Category Archives: Light

A photographer’s ‘fish story,’ literally

When the full moon is set to rise into a clear sky over a calm ocean at dusk, you know it is going to be a great photo opportunity.

Bring on the golden-red colours, the presaging glow, the first glimpse of the sliver of disk peeping above the watery horizon, the silver path of light on the rippling waves!

This was the moment on Tuross Head Beach last weekend, with the moon at its fullest and rising into the fading light of an early mid-winter evening.

Sedate but swift, the moon rises

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On this magic night the moonrise is a slowly moving visual feast, clinging to the ocean with a spectacular light-drop effect, then drifting upwards between the layered clouds. As I take photo after photo, the sky darkens, and the moon begins to lighten.   While it still beams with a deep russet glow, I frame up the cloud-bespeckled moon yet again, trying to keep the barely visible sea horizon level. I take the photo.

Suddenly, an eagle

A split-second later, with the sound of my camera shutter still ringing in my years,  an Australian sea eagle dives directly in front of the full moon, head down and wings spread, magnificently limned in the moon’s ruddy glow.

The eagle plunges into the sea like an arrow, to snatch one last fishy meal from the water as darkness closes in.  And I missed the perfect photo by milliseconds!

Later, reviewing my images, I did find at least photographic evidence of my story of the ‘one that got away’ – this lovely photo of the eagle making its approach from just above the moon.



The moons sails the night sky

Soon the moon fades to silver-white, lighting the waves and heading off on its journey to illuminate the night for other entranced viewers.




Photographs: All photos by Sabrina Caldwell. Other than resizing for web-use, no alterations have been made except that ‘Sea eagle over moon’ photo was cropped.

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Flaming molecules

Photographing a fire can be both challenging and satisfying.  Challenging, because those flickering flames just won’t stay put for more than an instant at a time. Satisfying, because a crisp photo of a campfire or bonfire is a thing of golden beauty and lasting memories.

Campfire, April 2008

Campfire, April 2008

Photos of fires evoke special memories of star-spangled nights encircling a crackling fire with friends and loved ones. Looking at these photos years later helps us remember each campfire as a unique event, a particular kind of fire that feels different to the memory than any other.

But what exactly are we photographing when we frame up the flames in our viewfinders? Light, surely, but from where? What makes a fire burn? We all know it is the wood that is burning, but why do flames appear above and around the wood, not just on its surface? Conversely, why are there glowing coals inside the flames?

And why does putting water on a fire douse it? Why does putting a fire blanket on it smother it?

I’ve done some investigating into how wood burns, and have discovered that fires are both simple and complicated at the same time. And it is all to do with a chemical reaction that is self-sustaining once it gets going.

Secondary Combustion

It may seem counter-intuitive to start with Secondary Combustion; no doubt you are thinking “but what about Primary Combustion?”  I’m starting with secondary because I find it easier to think first in terms of the process by which a fire starts, rather than thinking in terms of fuel.


Plants build wood from CO2 and H2O, and fire turns wood back into CO2 and H2O again.

Wood is made up of molecules that are remarkably like sugar.  Like the sugar we eat, these molecules are an easy source of energy. While there are many types of molecules that make up what we call a piece of wood, the main molecule in wood is cellulose. In a live fire, cellulose molecules (C6H10O5) break down into carbon dioxide and water. [1] This chemical reaction is the reverse of photosynthesis, in which plants assemble cellulose from carbon dioxide and water. Plants build wood from CO2 and H2O, and fire turns wood back into CO2 and H2O again. Sounds simple, doesn’t it?  But it gets more interesting from here.

Firstly, it isn’t really the cellulose inside a piece of wood that catches fire and starts the whole chain reaction going. It is gaseous cellulose and other volatile molecules floating above the wood.

