Scattered light and diffusion , a different angle

[Rusty]

While experimenting with my home-made diffuser , photographing a crumpled foil ball , soon realized that no matter how diffused the light source is there are always some parts that are blown

The diffuser was a flat reflector ( at +-20 degrees to the flash head) to spread the flash hot spot and then reflected through 2 layers of spaced diffusion material (from lcd panel)

This made me realize the main problem might not be the amount of diffusion that will solve the problem but the amount of light the blown spot "sees"

Every single spot that has a curve allowing the spot to "see" the whole light source are likely to be blown , where the surfaces with a gentle curve "sees" only part of the light source and stay within limits

If you photograph a flat reflective surface with a shiny ball parked on the surface with any light source, there will be a small high spot on the ball because the ball " sees" the whole source (like a wide angle lens) but the flat surface does not

In my opinion the problem is because the light is diffused and scattered
What is really needed is a light source that covers an area somewhat larger than the subject to be photographed
The light source must be of even illumination (does not have to be diffused)
The light rays must be parallel to each other

When illuminated this way every part of the subject can only receive light from one direction (which is constant over the whole area)
and cannot act as a small curved mirror to reflect the whole light source, causing a high point

May this be a doable solution ? Please feel free to correct if i made a logical or technical blunder some where

[rjlittlefield]

The light rays must be parallel to each other

Well, this part is clearly not correct. When the light rays are parallel to each other, the situation is equivalent to having the light come from a small source far away. If the subject is a curved mirror, then the camera will receive no light except from spots on the subject that happen to be angled so that the camera can see the reflection of the light source. The subject will render as black except for tiny very bright specular reflections -- the complete opposite of what you are trying to achieve.

The rest of your description seems more on track, but given the above I'm not sure that I am reading your words the same way they were intended.

A good way to think about illumination is to imagine how things look from the standpoint of the subject. The lighting will be most even ("flat", free of shadows and blown highlights) when light comes evenly from all angles, everywhere around the subject. Think of the subject being suspended in the center of a translucent white ball that is evenly illuminated on the outside.

In this case, if the subject is a curved mirror, then every point on the subject will reflect a different region of the illumination source. But because all regions of the source are equally bright, the reflections will be equally bright also. The mirror will appear to be evenly illuminated.

In fact if this is done perfectly, the mirror will not even appear to be a mirror, but rather a featureless gray shape that could equally well have a matte surface! Fortunately this level of perfection is very hard to achieve.

In a studio environment, what works well to minimize harsh reflections is to surround the subject with a "light tent" made of diffusing material. This is often not practical in the field, so what's usually done there is to place a large diffuser as close to the subject as possible, so when the flash goes off, the subject sees "the whole sky" get bright.

As an example of the light tent approach, see http://www.photomacrography.net/forum/v ... php?t=6816 . The subject there is well described as a "curved mirror" -- it's a piece of highly polished metal that used to serve as the filament in an automobile headlight. 'Twas the devil's own time to photograph! In the end I had to essentially wrap the thing in tissue paper, leaving a hole just big enough for the lens to shoot through. If you go to the top of page 2 of the thread, you'll see something of what the subject looks like with more directional lighting.

I hope this helps. Happy experimenting!

--Rik

[DaveW]

I was always taught, back in the steam age days of black and white negatives and printing, that all whites should be slightly veiled apart from specular highlights (mirror like reflections off chrome or water etc) which should record as paper base white = clear film, or in present terminology as blown highlights.

What puzzles me therefore is should highlights reflected off say the transparent shiny wings of hoverflies have a slight veiling or be regarded as specular highlights and burn out?

With your tinfoil ball I would think, like chrome, you would get some specular reflections which in Ansel Adams time would have been allowed to burn out and be clear film. However I suppose a monitor screen is brighter than a print, so the base white of the print sets its reflectivity anyway?

Do we need therefore to adopt different criteria for say printing out, or digital projection where the light is reflected off the surface to that in the case of monitor screens where the light comes from behind so is much brighter?

A few quotes off the Web for printing:

"Specular Highlights. Specular values refer to highlights that are pure paper-base white and have no image detail. Specular highlights act like mirrors."

"Your prints should have some white. Specular highlights should be pure paper-base white. Everything else white, say white walls near a lamp, should show the slightest trace of density. You should see a trace of grain there."

"Specular highlights have no density and are reproduced as pure paper-white, adding brilliance. Diffuse highlights are bright and have a delicate gradation"

For Ansel Adams zone system:

"X (zone 10)- Reproduces as paper base white, no image recorded. In print, will appear as specular highlights, sun reflection from chrome bumper, sunlit drops of sparkling water, etc."

http://community.spiritofphotography.co ... opic=474.0

Or throw the books away and use your own eyes?

http://www.westcoastimaging.com/wci/pag ... tip13.html

http://photo.net/photography-lighting-e ... rum/007tNJ

Can't see me using this one on hoverflies though:-

http://www.scielo.br/scielo.php?pid=S01 ... ci_arttext

DaveW

[AndrewC]

Light Science and Magic by Fil Hunter and Paul Fuqua is a good read

[Rusty]

Rik wrote

Well, this part is clearly not correct. When the light rays are parallel to each other, the situation is equivalent to having the light come from a small source far away. If the subject is a curved mirror, then the camera will receive no light except from spots on the subject that happen to be angled so that the camera can see the reflection of the light source. The subject will render as black except for tiny very bright specular reflections -- the complete opposite of what you are trying to achieve.

