Thanks ... my analogies usually "bomb" around here, so it's always nice to know that one connected along the way.

At the risk of inserting another one that doesn't fare as well ... think about what happens to the billiard balls following a break. The applied energy disperses in a radial direction ... yet each ball continues to follow along its individual path, reflecting off the rails iaw with AI=AR. If it weren't for the force of gravity and the friction of the felt, as well as the energy absorbed by the rails they are reflecting off, they would continue indefinitely along the straight line path from which they originated and subsequently acted upon by the rails iaw AI=AR.

The ricochet from a shotgun blast also might aid in the concept. The initial dispersion of the shot from the barrel will radiate (in concert with the choke), but the path of each of the pellets will continue on a straight path, each one at a different angle from the others. Should one of those pellets strike an object and ricochet (i.e. reflect), it will continue to follow a straight line path iaw. AI=AR. It too would continue indefinitely, if it were not for the force of gravity to pull its vertical axis down to the ground. Light, however requires much more than the earth's gravitational pull to alter its straight line path, and as such continues along its straight line path, except as acted upon by other objects, where AI=AR continues to apply (index of refraction, also).

Point being ... ISL pertains to point source dispersion, not reflection (as Guari and others have duly noted) ... and camera to subject distance is NOT responsible for exposure due to ISL theory being applied to reflected light.

You might want to read up on reflection of light from diffused surfaces. Here is a quick link to a wikipedia artile, but I am sure there will be other better articles on the net:

http://en.wikipedia.org/wiki/Diffuse_reflection

Mirrors reflect light only in one direction such that a light coming in at one angle leaves at exactly that angle on the other side. This is not true of non-mirror i.e. diffused surfaces. Diffused surfaces reflect light in all directions uniformly - pretty much like the light coming out of a point light source. This is why for example that if you are looking at a uniformly lit wall then its brightness would seem the same if you look at it from a slight angle versus straight on. Every point on the wall reflects light uniformly in all directions.

Now every real world object has some diffusion reflection as well as some mirror-like reflection. However for non-mirror objects most of the reflection is diffused reflection. As the article would tell you "The visibility of objects is primarily caused by diffuse reflection of light". I would also suggest that you look at the first image on the top right in the article which shows diffused reflection of light from a point on a surface, and then imagine a lens in the path of these light rays and consider what would happen when you move the lens forward and backwards.

I understand the example of the like spokes. However that spoke is representing just one ray of light. A lens captures multiple rays of light from the same point (again refer back to that image and imagine a lens in the path). That's why we build lenses at the first place, a lens gathers lots of light rays from a point on the subject and focuses them on a point on the imaging plane (hence we call it focusing). That is also why lenses with larger apertures are brighter. Every point on the subject is sending rays in multiple directions and a larger lens aperture has a larger surface area to capture more of those rays, and thus get a brighter image of that point. When you move the lens backwards, the number of rays from the subject that hit the lens surface decrease and you capture less light from that point on the subject. And thus the ISL still hold.

RustyBug wrote:
Curious80

Take your 5 ft @ 2x2 scenario and errant application of ISL theory and rather than try to use it to explain what you think you are observing when you move back to a 10ft @ 4x4 area ... instead, move to 2.5 ft @ 1x1 for both walls . Both walls would still be filling the frame (as your 2x2) and according to your theory of ISL applicability, the exposure would need to be changed due to your increased proximity to your subject. Move closer yet again, and your exposure (according to your ISL application theory) would again change. Here is where your theory of applying ISL as you've suggested ... falls apart.
.....



No as per what I explained the exposure would not change. The intensity of light arriving from each point in the wall to the lens has gone up by a factor of 4. However you are now getting the light from just 1/4th of the area of the wall (1x1 ft compared to 2x2 ft). So the total amount of light getting to the lens is the same. In fact if the ISL was not in action THEN exposure would have changed.

curious80 wrote:
Every point on the wall reflects light uniformly in all directions.


Ummm ... AI=AR

If a point on the wall is receiving light uniformly (i.e. diffuse light) from all directions, it will reflect light uniformly in all directions in accordance with AI=AR.

This is not the same as a point on the wall being a point light source that disperses light radially iaw ISL. Recognizing and acknowledging that the camera to subject distance DOES change the angles involved and may / may not include / exclude differing / offsetting amounts. But again, that is not ISL.

Light Science & Magic ... I'd recommend it over Wikipedia for a deeper read. Also, while the Wikipedia article introduces elements of translucent and refractive index properties of non-opaque objects, it does not come remotely close to presenting the issues fully. But, even here a thorough understanding of it will reveal that the substructure reflections still follow AI=AR (in conjunction with refractive index of translucent medium being passed through).

RustyBug wrote:
curious80 wrote:
Every point on the wall reflects light uniformly in all directions.


Ummm ... AI=AR

If a point on the wall is receiving light uniformly (i.e. diffuse light) from all directions, it will reflect light uniformly in all directions in accordance with AI=AR.

This is not the same as a point on the wall being a point light source that disperses a single ray of light radially iaw ISL.
...



