Photography Basics - Why Use A Light Tent?

[jrista]

I don't think we should eliminate "diffused light" from the discussion. There are diffuse materials, and there is diffuse light. You've talked about diffuse materials...which create diffuse light! Diffuse light itself matters as well. The very simplistic geometric example of just three rays doesn't really demonstrate the nature of rays with diffused light, as it would be less coherent than the example you demonstrated in your previous post... Your three incoming rays are fairly coherent, whereas with diffused light coming from all directions, there wouldn't be that neat coherence producing a specular highlight as your example demonstrates.

I understand all the theory you are describing, and I completely agree with your description of why you used the matboard. I was trying to say the same thing...you are creating a band of brighter light that creates a "neat" reflection. I don't disagree with that assessment.

There is such a thing as diffused light, though. The very word "diffuse" means to "spread out over a large area"...i.e. to create a "large" light! I think the commonly understood term here IS diffuse light. That's what a light tent does...through the process of diffusion, it enlarges the area around the subject from which the wavefront propagates. I have two large lights...around 3 feet tall or so, and about a foot and a half wide. Without their diffusers attached, they are largely a small, 5" diameter light source. With the diffuser attached, they produce a fairly evenly illuminated 3.5'x1.5' bright box of light.

I still get what you are saying, that moving a light source back can also enlarge it when using a tent. The wavefront expands outward from the source (what we also call the inverse square fall off of light, which means that while the wavefront is larger, its also less intense...meaning the farther away your light source, the more of an impact it will have on your exposure settings). So yes, moving a light farther away will enlarge the light source within the tent. But it only does so, because of the diffusion the tent applies to the wavefront... So its definitely about diffuse light. Without the tent, a distant point light source...is still a point light source... (By point, I mean small, not technically a true "point" in the mathematical and optical sense.)

 

[Sylvanite]

The very simplistic geometric example of just three rays doesn't really demonstrate the nature of rays with diffused light, as it would be less coherent than the example you demonstrated in your previous post...

I only drew three rays, but please imagine that instead, the picture showed an infinite number of rays coming from every possible angle that reflects off the curved surface into the camera lens. THAT is what I was trying to show in order to illustrate the need for "large light".

Again, in pen photography, the range of angles that a source emits light is irrelevant. What matters is the range of angles from which the pen is illuminated. Whether a light source is diffuse or not is meaningless. Whether it is small or large is paramount.

There is such a thing as diffused light, though.

Ok, I was trying not to say this (to avoid argument), but here goes: there is no such thing as "diffuse light". A light source may emit light in a small range of angles (a.k.a. not diffuse) or a wide range of angles (a.k.a. diffuse), but the light itself is no different. Ponder this. The sun emits light in every possible direction. Therefore it must the the single most diffuse light source on earth. Yet, you would hardly call direct sunshine "diffuse light".

The very word "diffuse" means to "spread out over a large area"...i.e. to create a "large" light!

"Diffuse" is not the same thing as "large". The sun emits light in every direction (spread out over the largest area possible), but the range of angles of light that actually hit the earth is quite narrow. Therefore direct sunlight is actually a "small" light source.

I think the commonly understood term here IS diffuse light.

And, the commonly understood term is woefully misleading. Adding diffusers to one's lights will not fix a poorly lit pen photograph. What matters is the relative size of the source light, not it's diffusion.

That's what a light tent does...through the process of diffusion, it enlarges the area around the subject from which the wavefront propagates. I have two large lights...around 3 feet tall or so, and about a foot and a half wide. Without their diffusers attached, they are largely a small, 5" diameter light source. With the diffuser attached, they produce a fairly evenly illuminated 3.5'x1.5' bright box of light.

One last time - although diffusion may be the process by which a larger light source is created, it is not the diffusion that matters, but the size.

 

[jrista]

I only drew three rays, but please imagine that instead, the picture showed an infinite number of rays coming from every possible angle that reflects off the curved surface into the camera lens. THAT is what I was trying to show in order to illustrate the need for "large light".

Again, in pen photography, the range of angles that a source emits light is irrelevant. What matters is the range of angles from which the pen is illuminated. Whether a light source is diffuse or not is meaningless. Whether it is small or large is paramount.


Ok, I was trying not to say this (to avoid argument), but here goes: there is no such thing as "diffuse light". A light source may emit light in a small range of angles (a.k.a. not diffuse) or a wide range of angles (a.k.a. diffuse), but the light itself is no different. Ponder this. The sun emits light in every possible direction. Therefore it must the the single most diffuse light source on earth. Yet, you would hardly call direct sunshine "diffuse light".


"Diffuse" is not the same thing as "large". The sun emits light in every direction (spread out over the largest area possible), but the range of angles of light that actually hit the earth is quite narrow. Therefore direct sunlight is actually a "small" light source.


And, the commonly understood term is woefully misleading. Adding diffusers to one's lights will not fix a poorly lit pen photograph. What matters is the relative size of the source light, not it's diffusion.


One last time - although diffusion may be the process by which a larger light source is created, it is not the diffusion that matters, but the size.

I think you missed my point.

Without diffusion, a lamp, like yours, is small, no matter how far away you place it. Kinda...like the sun! (Good analogy!)

