Family of Angles.

[HaseebM]

Just received my copy of Light: Science and Magic and so far its been an interesting read. Right now I am on the chapter which discusses 'family of angles' and cites a few examples but in each case the examples are that of either a painting or flat surface. Maybe as I progress through the chapters this may become clear but meantime just in case, thought I could ask here.

- Is the family of angles presumably one that reflects outward depending on the size of the object?
- I photograph suits a lot as well as swatches so kinda confused as to the angle outside which I should place the lights to avoid direct reflection.

Here is an example of the type of pictures I take for my cousin's use. Nothing professional but gets the job done for him.



So basically trying to understand if I inadvertently shot outside the family of angles or if I did not, how to judge the family of angles?

[The Blue Boy]

Hi Haseeb,

Here's a good article that I stumbled across a good while ago and should set you well on the way to understanding what is meant by this term...

http://www.digitalphotopro.com/gear/...l#.VB2YTxa5-So

Hope this helps,

[HaseebM]

Hi Mark, I am reading the book from which the article has posted excerpts from. I am trying to understand family of angles for the suit and for general objects. I have a rough idea but not really 100% certain.

[revi]

The family of angles is mostly relevant for specular reflections (mirror-like, if you prefer), where the angle of the reflected light ray is 180° - angle of the incoming light ray .
With most tissues, like here, you have mainly diffuse reflection, for which there is no family of angles, as there is no well-defined reflection angle for any incoming light ray. By definition, diffuse reflection scatters incoming light over all possible angles.

"Light: science and magic" discusses mirror reflection and diffuse reflection just before discussing the family of angles.

[Kodiak]

The family of angles is mostly relevant for specular reflections (mirror-like, if you prefer), where the angle of the reflected light ray is 180° - angle of the incoming light ray .
With most tissues, like here, you have mainly diffuse reflection, for which there is no family of angles, as there is no well-defined reflection angle for any incoming light ray. By definition, diffuse reflection scatters incoming light over all possible angles.

+1

Nevertheless, the camera right side is well lit but the left is too close to 255!

[HaseebM]

Revi, so that basically means I need not worry about the family of angles when I use diffused light?

Kodiak, the light was on the left and I used a gold reflector on the right.

Thanks.

[Shadowman]

Haseeb,

I like your effort, the exposure and background treatment was done very well.

[Manfred M]

Revi, so that basically means I need not worry about the family of angles when I use diffused light?

No, this has nothing to do with the light source, but rather the subject matter. If the subject matter is reflective (a piece of metal, shiny plastic, glass, a mirror, etc.) , a light source inside the "family of angles" will show up in your image. A light source outside of the familiy of angles will not.

When shooting a textured subject, like the jacket in your image; the subject is not highly reflective so you really don't have to worry about getting reflections in your image. On the other hand, we can see that the light is rather unbalanced, tending to being too bright on the left side of the image. Unless I have two identical light sources for this type of shot, I use an incident lightmeter to set these up to ensure my lighting is effective. If this is a catalogue shot (and it looks like one), balanced light from both sides is probably desirable..

[xpatUSA]

While the posts in this thread make it quite clear what the 'family of angles' means, I find that the phrase itself is akin to the so-called 'circle of confusion', in that the phrase conveys little of what is actually meant.

Plus, should they not be solid angles, entirely dependent on the shape of the object itself?

Just my pedantic tuppence worth

[Loose Canon]

I know I’m going to be sorry I jumped in here…

No, this has nothing to do with the light source, but rather the subject matter. If the subject matter is reflective (a piece of metal, shiny plastic, glass, a mirror, etc.)...

Actually all surfaces are reflective. Otherwise you couldn’t see them!

It’s more a matter of how they reflect light. And really whether or not they are textured isn’t so much the issue either. Textured glass is going to reflect light differently than a textured piece of cloth (the jacket).

When shooting a textured subject, like the jacket in your image; the subject is not highly reflective so you really don't have to worry about getting reflections in your image.

If the jacket is over exposed on the left it is because there is too much reflection from too much light. So to say that you don’t have to worry about getting reflections might be confusing. To say that the jacket produces diffuse reflection might be more accurate.

The family of angles is mostly relevant for specular reflections (mirror-like, if you prefer), where the angle of the reflected light ray is 180° - angle of the incoming light ray .
With most tissues, like here, you have mainly diffuse reflection, for which there is no family of angles, as there is no well-defined reflection angle for any incoming light ray. By definition, diffuse reflection scatters incoming light over all possible angles.

This is technically accurate.

