inverse square law

Feathering your light

For my fine feathered friends!

feather.jpg

Back again to help clarify studio lighting terms. This time I am taking on the concept of feathering. Feathering is pointing the light ahead of your subject instead of straight on to your subject. Feathering usually produces a nicer looking image. People often say a "softer" light on the image, but that isn't quite correct. Remember that the quality or character of light is determined by the size of the light in relation to the subject. You see this in the shadow edge transition. The larger the light source is the smoother the diffuse highlight to shadow transition is and the softer the light is.

Let's take a look at what happens when you feather the light. Here are photos of a 2x2-foot softbox straight on and the same softbox feathered off to one side. What do you notice first? I hope you see that the feathered light is SMALLER than the direct light. So, if the light remains the same distance from the subject, it is potentially slightly harder when feathered. But at the same time, it spreads out more across the face of your subject, giving a more pleasing look. 

Straight on the softbox is larger than when feathered

Straight on the softbox is larger than when feathered

Hard Light

Working with a softer (larger) light source it isn't so easy to see what is happening. Let's start with a harder light. This time a 7-inch standard metal dish reflector. 

7-inch reflector straight on and feathered 

Take a look at the 3 points labeled in the photo. A (the nose), B (the lower lip), and C (the ear).  A is the same brightness direct and feathered. Why? Because it is a specular reflection and is the same brightness as the light source, no matter the distance. If the light was moved in closer the brightness would stay the same but the reflection would get bigger. If the light source was moved back the brightness would again be the same, but the reflection would get smaller. The feathering doesn't have much of an effect here.

B is somewhat similar with the small hard light source. Basically the same straight on or feathered. At point C we can see that pointing the light more in front of and across the subject a little bit of light catches the ear. I would probably retouch out that little highlight on the ear. 

Go back to point A and the nose. This time look on the shadow side of the nose. The edge of the main dark shadow is about the same. With the feathered light the shadow is not as dense, but the edge transition line is pretty much the same. The feathering lowered the contrast, but didn't make a softer transition. It didn't soften the light.

Soft Light

Now let's work with a softer light. This time a 2x2-foot softbox. 

24x24-inch softbox straight on and feathered

The differences are more subtle, but they are there. Look at the same three points as before. The brightness of the specular highlight on the tip of the nose is the same brightness straight on and feathered. But this time there is a difference in point B on the lower lip. Feathering the light changed the shape of the wide highlight on the lip, but the brightness is still the same. The larger light also allowed more  light to reach the ear than with the 7-inch dish above. 

Again, I invite you to look at the shadow side of the nose. There is very little difference in the shadow edge transition. Feathering did not make it softer.

Comparison sliders drag to the left to see the feathered light, drag to the right to see the direct light

Hard vs Soft

Below are the feathered light images with the small and the larger light sources

Comparing the 7-inch and the 24-inch light sources

Look at the same points again. A is pretty close in brightness, but the shadow edge on the shadow side of the nose is very different. That comes from the difference in the size of the light sources, not from the feathering. The larger light makes for a nicer looking highlight on the lower lip (B). If I had move the softbox back further away from the face it would have been smaller and brighter. Brighter because the rest of the face would have received less light and I would have to have compensated by turning up the power of the light (strobe) or used a longer shutter speed (constant light, which is what I used for these demonstrations). As noted above the specular highlight remains the same brightness, but I would have had to brighten up the entire image, including the specular highlight to maintain proper exposure on the face. I know that  this can be confusing. So let's look at another illustration.

Controlling the size and brightness of specular highlights

Again I am using a constant light, so I am controlling my exposure via my shutter speed. If I was using flash here I would vary the power of the flash to maintain the same exposure on the subject of the photo. With the light 40-inches from the can the highlight is thin and bright--too bright to read the text on the label. As the light is brought in closer a couple of things happen. 1. The highlight gets wider because the light is closer and therefore larger in relation to the subject. 2. I have to lower the overall exposure so the diffuse reflection of the can is properly exposed. This brings down both the overall exposure and the specular highlight brightness, bringing them closer in line with each other. In the third image the light is very close and its reflection is even less noticeable because it is spread out over a larger area and the brightness of the rest of the can has been brought up closer to the brightness of the highlight (and then the overall exposure was lowered to bring it all back to give the proper exposure of the overall subject). I know this can sound counter-intutive. Bringing the light in closer reduces the mirror-like specular reflections.

We see the Inverse Square Law doing its thing here. As the light is moved closer to the subject it falls off faster and the background gets darker. As the light is moved in closer the specular highlight remains the same brightness, but gets larger. As the light is moved in closer the overall diffuse reflection exposure gets brighter and we have to lower the intensity of the light.

