Family of angles question

<p>Hi, I was reading Light Science & Magic, and had a question about family of angles. From what I understood, an object at a certain position HAS its own family of angles for direct reflections, so in order to avoid direct reflections you should not put a light source inside that family of angles. But I've come across a sentence that does not make sense to me: </p>

<p>"Moving the lights closer to the camera axis, for example, means moving the camera farther away from the subject(and using a corresposdingly longer lens to get a similar image size.) This<strong> creates a smaller family of angles</strong> that causes direct reflections and allows more freedom in choosing the angle to light the subject."</p>

<p>From these sentences, it sounds like you can create a family of angles by moving the light source. Two things don't quite come together: 1. You can decide to or not to place a light source inside a family of angles. 2. You can create a family of angles by moving a light source. </p>

<p>Can you explain what I am not getting here? Thank you in advance. :)</p>

<p> </p>

<p>I'll take a whack at it:<br /> The passage refers to the family of angles <em>associated with the camera</em>. A longer lens means a narrower field of view (or family of angles). So the direct reflections off the object are narrower (think of the camera as being the source, and the narrower the "cone" of angles from the camera, the narrower the reflections).</p>

<p>It really wants a sketch...</p>

<p>The main idea is that if you want maximum flexibility for lighting position with no direct reflections off the object, your options for light position are greater with a longer lens (which has a narrower family of angles).</p>

<p>A light source, of course, has its own family of angles, but that's not what's being discussed here.</p>

<p>Let's see if I can post an image here...yes.<br>

To get no direct reflections, you want your light source positioned outside the family of angles associated with the camera.</p><div></div>

<p>It's been a while since I read the book (and I should reread it, really), but if I am not mistaken, the diagram above makes a right point. In my recollection, the family of angles is <strong>not</strong> what you describe as "<em>an object at a certain position HAS its own family of angles for direct reflections, ....". </em>The object itself does not have a family of angles<em>.<br /></em><br /> The family of angles is the angles in between which there will be direct reflections, so it is dictated by the positioning of the light(s) relative to the object. If you move the light, the incident angle of the light will change, and as a result, the angle under which direct reflection is observed will change: the family of angles will change.</p>

<p>Note: I really could be very mistaken, and I do not have the book at hand at the moment, but this is how I remember it.</p>

<p>Ok..it's not too hard to make myself think that the family of angles changes relative to the position of the light source. It does seem to make sense. But the book focuses on "whether to <strong>position</strong> a light source <strong>in</strong> a family of angles or not" It is this part that made me think that "an object at a certain position has its family of angles for direct reflections" When the author says whether to put a light <strong>in</strong> the family of angles or not, does he mean the family of angles created by the ambient light that was already there before the main light we are trying to position? </p>

<p>Can you guys tell me what I'm thinking right now is right? I think I've got it figured out.</p>

<p>When you light a subject from a certain angle, the subject creates a family of angles for direct reflections. The position of the light source at that moment could be inside or outside of that family of angles. As you move the light source, the family of angles changes, and the light source could happen to sit inside or outside that changing family of angles.</p>

<p>The term "family of angles" is used throughout the book. It's important to thoroughly grasp what it means. Think of a camera. Basically, it can see everything within a cone whose apex is at the camera and whose base is at infinity. The central axis of the cone is a ray of light extending forward from the film plane, through the lens and intersecting the subject.</p>

<p>That cone delimits a "family of angles"...rays of light which include the ray perpendicular to the camera's focal plane (which is the axis of the cone) and all the rays at angles to that central ray, out to the edge of the cone (which is defined by the focal length of the lens...shorter FL means wider apex angle of the cone).</p>

<p>Where that family of light rays reflect off a subject, a new, truncated cone is generated. Anything within its volume will be visible to the camera as a reflection off the subject.</p>

<p>The family of angles just describes the portion of the world visible to the camera, both directly and by reflections off the subject. A light source can also have a family of angles, but the family of angles mentioned in the passage you quoted is the family of angles of the camera.</p>

 

<p>Thank you for all the help. I appreciate it. I need more studying and practicing before I can shoot seriously.</p>

<p>Keep at it Dan, I also had difficulty wrapping my head around the "family of angles", but once I grasped it, the whole book makes complete sense. I could not put it down and read it in one (or two) long sittings. It truly is an exhaustive and very clear explanation of studio lighting.</p>

<p>Perhaps a small setup with a couple of low cost remote controlled strobes bounced off foamcore would help? There are a number of setups described which should be easy to duplicate. You could then vary the setup and observe the resulting effects.</p>

<blockquote>

<p>The family of angles just <em>describes the portion of <strong>the world visible to the camera</strong>, both directly<strong> and by reflections off the subject.</strong></em> . . . the family of angles mentioned in the passage you quoted is the family of angles of the camera.</p>

</blockquote>

<p><strong> </strong><br>

Yes.</p>

<p>***</p>

<p>Daniel, using Peter’s diagram as a template, you can make a test shoot to illustrate how moving the Camera and changing the Focal Length of the Lens will narrow or widen the Family of Angles and where you can place the Lighting Source so as to NOT get Direct Reflections.</p>

<p>You need:<br>

> a big mirror, like a mirror on the wardrobe doors in a bedroom<br>

> a Subject with markers – if it is a person then the persons shoulders’ width, will do as the markers<br>

> two poles or any two skinny objects that will stand upright<br>

> an helper (maybe)</p>

<p>What to do:<br>

Place the Subject hard against the mirror and facing the camera<br>

Zoom to the WIDEST Focal Length and frame a shot that just crops at the Subject’s shoulder width<br>

Look in the <strong><em>reflection of the mirror</em></strong> and get your helper to place two sticks about 10ft behind you at the limits of the frame that is in your viewfinder</p>

<p>Next:<br>

Step back about 8ft (exact distance is not critical)<br>

Zoom to a longer Focal Length so you FRAME the Subject just so it crops the two shoulders as before.<br>

Look in the reflection of the mirror and you will notice that your sticks can be moved inward, closer to the Lens’s Axis</p>

<p>Those sticks represent approximately the closest position to the Lens’s Axis where the LIGHTING could be placed so as to NOT give a ‘direct reflection’.</p>

<p>WW</p>

<p>EG:<br>

<img src="http://d6d2h4gfvy8t8.cloudfront.net/18087798-lg.jpg" alt="" width="1280" height="960" /></p>

<p>Thank you. I will try this.</p>

<p>And I'll pick the book up again as it's obvious enough its knowledge did not sink in deep enough :-) Thanks for the additional explanations!</p>