightest element in the image, and it becomes very apparent that her right arm is much brighter than her left. This is not what I normally want; usually, I want the face to be brighter than the rest of the body.
Here’s a more subtle example; the outside of Saya’s right forearm has a hotter light intensity than any other part of her body.
One of the more maddening situations that the inverse-square law influences occurs when you are lighting more than one subject:
You can probably guess what happened here; the light is off to camera left, relatively close to Saya Tokido, which means it is rather far from Kyou Fujibayashi. As a result, Saya is properly exposed while Kyou is mired in murkiness. Exposing the image for Kyou would obviously cause Saya to become overexposed. What can you do about this?
One easy thing you can try is to move the figures closer together. This brings the light intensity on each figure closer together and is also usually more appealing from a compositional standpoint. However, you may not be able to do this, depending on the construction of the figures you’re shooting. What else can you do? The obvious thing to do is to move the light further away; that will even out the exposure on each figure. However, you might wonder whether that will cause the light to become harder, as moving it away turns it into a relatively smaller light source. Indeed, that is what will happen; if you want to maintain the same level of softness, you’d have to use a bigger softbox (or other diffusive device). Photographic lighting often involves making compromises, and there is no free lunch to be had here.
Or, well, actually, there is; what I usually do is fix up the exposures in post-processing software. For example here’s what that Sena Kashiwazaki photo looks like with Lightroom’s graduated filter applied to her right arm with a 0.3 stop exposure reduction and a -70 Highlights value:
Lightroom users will note that the adjustment brush would obtain more localized and accurate results, but the graduated filter does get the job done quickly.
Alternatively, you could add in another light to light up your second subject:
Here we have a version of the header image (which was basically just me noodling around) with an overhead light added into the mix; this light causes the background Asuka to look about as exposed as the foreground Asuka.
Thus far we’ve made it sound like the inverse-square law is something that will plague and bedevil your photographic pursuits. Indeed, the falloff of light that we’ve described can present significant complications, but it also provides for interesting creative possibilities. For example, we’ve talked about how the near side of the figure can become overly bright; by the same token, you can control how dark the far side of the figure appears. For example:
In this image, the light is coming from the left side of the image; placed fairly close to the figure, the shadows on the far side of Lacia’s body are deep and rich, adding dimensionality, drama, and, in my view, appeal. In my experience, not many people seem to take advantage of this phenomenon with their figure photos, and thus this is one thing that can really lift your pictures.
Things You Should Take Away
This topic can be a little counter-intuitive, so I’m going to wrap it up by providing some straightforward answers to some common situations.
I want my figure to be brightly lit but the background to be dark, so that the contrast adds drama and focuses attention on the subject.
- Move your light close to your figure. If you can, move your figure away from the background (note that this will not always be possible, depending on compositional concerns).
I want my figure to be lit at about the same value as the background, which gives it a more natural look.
- Move your figure closer to the background if you can, and move the light further away. Depending on what sort of background you’re using, I find that shooting with a narrow aperture can also emphasize the realism of the shot.
I want to have deep shadows on my figure, instead of flat lighting.
- Move your light source close to the figure; I often like to place it as close as I can get it without it appearing in the photograph. Using a longer focal length will allow you to get it closer (as such a lens obviously provides for a narrower field of view). Using a split light style will also emphasize the shadows, as they will appear on the opposite side of the figure, rather than being cast behind the figure (as they would be if your light source were placed closer to the front of the figure).
I want to light a group shot.
- I really hate lighting more than one figure; the problems we’ve discussed here are a big reason why I seldom take group shots. However, when you do need to include two or more figures in a shot, there are a few things you can do. We’ve mentioned moving the light source further back to even out the light intensity on each figure. If one figure is wearing dark clothing or has darker skin, placing it closer to the light will cause the difference in brightness to be less noticeable. Using a fill source with the figure that is furthest away (but not on the one that is closer) can also even out the brightnesses.
Personally, I usually just light each figure separately; the left-side figure gets a split light to camera left, the right-side figure gets a split light to camera right. It certainly doesn’t look natural, since each figure is being lit from a different direction, but it gets the job done.
This wraps up our discussion of the properties of light; next time, we’ll look at shaping and controlling light with light modifiers.