BOTTOM LINE

Feathering your light can change the size and shape of specular highlights, but the brightness will stay the same. Feathering your light will help even out the exposure across your subject, but won't have much effect on the shadow edge transition (softness). Feathering your light is the right thing to do. If you want to tone down a hotspot you need to move your lights in closer. But that also affects the depth and spread of the light. So many tradeoffs in photography. More details about the tradeoffs are in my book, Anatomy of a Studio Portrait.

Thanks again for stopping by!
John Cornicello

 

Light Depth

Inverse what?

In our last exciting episode, I wrote about adding diffusion to your lights. In that article I made a few references to the depth of your light and why you want to have various size light modifiers instead of moving your lights closer or farther from your subject. I promised that I would write more about this in a follow-up article, so here we are.

As noted previously, the quality of light (hard or soft), is determined by the size of the light as seen by the subject. When first starting out, you might only have one light modifier, such as a 24x30 softbox. So the temptation there is to move the light in closer or move it back farther from your subject to control the size of the light, and thereby the quality of the light. This does work. However, it has a secondary effect on your image. Moving the light changes the depth of the light.

Depth is one of the four main things I think about when designing my lighting for a portrait. The others are the depth of field that I want (how much is in focus), the size of the light (the shadow edge quality), and the amount of power I need (to get the aperture I want for the depth of field I want).

The depth of the light is based on a law of physics called the Inverse Square Law. This law states that a specified intensity (in our case, light) is inversely proportional to the square of the distance from the source of that light. As an example, if you double the distance between the light and your subject the amount of light falling on the subject will be 1/4 (at twice the distance the light has to spread out to cover 4x the area, see the illustration below). If you move the light back to 3 times the distance, the amount of light will be 1/9th the power, and so on.  

inverse-square-1
inverse-square-simplified

Another part of this law, that seems to be often forgotten or ignored, is that it applies to a point source of light, something we rarely, if ever, actually use. As you put modifiers on your lights, such as softboxes, umbrellas, or add a lens, such as in a spot light, we start drifting away from the law. So don’t get hung up on the inverse square law or on measuring the distance between your light and subject. You only need to remember one simple rule: The closer your lights are to your subject, the darker the background will be.  

OK, sounds simple enough. But it is also very powerful knowledge. With this knowledge you can take a simple white seamless paper background and make it appear white, black, or any shade of gray between them. It also allows you to light your background separately from the subject to control the apparent depth of the scene. More on that later.

So, let’s look at some examples. In this series of images the subject is 65” in front of a white seamless paper background. The single light is a 24x30 softbox. The only changes between the images is the distance from the softbox to the subject and the power setting on the light to maintain the same f/4.0 exposure. As indicated in the images, the distances are 12”, 24”, 48”, and 96”. The first thing I think you will notice is how the tone of the backdrop changes. 

light-depth

You will also want to take a look at highlights on the subject. Look at the cheek. Specular highlights are mirror-like and at first seem to not follow the inverse square law. In actuality they do follow the law, but in a reverse way. Specular highlights maintain the same brightness, no matter what the light to subject distance is, but they change in size inversely proportional to the distance. As your light is moved farther from your subject that specular highlights get smaller and appear brighter because the rest of the image is getting darker while the highlight stays the same brightness.

specular-highlights

Now, about lighting your background separately from the subject. This involves adding more lights. In this case I added one light on my white background, a flash with a 7” silver dish reflector with a red gel. With the main light (the light on the subject) 96” away from the subject you can make out a pinkish glow on the background. But when I move the light in to 12” away from the subject the white background goes to a dark gray (as shown above) and the red gel on the background light can now be seen. If I had used a dark gray or a black background paper the red would be a richer tone than the pink that you see here. The background light was on its lowest setting. If I could have lowered the power more, it would have been more red, too.

background color

A practical application: Group Photos

So far, I’ve been talking about light falloff from subject to background. We can also think about group photos and fall off from subjects different distances from the light. By combining our knowledge of light falloff with a lighting technique called feathering we can learn to light a group of people. Feathering is when instead of pointing the light directly at our subject(s) we point it a bit off so that the light skims across our subjects. A fill card (white foamcore or a pop-up white disc) helps fill in the shadows from the right side.

group-lighting-1

As an example, if our light is on the left side of our camera we would point the softbox or umbrella at the person on the right side of the group. The main intensity of the light will now be aimed at the person farthest from the light and the lower intensities from the edges of the light will be lighting the person closer to the light, helping to even out the light across the group. If the light is still too uneven, we use our knowledge of inverse square to tell us to move the light back, farther away from the group to even out the illumination across the group.

group-lighting-2

Here is a group photo of my friends at CreativeLive posing for me. This image was lit with one studio flash in a 60" Photek Softlighter on camera left and a fill card on the right. The Softlighter was backed up a bit to help even out the exposure across the four individuals.

softlighter-group-photo

That's all for today. Thank you for following along.


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John Cornicello