Tuesday, September 7, 2010

Family of Angles

For today's post we will cover a topic that isn't discussed a lot, but is fundamental to lighting. The term “family of angles” doesn't come up often, if at all in most circles. Is it useful for you? Read on to find out...

Terms Defined
Before going any farther, I want to define some of the terms that are related to this discussion, just to ensure everyone gets this bit of background. These are my definitions, feel free to consult the Wikipedia or your favorite source for better terms.

Direct Reflection Reflections that mirror the light source, as one typically gets from a hard, smooth surface, such as glass, plexiglass, metal, plastic, etc., materials that are shiny. Direct reflection does not diffuse the light in any significant manner.

Diffuse Reflection Reflection that gets diffused, that is, scattered all directions rather than simply mirroring the source. Diffuse reflection is typically associated with textured surfaces like cloth, skin, dirt, uncoated paper, etc., materials that are not shiny.

Angle of View Abbreviated as AOV. The angle extending out from a lens along the path of what the camera can see. It is worth noting that while focal length affects the field of view, the sensor size also affects Angle of view. For example, a 100mm lens with a 35mm sensor will yield a wider AOV than the same lens with a 23mm (APS) sized sensor. See the image below for a graphical depiction of this.

Field of View Abbreviated as FOV. The rectangular area describing what the camera can see at a particular focus point. This is similar to AOV except it describes the rectangular area at the focal point, so the FOV increases/decreases correspondingly with an increase/decrease of the focal point. Because FOV is associated with AOV, it too is affected by sensor size

The Basic Premise
The term “family of angles” refers to the angles from which if light is sourced, a reflective surface will direct light back into the lens creating visible light spots. For a flat surface, the family of angles are the same as the lens' angle of view, but coming back toward the camera. Throughout this discussion it is assumed that the surface being photographed is flat and reflective, which could be glass, plexiglass, metal, plastic, etc.

In the above diagram the dotted line represents the family of angles and is equal to the angle from the lens' field of view. When light is sourced anywhere within this family of angles, it will reflect off of the surface being photographed and back into the lens.

From this diagram you can see that using a longer focal length from a greater distance will reduce the family of angles. Now that the lights are outside the family of angles light will not reflect back into the lens, thus eliminating direct reflections.

What's The Point?
The point is, knowing about the family of angles takes some of the guess work out of lighting. It is actually an intuitive kind of thing, but giving it a name makes it easier to discuss and for me, modeling it on paper drives it deeper into the gray matter.

Knowing about the family of angles helps me to control reflections.

I can add them when they're wanted...

...and eliminate them when they're not.

When shooting materials that are somewhat reflective, and especially for those that are highly reflective, if I'm getting unwanted reflections, I know to either move the lights outside the family of angles or use a longer focal length and move the camera back. There are times I may want reflections, like filling a pair of sun glasses or the entire side of a metal object. To do this, I know to place a light source inside the family of angles.

By now someone has no doubt noted that not all objects are faced perpendicular to the camera and not all objects are flat. When the subject is turned, then the family of angles move accordingly along with the subject. This concept can also be applied for non-flat surfaces; you need to adjust for whatever curvature you encounter. A concave surface will reduce the family of angles whereas a convex surface will increase the family of angles. If photographing a sphere you are out of luck, as all angles reflect back into the lens, so the family of angles are 180º. There are ways of dealing with it, but that discussion is beyond the scope of this little post.

Read This!Publish Post
I have only scratched the surface for how the family of angles plays into photographing various materials. If you care about lighting, please do yourself a favor and get a copy of the book “
Light: Science and Magic: An Introduction to Photographic Lightingby Fil Hunter and Paul Fuqua. It covers this subject and much more, in-depth and in a way that makes it interesting and easy. Google books has an excerpt here.

Coming Next
The next post covers a little product photography from a recent shoot. Come on back and I'll show how I approached lighting a larger than tabletop object.

Until then...


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