Cellulose molecules are linked tightly to each other through an oxygen atom.  Fires catch alight only after wood is heated enough that some cellulose molecules break away from their oxygen links. Once in the heated air above the wood, the unstable molecule reacts with nearby oxygen molecules. In this reaction, the atoms in the cellulose and the O2 swiftly recombine into carbon dioxide (CO2) and water (H2O) molecules. CO2 and H2O molecules have a lower energy state than cellulose, and the extra energy is released in the form of light and heat. [2] Voilà, light and flames and photographs!

Ironically, it takes heat to start a fire, because without heat, the cellulose will just stay in the wood and not combust with the oxygen.  However, once the initial heat is applied and the process starts, it is self-sustaining; the heat generated by the chemical reaction causes more cellulose to be liberated from the wood which then combines with O2 with the side effect of producing more heat which liberates more cellulose and so the cycle goes on. The fire will continue until the wood is exhausted or the fire is put out, whichever happens first.

However, what I’ve just described is only half the story. The flaming molecules of cellulose and oxygen are what cause the flickering flames in which we delight.  But what about the glowing wood, the motes flying through the flames, and the hot coals that toast our marshmallows? That’s where primary combustion comes in.

Primary Combustion

Primary combustion is what creates the coals and embers of the fire. This is direct burning of the solid wood rather than the burning of gases given off by the wood as happens in secondary combustion.

Camping at Wombeyan Caves 25 April 1999

Campfire burns low at Wombeyan Caves 25 April 1999

As the volatile molecules are used up and the fire dies down, the temperature around the wood drops, and the combustion of molecules such as cellulose no longer takes place as rapid gas combustion above the wood, but as slower burning directly within the wood. Because it takes place at a lower temperature and with less exposure to atmospheric oxygen, the solid wood burns more slowly, and tends to retain its carbon to become charcoal.

If the fire burns hot enough for long enough, all of the charcoal will also be used up and there will be nothing left of the wood but ash, which is all the components of wood that cannot burn.

Of course primary combustion is usually taking place at the same time as secondary combustion, which is why the firewood glows even while the flames flicker.  And as bits of wood pop and crackle, tiny pieces of wood undergoing primary combustion break away and fly up into the flames as motes of glowing coals.

Stopping the fire reaction

Bobby controls tree-side of bonfire Michigan, 2014

Brother Bobby hoses down tree-side of bonfire
Michigan, 2014

We all know that if we pour enough water over a fire, it will go out.  But why? It is because water reduces the amount of energy available to the chemical reaction of fire. It takes heat to make cellulose molecules break away to combust in the air. When water is added to a fire, it soaks up all the heat near where it has fallen, and the cellulose molecules in that area aren’t hot enough to break away. Unless a new source of heat is applied, the fire reaction can’t recommence.

Water is also the reason that fresh twigs and logs are often difficult to use in a fire – the water inside unseasoned wood interferes with the heat buildup that commences the fire reaction. If the fire is hot enough, and there is enough moisture in the wood, water escapes from the wet wood as steam. As the wet wood dries, it becomes usable fuel for the fire.

Smothering a fire with dirt or a fire blanket targets a different aspect of combustion. The goal with smothering a fire is to remove the fire reaction’s access to oxygen. Without access to oxygen molecules, wood molecules such as cellulose cannot react and recombine into CO2 and H2O and so no new heat is produced and the fire reaction cannot sustain itself.

So when we take out our cameras to photograph a fire, there is so much more to know about what we are recording than just the beauty of the flames. A fire photograph shows how much primary and secondary combustion is taking place, the speed of combustion, the quantity of coals being created, how wet the wood is, and how hot the fire is. These are the characteristics that form the uniqueness of that particular fire, and even if we don’t consciously recognise them, they are the characteristics that create our memories of that moment of fire.

All photographs by Sabrina Caldwell; other than re-sizing for webuse, photos have not been altered in any way.

The cycle of Cellulose and Oxygen CO2 and H2O diagram is a composite of six photographs, manipulated arrow clipart and text.  Many thanks to Harry Ward for his endless patience in throwing molecule models in the air for me to photograph!


