Photography is a balancing act
Yes, a balancing act in many ways. Shutter speed vs aperture vs ISO vs amount of light is what most people probably think of. But it also plays a big role in the character of the light we use on our subjects. Today is a tale of two umbrellas. The first is a 84-inch reflective and the other is a 36-inch reflective. As you can see, the larger umbrella surface is a bit more diffuse than the smaller, satin-finish umbrella. But I am not concerned with that today. For now, I want to talk about size vs distance.
Let's start with a refresher on the Inverse Square law. "the intensity of an effect (in our case, light0 changes in inverse proportion to the square of the distance from the source." For example, if we have a point source light at 2 feet from a subject and we back the light up to 4 feet away (doubling the distance), we end up with 1/4 the amount of light on the subject, a loss of 2 stops. Double the distance equals one quarter the light. 4 is the square of 2.
Have I lost you yet?
In practice you don't have to remember any of the math. For one thing, we rarely work with a point source of light in our photography. Our lights are either larger on their own or are modified with softboxes, umbrellas, diffusion screens, etc. So moving these larger lights closer or farther from the subject has less effect than the law would suggest. I think that photographers can narrow and simplify the meaning of inverse square down to saying that the closer your light is to your subject, the darker the background will be. Or you can turn it around and say that the farther your light is from the subject the brighter the background will be. Is the glass half full or half empty?
Here is a set of images photographed against a white background. The subject (yours truly) is 24 inches away from a white seamless paper backdrop. The photos labeled 1 are lit with the 84-inch umbrella and the photos labeled 2 are lit with the 36-inch umbrella. The A images are with the light at 84 inches from the subject and the B images have the light at 36 inches away.
There are a few things that I want you to take notice of in these photographs. Starting with 1A and 1B you can see that with the big umbrella backed up away from the subject (1A) the face is lit flat and the catch lights (the reflection of the lights in the eyes) are small and bright. When the light is brought in closer (1B) the face start to have more contouring as the light falls of quickly in intensity and the left side (camera right) of my face is darker. At the same time, the catch light became larger and much less bright. Because the umbrella is so large the changes are a bit on the subtle side. In the closeup crops you can see that the background on 1B is a tiny bit darker than in 1A.
Now take a look at the bottom row. With the small umbrella backed up (2A) again the face is lit flat with small bright catch lights. In 2B with the light in closer the face again has more shadow area and a more defined transition from the properly exposed parts of the face to the shadows. 2A has a distinct shadow on the white background because the 36-inch umbrella 7 feet away is a relatively small light source. When moved in closer for 2B it becomes larger in relation to the subject and helps fill in the shadow, but not as much as the larger umbrella fills in on 1B.
Let's go back to the catch lights. Catch lights are specular reflections. That is, they act like a mirror of the light that creates them. They are the same brightness, no matter the distance from the light. Does this counter the inverse square law? No! Specular reflections follow the inverse square law in a different manner. While their brightness stays the same, their size varies in relation to the distance. When the light is farther away (1A and 2A) the light is small and bright. In closer (1B and 2B) the light is larger and dimmer.
But wait! I just said that the brightness remains the same, how could it be dimmer? That is because when we bring the light in closer the overall exposure (the diffuse reflection--the skin, hair, etc.) gets brighter. The face got brighter while the catchlights stayed the same. To compensate for that and not overexpose the skin we have to lower the overall exposure. In this case with electronic flash, I turned down the power on the flash to bring the overall exposure back to normal. Turning down the power reduced both the diffuse reflection and the specular reflection, bringing them more in line with each other. Moving the light in closer also helps with other hot spots, such as highlights on the tip of the nose or a shiny forehead. Instead of lowering the flash power I could have stopped down the lens more. But that would have changed depth of field, which I usually do not want to do. If this was lit with continuous light I could have changed the shutter speed, as well, to control the exposure.
Bright vs Harsh
At first it may seem counter-intuitive. Bringing the light in closer makes it brighter, but it also makes it softer (unless it is a inherently small light, such as a speed light (camera flash). Bringing the light in closer makes it larger in relation to the subject and allows the light to come in from more angles, filling in its own shadows. Don't equate brightness with harshness. We control the brightness via our exposure settings. We control quality (hard vs soft) with the size of the light. We control the depth of the light with the distance. This is why modifiers come in so many sizes. That lets you decide on your aperture first for the depth of field that you want. Then you set your light to subject to background distances for the depth of light that you want. Then you select the modifier size for the character of light you want. And finally set your exposure via the power of the lights. It is all a balancing act.
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