Hit your physics books again and you will find out that "angle of incidence = angle of reflection" only applies to flat mirror surfaces. Most of the subjects have diffused surfaces and this law doesn't apply to diffused surfaces.

Even if the light is not diffused and is coming from a single direction, the reflection from a diffused surfaces is still diffused i.e. light gets spread in all direction. Look at the diagram again. It shows light coming from a single direction and spreading everywhere after reflection. If it was an ideal diffused surface then light will indeed spread uniformly in all directions much like a point light source. Real objects are not ideal diffused surface, but the light is still spread in all directions from the surface though in a non-uniform manner (you can look at goniometric diagrams to see how real objects behave).

But lets even ignore that. At least you agree that when an object is illuminated by diffused light coming from all direction then it will reflect light uniformly in all directions. Lets say you are taking the picture of such an object. So you will agree that the amount of light that your lens will capture from that object will depend on the distance of the lens from the object? because if you are closer to the object then you will capture more of the rays and if you are father than you will get less rays.

Ummm ... Wikipedia's illustration that you refer to is not indicative of ISL. It makes reference to an "ideal diffuse reflector" ... which is not the same as ISL. It is advocating what it would look like IF an object were an "ideal diffuse reflector" ... which would require exacting reflective design iaw AI=AR ... yet it still isn't ISL.

I'm quite aware that real objects are not ideal diffuse surfaces, but you are failing to realize that the reason for the various angles of diffuse reflections is attributed not only to the surface characteristic of multiple angles, but that light is also coming from multiple angles. AI=AR ... it really doesn't get any simpler than this.

I appreciate your passion to try and explain this as you have, but it just isn't an ISL issue / application that is being offset by camera to subject distance. It is AI=AR and the trigonometric impact associated with corresponding vector quantities as angles change.

Beyond this, I'd suggest it will require a course in physics from someone other than myself to make you believe other than you currently believe. I know a professor who designed night vision optics for the Pentagon that I could recommend if you're looking for a good one to learn from. Otherwise, Light Science & Magic is still a good read.

RustyBug wrote:
...
I'm quite aware that real objects are not ideal diffuse surfaces, but you are failing to realize that the reason for the various angles of diffuse reflections is attributed not only to the surface characteristic of multiple angles, but that light is also coming from multiple angles....


We have to disagree on that. But we have gone way off topic.

The simple point is this - the points in a scene or on a subject reflect light in multiple directions. It is not the case that all the light rays leaving a point on an object all travel in the same direction (if it was the case then we could only see that point from that direction and from no other direction). And if the light is traveling from a point in multiple directions then how much of that falls on the lens depends on lens area and distance of lens from the point. There can be no denying this simple principle.

curious80 wrote:

Hit your physics books again and you will find out that "angle of incidence = angle of reflection" only applies to flat mirror surfaces.


Sigh ... really

Then you've just totally trashed your Wikipedia article that shows AI=AR as a collection of the various surface angles for a non-uniform surface. If you read the rest of the physics books, you'll note that translucence and refractive index are part of the rest of the story, as well as absorption and form transfer of energy to heat ... but they don't negate AI=AR for that which remains to be reflected.

You've clearly made the point that you ardently believe what you believe ... but I simply cannot ascribe to your belief / attempted explanations that are foregoing the most fundamental elements of vector forces regarding the physics of light. I accept it as a lacking on my part to be able to convince you otherwise.

I'll rest my case ... if you choose to disagree, that's your prerogative, but I still think you've misunderstood / misapplied the concepts involved.

RustyBug wrote:
curious80 wrote:

Hit your physics books again and you will find out that "angle of incidence = angle of reflection" only applies to flat mirror surfaces.


Sigh ... really

Then you've just totally trashed your Wikipedia article that shows AI=AR as a collection of the various surface angles for a non-uniform surface.
....



Sigh form my side as well

Ok here is one last try.

Yes AI = AR holds but the surface of the non-mirror surafaces ensure that even if all the light rays are coming from the same direction, the rays get reflected in many different directions. So a real object with a not-perfectly-smooth-mirror-surface will be sending light off in all directions. Agreed?

And of course with a real light source all the rays will not be coming from the same direction because each point on the object will be receiving light from different point on the light source. So a real object illuminated by a real light source will be sending rays in all directions. Agreed?

And if that is the case then the amount of the light coming from that object and captured by the lens will depend on the lens area and the lens to object distance. I don't really know which part of that deviates from AI=AR or any other physics law.

You can choose not to believe it but that is the way the lens optics work. Lenses collect multiple light rays coming from the each point on the object. These rays travel from the point to the lens at various angles and hit the lens at various point. The lens then focuses all those rays from a point to a single point on the sensor (if the point is in focus). Since the light rays are coming at different angles so obviously the number of rays from a point that get collected by the lens depends on the lens area as well as the lens to point distance. You can ask your physics friend and he will tell you that this is right.

Okay, so you've got multiple linear light rays from multiple angles striking multiple surfaces at multiple angles being reflected ad infinitum iaw with AI=AR to produce a diffusion of reflections and subsequently being collected and refracted by lens optics.

