Petteri's Composition Class: Light (1/4)

Started Mar 12, 2003 | Discussions thread
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Petteri Sulonen Forum Pro • Posts: 24,585
Petteri's Composition Class: Light (1/4)

Well, I finally got inspired to finish the "Light" lesson I'd been working on for a while, and boy, is it a doozy. Meaning, long. I hope at least someone has the patience to wade through it!

PART ONE: WHAT IS LIGHT?

"Photography is 50% photographer, 40% light, and 10% equipment."

Photography means drawing with light. You don't actually take a picture of something: you record the light emitted or reflected from something. Understanding something of light can greatly improve your photography. This Lesson covers some theory about light, and follows up with some practical advice on how to deal with specific kinds of light.

The most important classification of light in photography is the division between available light and artificial light . Available light is the light that happens to illuminate the scene the photographer wants to photograph: it can be daylight, moonlight, starlight, streetlight, or interior lighting. Artificial light (in this sense) is light that the photographer controls to achieve a specific purpose in the photograph. It can be on-camera or off-camera flash, studio lights, or just any old light hauled up for the photo. A reflector is something in between: a device the photographer uses to modify available light to improve the picture.

For most purposes, available light should be used whenever possible, and artificial light should be made to look as "natural" as possible... unless the photographer has some very specific artistic or aesthetic purpose in mind.

With these things in mind, let's get down to the business at hand. What is light? What characteristics does it have? How does it get to the camera? And what can we do about it?

Characteristics of light

The three most important characteristics of light are brightness, colour, and temperature. Brightness does not need much explanation, but colour and temperature are slightly more subtle concepts.

Light is electromagnetic radiation visible to the human eye. It consists of different wavelengths, which are perceived as different colours. Very long wavelengths are perceived as red, and very short ones as violet. In between are orange, yellow, green, blue, and indigo. Beyond red are infra-red, microwaves, and radio waves; beyond violet, ultra-violet, X-rays, and gamma rays.

Light that has a specific colour is emitted on a narrow band of wavelengths. For example, the yellow sodium streetlights only cover one wavelength -- that emitted by excited sodium atoms. The same is true for most coloured light, such as the different-coloured flares used in fireworks, neon signs, Christmas lights, and so on. coloured light can only show up tonality (dark to light) in those colours that reflect the colour of the light. For example, a blue object in sodium light appears completely black: try it out for yourself if you don't believe me. However, a green object also reflects yellow light, so it will not look black -- and an object that is exactly the same colour as the sodium lights will look identical to a white object.

White light is a combination of many wavelengths. It can show up tonality in all colours. However, the wavelengths may not be evenly distributed: the light may have a colour cast. For example, normal incandescent interior lighting is heavily weighted towards the orange-red end of the spectrum, and fluorescent light has a green cast. This is where white balancing comes in: it corrects for the distribution of wavelengths in the available light, and gives objects their "actual" colour. The human eye white-balances automatically -- that's why we perceive a white sheet of paper as white even when it's lit by orange incandescent light... but if we photograph it in daylight white balance, the photo will appear orange. (Or vice versa: if we take a picture of a white sheet of paper in daylight using incandescent white balance, it will appear blue.)

The colour cast of white light can be unambiguously expressed as a temperature, usually in the Kelvin scale. It is a physical fact that if you heat an object to a given temperature, it will emit light with a certain colour cast, no matter what material the object is made of. However, this light is still "white" in the sense that it contains light of all wavelengths: therefore, it can reveal tonalities in all colours, and it is possible to correct for the colour cast by white balancing.

In a nutshell: you can white balance to correct colour casts in white light, but not for coloured light. There is no way of getting natural-looking colour in a scene lit by sodium streetlights: there simply aren't any blue or green photons to carry the information.

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