[1] Curkeet, Rick. Wood Combustion Basics:
[2] More than you ever wanted to know about wood combustion:

Clipart arrows used in diagram from


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The light we photograph: spacelight

If you have read The light we photograph: daylight then you will know that little we see on the Earth is more then a handful of nanoseconds from the past. But what about when we turn our eyes (and our camera lenses) towards the night sky and star-spangled darkness of space?  From when does that light come?

Nearby things – very recent light

Moon over the ocean, early evening 18 July 2008

Moon over the ocean, early evening 18 July 2008

When we photograph the moon in the night sky, we are capturing it as it appeared about a second earlier. The light that describes its appearance required only about 1.3 seconds [1] to travel the average 384,000 kilometers from the surface of the moon to the surface of our lens. Using the same logic, we see Mercury as it appeared 5 minutes ago, Venus 2.3 minutes ago, Mars 4.3 minutes ago, Jupiter 1/2 hour ago, Saturn over an hour ago, Uranus almost three hours ago and Neptune four hours ago.

To try to imagine the speed needed to travel such vast distances in such a short time, consider this: on average, the time it takes for a human to blink is about 1/3 of a second.[2]  That means that light from the moon speeds across 384,000 kilometers of space to you in the time it takes you to blink at the moon 4 times in a row. (Try it one day, it’s fun.)

But now consider this: when we look at the moon between blinks, our eyes are being touched by light photons that only recently flew off the surface of the Sun.  The moonlight we see connects us in a very real way to the Sun and the Moon. That soft glow comes from light that left the Sun’s surface, sped 8 minutes and 150 million kilometers through intrasteller space to the Moon where it ricocheted off some dusty rock in our direction across another 384,000 kilometers in about a second, and that’s what just landed in our eyes.

Far away and long ago things

When we look up at the night sky or point our camera at a constellation, it is a moment of high drama. At that instant of time our eyes are intercepting a stream of light photons that may have been traveling towards us since before we were born, or before the first Pterodactyl took to the air, or even before our solar system settled out of the gas cloud from which it was born. More than just a temporary flare of light on the rods and cones at the back of our eyeballs, these photons are a physical connection between us and distant suns of our galaxy, and galaxies extending into the depths of space and time.

Imagine for a moment a relatively nearby constellation, the Southern Cross. Its stars range from 88 to 570 light years away. For those of us in the southern hemisphere, it is a special feature of the night sky. But consider for a moment the journey of the light we are seeing. The light of the furthest star we see burst from its sun’s surface and headed towards us at about the same time King Henry VI founded Eton College in the 15th century. By contrast, the light from the nearest star began its journey only a few years before the New York stock market crash of 1929.

Photograph taken in 2006, using 30 second exposure

Southern Cross constellation, photograph taken in 2006, using 30 second exposure

These timeframes are so extended that we cannot be sure that the objects we see in the sky are still there.  It would not be at all out of the question for one of these stars to have exploded in a supernova 80 years ago and we’re still waiting to get the news.

But of course 88-570 light years away is not as far away and long ago as we have been able to peer into the universe around us. Recent deep field photographs from the Hubble telescope have revealed light from stars in galaxies from more than thirteen billion years ago. [3]

Video still from HubbleSite

Video still capture from HubbleSite [4]                        

The more one contemplates the nature of spacelight, the more dizzying it becomes.  For example, when we see a planet such as Saturn, our eyes are being impacted by light photons that travelled from the Sun and made a billion kilometer each way trip to and from Saturn. Think of it, when we see Saturn in the night sky, photons of light that actually touched the rings of Saturn an hour earlier are now touching us.

On the night I took the photograph of the Southern Cross, other photons of light from these same stars (the ones that didn’t hit Earth) whizzed past us. Those photons are now eight light years (about 80 trillion kilometers) away and streaking off a rate of 27 billion kilometers a day. Perhaps on some clear night thousands of years and heptillions of kilometers distant those photons will impact upon the alien eyes (or photographic device) of an inhabitant of some other planet.