So, whether we accept whether there is such a thing as "diffuse light" or not (it is an extremely common term in photography, so I use it and I think people generally understand it; personally to me, its more about what happens with the wavefront, but most people don't actually understand light in that manner), the point I'm trying to make is that the tent diffuses the light, and that diffusion is what makes the light "large."

Diffusion is the process. As such, most people call what a light tent (or other diffusers involved in lighting) as making the light "diffuse"...spread out over a large area. In that sense, yes, I would say there IS such a thing as diffuse light. Its what we call the nature and size of the light after a light tent does its job. We can get overly technical, or we can stick with terms people generally understand.

But my point was, without diffusion, the distance of a simple bulb in a lamp really doesn't matter. The light doesn't become larger, because its not diffused (spread out). The sun's light is scattered by the atmosphere, which in effect diffuses it, which is why the sun doesn't look the same here on earth as it does in space. In space, the sun is very much more like a small light (small bulb in a lamp), but on earth it lights up the entire sky (with or without clouds really). Diffusion spreads light out...and what people colloquially call that is diffuse light.

I think there are other ways to describe light as well. Direct, which is what I would call actual sunlight (vs. say "daylight" which would include all the scattered light from the atmosphere), and indirect. Large vs. small. Diffuse vs. hard. There are a lot of terms we can use to describe light. Its a means of communicating in the end.

Anyway. I think the points are made. We use different terms, but I think we are on the same page. I come from an astrophotography background mostly, since ~2013, and in that field, we are much more concerned about the wavefront, and how the optics and filters and internal obstructions affect that wavefront in its entirety. Its not just diffraction, its all the various aberrational effects...so, I think a lot in terms of how the wavefront is affected. Diffusion makes a lot of sense when you think about the wavefront...

 

[MRDucks2]

By the way, I really enjoy people who are smarter than me discussing different perspectives of something they know more about than I do. Wonder education tool. Keep going.

 

[Sylvanite]

I think you missed my point.

And I think you missed mine.

Without diffusion, a lamp, like yours, is small, no matter how far away you place it.

Not so. A diffuser is a light modifier that can be used to enlarge a light (make it physically bigger and move it closer), but it does not necessarily do so. Nor is it the only light modifier that can make a light larger (or smaller).

So, whether we accept whether there is such a thing as "diffuse light" or not (it is an extremely common term in photography, so I use it and I think people generally understand it... the point I'm trying to make is that the tent diffuses the light, and that diffusion is what makes the light "large."

Yes, it's a common term, but one that people generally misunderstand. When you tell people they need to "diffuse the light", you are telling them to use a particular process - not what that process is supposed to produce. When someone uses that process without understanding the goal, he is likely to fail (i.e. produce a light that although diffused is still small). The light illuminating a pen needs to be large, whether or not it is diffused. So, insisting on using the term "diffuse light" does not help.

I accept your dictionary definition of "diffuse" to mean "spread out". That's exactly what a diffuser does. It spreads the light out. As long as the light fully covers the pen, however, it does not matter how much more you spread the light out. To understand how to light a pen well, one needs to stop thinking about light from the point of view of the lamp, and start thinking about light from the point of view of the pen. Not "does the light cover a broad area", but rather "how broad is the area from which the light comes".

But my point was, without diffusion, the distance of a simple bulb in a lamp really doesn't matter. The light doesn't become larger, because its not diffused (spread out).

Not so. With or without diffusion, both the physical size and the distance of the light source matter. "Large" light is a relative term. The larger it is physically, the larger it looks. The closer it is, the larger it looks. Placing a small diffuser over a small light source diffuses (spreads out) the light, but does not make the it look any larger.

Large vs. small. Diffuse vs. hard. There are a lot of terms we can use to describe light. Its a means of communicating in the end.

Exactly! The goal is to communicate clearly. That is why it's best to use terminology that is not likely to be misunderstood. The term "large light" is much clearer and more accurate than "diffuse light".

Anyway. I think the points are made. We use different terms, but I think we are on the same page.

Maybe so, maybe not. But even if so, I want others to realize that in pen photography, one needs to have light coming from a wide range of angles (large light), not light that covers a wide range of angles (diffuse light).

I come from an astrophotography background mostly, since ~2013, and in that field, we are much more concerned about the wavefront, and how the optics and filters and internal obstructions affect that wavefront in its entirety.

Pen photography is very different from astrophotography.

 

[jrista]

Yes, it's a common term, but one that people generally misunderstand. When you tell people they need to "diffuse the light", you are telling them to use a particular process - not what that process is supposed to produce. When someone uses that process without understanding the goal, he is likely to fail (i.e. produce a light that although diffused is still small). The light illuminating a pen needs to be large, whether or not it is diffused. So, insisting on using the term "diffuse light" does not help.

I accept your dictionary definition of "diffuse" to mean "spread out". That's exactly what a diffuser does. It spreads the light out. As long as the light fully covers the pen, however, it does not matter how much more you spread the light out. To understand how to light a pen well, one needs to stop thinking about light from the point of view of the lamp, and start thinking about light from the point of view of the pen. Not "does the light cover a broad area", but rather "how broad is the area from which the light comes".

Alright... This is a good explanation. The last bit, how broad is the area from which the light comes, is the key point you are making I think.