Rik ..The way i understand it is , rays from the sun are almost parallel and the reason it will reflect as a pinpoint is because of it's relative size to the subject being photographed is much smaller than the subject due to the distance

Imagine moving the camera ( or the whole planet) towards the sun, so that the sun fills the whole sky
The subject can now "see" only one big source of parallel light larger than itself ( almost like a giant light tent from one horizon to the other
Would it then still reflect a pinpoint on a polished globe ?

If the rays are parallel and cover the whole subject, what would block of the transmission of light in certain areas to cause the dark portions between the specular points you mention ?

I am thinking of my proposed light source much alike a cool large sun very close to the subject
Am i still totally missing it ?

or at least making a teeny bit of sense ?

[Rusty]

@ DaveW....Thanks for the informative links and info!

It is evident that when controlling specular highlights the main factor is the size of the diffuser , a big hurdle for me whilst chasing shiny critters in dense vegetation

[Rusty]

Thanks Andrew......will take me a while to digest all that info

[DaveW]

If the light source is larger than the subject would all the rays then be parallel to it, or am I not understanding what you mean?



The situation is surely like the tomatoes in my earlier link. In the first they are lit by a comparatively small light source where the rays are more parallel but when a larger softbox is used the rays become less parallel from the edges of the larger light source?

http://photo.net/photography-lighting-e ... rum/007tNJ

Or am I on the wrong track?

DaveW

[Rusty]

@ Dave

The way i understand it is that all rays transmitted from a subject will do so at a right angle from the surface of the body

The reason why the suns rays are parallel is because of its size in relation to us the curvature of the suns surface is so gentle that the rays are very close to parallel
Where you have a small pinpoint source, the curvature is small and the divergence between the rays more pronounced

edit: this will not apply to any diffuser, which is designed to prevent the rays from escaping in parallel and rather to scatter in all directions

[scitch]

I host a science misconceptions podcast for teachers and I happen to be working on an episode about this exact topic. Turns out that the sun's rays are not exactly parallel when they arrive at our home planet. They are angled as much as a half a degree. It's easy to calculate, just get the distance from the sun to the Earth and the diameter of the sun. Draw a triangle from one edge of the sun to the earth and back to the other edge of the sun. Even at 1 Astronomical Unit, the rays are not quite parallel. Albeit, a half a degree isn't much.

I had originally heard this stated as, "Because the sun's rays travel parallel to each other, the shadow of an airplane is the same size as the airplane. It's not true. And a half a degree over 40,000 feet would make a difference in the airplane shadow's size.

BTW, it is definitely not true that all rays come perpendicularly out of a light emitter. If that were the case, we would not be able to see the entire sun. In Rusty's diagram, we would only be able to see that portion of the sun whose light struck our eyes and the rest would not be visible. Also, shadows would be perfectly dark if this were the case.

Light comes out of each spot of the sun at every angle. We only see those that intersect our position. That includes ones coming from the edges of the sun traveling 0.26 degrees from perpendicular and those coming out of the center of the sun traveling precisely perpendicular. DaveW's second picture was correct.

[Rusty]

Thanks scitch

BTW, it is definitely not true that all rays come perpendicularly out of a light emitter. If that were the case, we would not be able to see the entire sun. In Rusty's diagram, we would only be able to see that portion of the sun whose light struck our eyes and the rest would not be visible. Also, shadows would be perfectly dark if this were the case.

That makes perfect sense now, The closer one would go to the sun, the less parallel it would be because the visible diameter of the sun is larger , close to the surface the light will be very diffused , because it will be coming from all points on a very large visible disc

another point i missed was the deep shadows that would be caused if parallel lighting was used for macro
Exactly like the kind of midday sun we normally don't like to use

[DaveW]

"What is really needed is a light source that covers an area somewhat larger than the subject to be photographed."

In most cases this usually happens in macro work since the subject is so tiny. A normal flash gun provides harsh light for a portrait, but softer light for an insect because of its size in relation to it. With a hoverfly even a small flash's reflector is like using a softbox size light source.

The problem with macro would be getting small enough lighting to get the results a spotlight would produce in normal photography. To obtain the effect a spotlight would produce in normal portraiture for a tiny bug you might only need one fibre of a fibre optic light system.

We tend to name lights for the effect they produce on a human scale. However a floodlight for portraiture if shone on the side of an aircraft carrier would become a spotlight, similarly a spotlight shone on an insect becomes a floodlight. Lighting is all a question of size in relation to the object being illuminated.

I think the highlight effect of lighting for various sized light sources is answered by those actual examples on those tomatoes with gradually increasing softbox sizes, except the first bare flash is not diffused.

DaveW

[Rusty]

I think the highlight effect of lighting for various sized light sources is answered by those actual examples on those tomatoes with gradually increasing softbox sizes, except the first bare flash is not diffused.

Absolutely ! thanks

[DQE]

Light Science and Magic by Fil Hunter and Paul Fuqua is a good read

I have this book and also find it to be informative. It's helpful at both the practical and the moderately technical scientific level. It has many specific cases, such as how to photograph a very polished, shiny metallic object, etc. It helps one to separate the specular and diffuse components of one's lighting.

I believe that Strobist also recommends this book, which is how I learned of its existence and utility.