Moon over the ocean, Tuross Head, 2008 by Sabrina Caldwell; other than resizing for web use, no alterations have been done to the photograph. Southern Cross constellation, 2006 by Sabrina Caldwell; original photograph unaltered other than resizing for web use, overlaid with text information.  hubble deep field video still from HubbleSite


[1 ]Light travels at a speed of about 300 million meters per second. When this is applied to the 384,000 kilometers distance between the moon and the Earth, it means that it takes 1.28 seconds on average for moonlight to reach Earth.
[2] MadSci Network: Medicine Accessed July 2014
[3] Kramer, Miriam October 23, 2013 01:01pm ET Ancient Galaxy Is Farthest Ever Seen
[4] Hubble deep field video, Accessed July 2014.

Posted by on July 31, 2014 in Light


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The light we photograph: daylight

Photographers are very concerned with light: its colour tone, its angle of incidence, if there is enough of it, if there is too much. Why? Because while our photographic focus is on the subjects we wish to memorialise when we turn our lenses on them, we do not capture our subjects directly.  What we are photographing is the light that reflects from them, light that can and does very much assert its own characteristics, thereby aiding or interfering with our photographic intents. Understanding light is as important to a photographer as understanding water is to a scuba diver, or understanding air is to a pilot.

Beginning at the beginning

So where on Earth does all this light come from?  Leaving aside man-made light for the moment, the answer is of course that it doesn’t come from Earth at all. Our light streams continuously at us from out of the nearby 1.4 million kilometer wide nuclear fusion reactor we know as the Sun.

100K_years_8_minsOur daily starlight (better known to us as daylight) begins when energy is released deep within the Sun as hydrogen atoms are fused together to create helium atoms. For the next 100,000 years, that energy slowly migrates to the surface of the sun, where it flashes off into space at a speed so fast (almost 300 million meters per second) that it can traverse the 150 million kilometers between the Sun and the Earth in 8 minutes. [1]

A tiny fraction of all of this light heads in our direction, where it penetrates the sun-ward facing side of Earth’s atmosphere and encounters surfaces: clouds, ice, the wings of a bird, atmospheric gases, the leaves of trees, the ocean, a flower, buildings, a person’s face, our camera lens. When this happens, some light may be absorbed or passed through that surface, and the rest bounces off in a different direction at a speed of 3.3 nanoseconds (billionths of a second) per meter as indirect light.

Lion, San Diego Zoo, 2007 Photograph by Sabrina Caldwell

Instant of lion in light and shadow
San Diego Zoo, 2007

We do not see anything until these reflected photons of light and the visual information they carry reach our eyes (or our camera lens). And though the time lag is vanishingly small, it exists. If you are looking at someone from 5 meters away, you are in fact seeing this person as they appeared 17 billionths of a second ago.

We are so accustomed to this vast abundance of speeding light photons that we mentally perceive them as a whole (daylight), with a beginning (dawn) and an end (sunset).

But the nature of this light stream is not homogenous and static. It is volatile, filled with eddies and swirls, angles and transformations. It can be direct (should we be unwary enough to look directly at the sun) or (more usually) indirect. Like a river of water running down a wash, each succeeding instant of light refreshes our view of the world around us in its own likeness.

Light in a bottle

Of course we cannot see light as it travels because it goes too fast, right? Actually, that is no longer true.  Andreas Velten and Ramesh Raskar at Massachusetts Institute of Technology developed an experimental  camera in 2011 that records images at a rate of a trillion images per second and with this camera, light in motion can be filmed.  In practice, so far they have only managed something elementary, but still rather magical: a video of a pulse of light travelling through a plastic drink bottle.

Video stills from MIT femto-photograph of a single pulse of light

Video stills from MIT femto-photograph of a single pulse of light

You can see the video at but above are two screen captures from it to give you an idea. (The video is worth watching, it is less than 3 minutes and the pertinent bit is only 15 seconds from timestamp 1:47). The first image captures a pulse of light moving through darkness, and the second demonstrates how light scatters from and is shaped by a reflective surface (the inside of the bottle neck) upon contact.