That kinda sounds like the anti-thesis of a description for point light source radially emanating for which ISL theory applies.

Seriously ... pick up a copy of Light Science & Magic. It's a good read that merges physics with practical.

AI=AR

Time to give it a rest, Rusty. You've told curious80 that you think he's wrong, PeterBerressem has told curious80 that he thinks he's wrong, Guari has told curious80 that he thinks he's wrong, I've told told curious80 that I think he's wrong...

You can lead a horse to water...

Yes AI = AR holds but the surface of the non-mirror surfaces ensure that even if all the light rays are coming from the same direction, the rays get reflected in many different directions. So a real object with a not-perfectly-smooth-mirror-surface will be sending light off in all directions. Agreed?

A "mirror" surface differs from a "non-mirror" surface only by degree. The light rays your camera captures are a bundle of more-coherent-than-not rays coincident with the axis of the camera lens.

The more mirrorlike the reflecting surface is, the more rays will be in that more-coherent-than-not bundle...i.e., the brighter it will appear.

RustyBug wrote:
Okay, so you've got multiple linear light rays from multiple angles striking multiple surfaces at multiple angles being reflected ad infinitum iaw with AI=AR to produce a diffusion of reflections and subsequently being collected and refracted by lens optics.

That kinda sounds like the anti-thesis of a description for point light source radially emanating for which ISL theory applies.

Seriously ... pick up a copy of Light Science & Magic. It's a good read that merges physics with practical.

AI=AR

Man the point of discussion was whether the amount of light captured by a lens changes or not when the distance between the subject and the lens changes. Most of the posters here have expressed the opinion that the amount of light captured by the lens does not change with distance from subject because the light rays from the subject to the lens are collimated. I have gone through great lengths to establish to you that this is not the case and the light rays from any point on the subject go in all directions instead of being collimated (I hope you would realize that the only way you can see a point on an object form multiple directions is if it reflects light in multiple directions). Therefore the number of light rays captured by the lens from any point on the subject will change with the distance. (However the exposure dos not change because of the reason that I have already explained multiple times). The principle whether you call it ISL or not, is the same basic principle as applied to ISL and point light sources. Thats all. You can choose to believe it or not but thats what it is.

As for your repeated suggestions about physics reading, while I am not a physicist I have taken graduate school courses dealing with design of photographic systems, including not only simple camera systems but also with operation of more much more complex light field camera systems. All these issues of illumination, reflection and lens operation were topics that we dealt with in depth. This particular issue is a standard one which most people have difficulty in grasping but which we also discussed in detail. Again I suggest going back to your physics friend.

And if you are in doubt I would again invite you to think about the fact that the only way you can see a point on an object form multiple directions is if it reflects light in multiple directions. If I look at an object from straight on, and then shift to the left and look at it again from the left, it brightness seems identical to me. This is because it is reflecting light uniformly in different directions. Otherwise it should looking darker to me as I shift to a side. Once you understand this point, you will realize how the rest of what I said follows naturally from it. I leave you at that. Bye to all, I tried my best to educate you.

And if you are in doubt I would again invite you to think about the fact that the only way you can see a point on an object form multiple directions is if it reflects light in multiple directions. If I look at an object from straight on, and then shift to the left and look at it again from the left, it brightness seems identical to me. This is because it is reflecting light uniformly in different directions. Otherwise it should looking darker to me as I shift to a side.

It does change brightness as you shift to the side, it is not reflecting light uniformly in different directions. Notice the phases of the moon. For that matter, move your reflected light meter 180 degrees around a subject. It's not reflecting light uniformly in all directions.

curious80 wrote:
All these issues of illumination, reflection and lens operation were topics that we dealt with in depth. This particular issue is a standard one which most people have difficulty in grasping but which we also discussed in detail. Again I suggest going back to your physics friend.

"This particular issue is a standard one which most people have difficulty in grasping"

I'm with you curious80. You're just stating fact, right from the point where the ISL affects exposure over distance yet it makes no difference to exposure. Your first post I believe: http://www.fredmiranda.com/forum/topic/1162172/0#11083218

ukphotographer wrote:
curious80 wrote:
All these issues of illumination, reflection and lens operation were topics that we dealt with in depth. This particular issue is a standard one which most people have difficulty in grasping but which we also discussed in detail. Again I suggest going back to your physics friend.

"This particular issue is a standard one which most people have difficulty in grasping"

I'm with you curious80. You're just stating fact, right from the point where the ISL affects exposure over distance yet it makes no difference to exposure. Your first post I believe: http://www.fredmiranda.com/forum/topic/1162172/0#11083218


Thanks! I tried

I have to say that considering the length of this discussion and the rather stubborn viewpoints, you all should be commended for maintaining your civility. Seriously, this board (and other boards that will not be named) could learn a lesson in discussion from you all.
Interesting discussion by the way.

+1 @ curious80 presented well as a good natured FM'er in the spirit of Fred's code of conduct for us all.

A question to curious80, if I may :
an evenly lit flat e.g. board, metered with a 5° spotmeter at varying distances, achieves identical exposure values. How comes?