On quality of light

‘Quality of light’ is one of those phrases I use often, but perhaps should explain a little better. Similarly, we are all guilty of overusing the ‘good light/bad light’ phrase; but what does it (and we) actually mean? To a certain degree, we photographers are programmed both with preconceptions of what constitutes favourable light – based on our own or others’ historical work – and what constitutes ‘bad’ light. At the same time, we also have our own aesthetic biases and preferences – some of us may prefer flatter or more diffuse light as a consequence of spend childhoods at extreme latitudes, or be predisposed towards hard contrast because we’re tropical people. Here’s the kicker, though: I believe there is no such thing as truly bad or wrong light; there is only suitability for a given subject and set of aesthetic preferences.

I think the easiest way to decompose this is to break light down into its four qualitative components: intensity, directionality, color and spread. (If I’ve missed one, please feel free to chip in in the comments.) Let’s define them individually first, so we have a common understanding to start from:

Intensity is a proxy for contrast: how bright is your source, and thus how bright is it relative to the other ambient sources? The more intense a source, the stronger the shadows it will produce immediately behind it, and the greater the contrast between the lit face and the shadow face of that object. When the source is so intense and the contrast so strong it exceeds the dynamic range of the camera, parts of the scene will clip to white or black – this can be a pain if your intention is to depict all parts of the subject clearly, but it can also be of artistic value if you want to choose to hide or obscure certain elements.

Directionality describes diffusion: where is the light coming from? A point source is directional because all rays must emanate from that one point; a collimated source (e.g. slit or hole mask in front of a point source) is even more strongly directional because only light travelling in that single direction defined by the mask is allowed to pass through. In practical situations, a collimated source might be sun/clear sky shining directly through a window aperture or slatted blinds; projected shadows are strong and directionality is hard, strong or tight (your choice of descriptor). A diffuse, omnidirectional source could be an array of point sources behind diffusion material – neighbouring sources in the array make up for the single source’s single directionality, and the diffusion material forces further averaging. There is directionality, but effectively every point on the diffusion material facing the subject radiates 180deg in front of it.

Note: all light diffuses – and thus weakens – over distance; in a perfect vacuum this is not the case, but even interstellar space contains a few molecules of hydrogen or the occasional stray helium atom, which over millions of light-years is enough to create diffusion. The effect is obviously much faster in denser materials. Effectively, diffusion is the physical scattering of light by the medium.

Spread defines the width covered by the beam – it’s related to but not quite the same as directionality, because you can have a source that’s near to your subject and narrow, which results in a directional spot (and possibly high intensity if there are no other secondary sources) – or the same source could be wide and cover more of the scene. Thirdly, you could have a distant source that’s narrow but covers all of your subject anyway because of distance, angles and diffusion of the medium. (The four possibility of far/wide is no different to the third from the point of view of both the subject and the nearer camera.)

Color is pretty self explanatory: what is the dominant spectrum? Color of inbound light also of course affects color of outbound reflected light from a subject: the surface of any subject acts as a filter, absorbing some wavelengths and reflecting others. This is of course why subjects are not monochrome under white light – the nature of the material determines what is absorbed and what is reflected. If your inbound light is a wavelength that is absorbed by your subject, reflected light will be minimal and the subject will in turn appear dark – e.g. blue objects under red light and vice versa.

Even in scenes that have ostensibly only one source – say outdoors, daylight and the sun – that’s not entirely true. Every material on Earth is reflective to some degree, otherwise there’s no way we can see it. This creates some complexity because that means every surface is a source; fortunately, most of the time the surfaces are so rough and diffuse that the net output is fairly uniform and doesn’t really create clear secondary sources (and accompanying shadows). There is always some reflect light, somewhere. Even the atmosphere itself creates diffusion – think about fog, incense, smoke etc. For the purposes of photography, whether we worry about a source or not is determined simply by this: does it create a shadow? I believe the most important thing is to have a clear hierarchy of sources: you need to know where your key light is, and fill the rest (or make sure it’s filled for you).

Impossible light – the sun was the only source, but there were plenty of ambient reflectors…

Shadows are the only way we can project three dimensions into two and imply some semblance of spatial order; at larger scales, shadows tell us what is in front of and on top of what. They create foreground-background relationships. At the micro scale, shadows create texture – without micro shadows, you can’t tell if a surface is rough or smooth. Reflective surfaces appear reflective because there are no shadows (and thus no diffusion), which means you see what’s immediately in front of that surface – governed by the rules of reflection, of course. Reflections can be interesting: multiple apparent sources without much loss in brightness even though you only have one true primary source; skyscraper corridors in modern cities are a good example of this – it’s possible to have a subject lit from all sides thanks to all that mirrored glass.

Traditionally, I think when we say ‘good light’ what we really mean to say is that there is directionality, narrow spread and a good intensity differential; i.e. contrast and shadows. When there is ‘bad light’ or ‘no light’, we mean there is too much diffusion and thus no differentiation in intensity – no shadows. However, three scenarios come to mind here: firstly, in a situation where you are in full control of all sources – i.e. fast enough shutter or bright enough light to eliminate ambient – then you can really determine where light falls using positioning and modifiers. The second scenario is where the quality of light suits the subject: a clean, modern urban environment may look its best – or most aesthetically pleasing, at any rate – in hard directional light of morning or evening with clear skies; a portrait of an older lady requires a bit more diffusion, but still directionality. If you swap light and subject – diffusion in the urban scene and hard directionality on the lady – the aesthetics aren’t so well suited to the subject.

I’m going to leave you with the third scenario: ‘no light’. In a heavily overcast outdoor situation – think London in winter – there are ostensibly no shadows, but this does not mean no directionality. In a way, you can always ‘force’ it – work at the extreme ends of the day just before sunrise and sunset; at night with artificial light; or if you can move the subject, use archways and porticoes because you will then only have a diffuse source from one side only – which is ideal for many kinds of work because it’s the sort of flattering light that hides flaws. Lastly, there’s also the option to throw spatial arrangement out of the window altogether: just as we can use strong shadows to blur the line between real and projected, we can use an absence of shadows to do the same; I think of it as painterly light 🙂 MT

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