Photographing light

Rainbow over Khancoban, NSW September 2004

Rainbow over Khancoban, NSW
September 2004

There are implications inherent in understanding our photographs as being constructed from light.

Firstly, it implies that our images are a fusion of the nature of the light falling on our subjects and the light qualities of the subjects themselves. This means that we should be aware of and understand the relationship between light and subject. What proportion of our photograph is about the light representing the subject in comparison to the light representing itself?

In the images from the MIT video above the object was to capture an image of light itself.  Another example of photographing light for its own sake that may be more familiar to us might be a photograph of a rainbow, where the goal is to capture the prismatic colours of the sun’s light reflected by moisture filled air.

Then there are hybrid photographs in which the quality of the light is foregrounded almost as much as the subject.  For example, the ‘golden hours’ in the early morning and late afternoon are so called because the sun’s light enters our atmosphere from an angle roughly parallel to the Earth’s surface, meaning that it has to travel through much more atmosphere than when the sun is overhead. This reduces direct light and increases indirect light, creating softer shadows and gentler highlights. As a result, subjects photographed during these golden hours often seem to glow. In these moments, the quality of the light is a very important component of the photograph, merging with the representation of the scene.

But for the most part, we use light as a sort of intermediary, a medium that carries detailed visual information about objects in the landscape to our eyes and to our camera lens. The medium, light, takes a back seat to the information it contains, much like the silicone of a mask is not as important as the face it molded.

Silicon mask of model's face "Making a silicone mask with Skye Wild"[3]

Video still of silicone mask of model’s face
“Making a silicone mask with Skye Wild”[3]

This brings us to the second implication, and that is that our original photographs are the closest representations of reality that we can achieve with a camera.

Each pixel of our photograph is an expression of the light that fell on an element of the camera sensor in that moment, and provide the highest degree of faithfulness to the existing light at the instant of the photograph. The photograph is an imprint of light, mediated by our camera sensor and settings we choose.

Any subsequent ‘tinkering’ with the photograph detracts from the essential ‘reality’ of the photograph. That is not to say that it is always inappropriate to manipulate photographs, merely that we need to understand that each operation we perform on our photographs takes them a step away from being a photograph and a step closer to being photographic art.

Lastly, on an existential note, the fact that we are unable to see something or someone until the light that first reached them reflects from them and reaches us suggests that it is not possible for us to see or photograph anything in the absolute present.  Put another way, everything and everybody we see and photograph is just ever so slightly from the past.

Silver Gull in 'golden hour' light Lake Macquarie, NSW 2007

Silver Gull in ‘golden hour’ light
Lake Macquarie, NSW 2007

So it behooves us to remember when we photograph our surrounds that we are at a visual remove from the objects in the landscape, linked only by the light rays that flooded the landscape and reflected from its elements.

Perhaps we should give more consideration to how the specific quality of the ambient light illuminates the scene and the role it plays in helping us or hindering us in pursuing our photographic goals. And perhaps we should think twice before manipulating our photographs, each of which is a unique record of the light that existed at a moment in time and space, to which we alone were privy.

We should treat the light that makes our photographs possible with great respect. After all, it took over 100,000 years to get to us.

Rainbow over Khancoban, NSW, 2004; Instant of lion in light and shadow, 2007 and Silver Gull in ‘golden hour’ light, 2007 by Sabrina Caldwell; other than resizing for web use, no alterations have been done to the photographs.


[1] Karl Kruszelnicki, ABC Science. (2012) Sun makes slow light. Accessed June 2014
[2] Andreas Velten and Ramesh Raskar, MIT, Visualizing video at the speed of light – one trillion frames per second. Video by Melanie Gonick Accessed July 2014
[3] Skye Wild, Making a silicone mask with Skye Wild.  Accessed July 2014


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Posted by on July 4, 2014 in Light


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