[Manfred M]

Terry - technically you are 100% correct. I was trying to write using slight less technically correct language in an attempt to be a bit more clear (I know that sounds strange). The semi-technical language in the book was obviously unclear to Hasseb and lead him in the wrong direction. I've done a lot of technical writing throughout my career, and the first question I always ask is "who is the audience". A layman often needs an explanation that a scientist or engineer would suggest is technically incorrect, and when writing, I always try to use language and descriptions that the audience, rather than the expert, will be comfortable with.

[Loose Canon]

Okay. Fair enough Manfred. I certainly don't want to confuse the issue. I see enough of that as it is!

So my apologies to you and Haseeb.

Haseeb, Manfred is right in saying that it is not the type of light you are using (diffuse or not).

It is the type of surface you are lighting and how it reflects light that makes the determination.

[jcuknz]

The point of a circle of confusion is that it is precisely that. The illustration shows a lack of balance between the two light sources, nothing to do with any family of angles...

[Mike Buckley]

The discussion that has taken place so far is really great, as it has clarified the issues. I have nothing to add other than that...

http://www.digitalphotopro.com/gear/...l#.VB2YTxa5-So

As I was very quickly glancing through parts of that article, I was thinking to myself, "Wow! That's an unusually good and accurate article compared to so many pieces seen on the Internet. Then I got to the very bottom of the article and saw that the text and diagrams were provided by the three authors of Light: Science & Magic. That explained why the article is so good.

[Mike Buckley]

Is the family of angles presumably one that reflects outward depending on the size of the object?

I just now realized that that question has not yet been directly answered, Haseeb. Turn to page 42 of the book. In diagram 3.7, you see that the size of the area inside the angles will change if the size of the subject changes. To that extent, the answer to your question is, yes, the family of angles depends on the size of the subject.

That's important to understand because if you want a light source to fill the family of angles, you need to know the limit of the family of angles. Conversely, if you want to ensure that the light source is entirely outside the family of angles, you need to know the limit. That limit changes depending on the size of the subject (as well as the position of the camera relative to the position of the subject).

It might be helpful to know how I intentionally overly simplified the explanation of diagram 3.7 so my pea-size brain could always quickly understand it without having to take the time to find and read the pertinent area of the text. First, I added the designation "B" to the light source on the right side of the diagram. (It had inadvertently been omitted.) Then, I added the following note to the page margin: "Light produced by Source A will not be seen by the camera because it is not placed within the family of angles. Light produced by Source B will be seen by the camera because it is placed within the family of angles."

[Mike Buckley]

While the posts in this thread make it quite clear what the 'family of angles' means, I find that...the phrase conveys little of what is actually meant.

I think the phrase conveys the totality of the meaning reasonably well. To quote from Light: Science and Magic beginning at the bottom of page 41: "Each surface is made up of an infinite number of points. A viewer looking at a surface sees each of these points at a slightly different angle. Taken together, these different angles make up the family of angles that produces direct reflection." (The italic font that provides emphasis is in the book.)

By the way, it is not lost on me that I have written two posts since I wrote the post explaining that I had nothing to add to the discussion.

[HaseebM]

Thank you John, Manfred, Ted, Terry, Mike...I understand now after reading all your posts.

Manfred, I understood you the first time and you are right, I am uncomfortable with technical terms and your explanation was simple and clear which I guess Terry further clarified.

[HaseebM]

I just now realized that that question has not yet been directly answered, Haseeb. Turn to page 42 of the book. In diagram 3.7, you see that the size of the area inside the angles will change if the size of the subject changes. To that extent, the answer to your question is, yes, the family of angles depends on the size of the subject.

That's important to understand because if you want a light source to fill the family of angles, you need to know the limit of the family of angles. Conversely, if you want to ensure that the light source is entirely outside the family of angles, you need to know the limit. That limit changes depending on the size of the subject (as well as the position of the camera relative to the position of the subject).

It might be helpful to know how I intentionally overly simplified the explanation of diagram 3.7 so my pea-size brain could always quickly understand it without having to take the time to find and read the pertinent area of the text. First, I added the designation "B" to the light source on the right side of the diagram. (It had inadvertently been omitted.) Then, I added the following note to the page margin: "Light produced by Source A will not be seen by the camera because it is not placed within the family of angles. Light produced by Source B will be seen by the camera because it is placed within the family of angles."

Thanks Mike, that's very helpful indeed. So based on the area of the object, the photographer needs to roughly calculate or guess the family of angles which I presume will be learnt through trial and error.

[Mike Buckley]

So based on the area of the object, the photographer needs to roughly calculate or guess the family of angles which I presume will be learnt through trial and error.

Correct. If you want to take the time to determine the exact limit of the family of angles, see the explanation of how to do that beginning on page 121. (You might not have gotten that far along in the book yet.)

[HaseebM]

That's right, on page 70+ at the moment. Thanks.