The world of lighting in front of the lens

I’m sat by the Yonne river in Burgundy, squinting at the sun in front of me. Specular highlights on the brick next to me; the reflection of the water tickling the side of the ancient bridge; shade under the trees on the opposite bank. I’m thinking of brightness and colour relationships between the various sources, and how knowledge of these can be used if I ever wish to recreate some of the feeling of the light I’ve seen here today.

In my article Behind the Translation I outlined my view that we are judging our lighting solely through monitors and EVFs, and that this is causing us to lose touch with the physical reality of how the light is playing in the scene. The monitor can be a useful lighting tool, but if we are bound to it, we are severely limited in our ability to visualise, make proper use of a recce, pre-light, create lighting lists, achieve consistency, and ultimately, get what is in our heads onto the screen in a reliable manner. Your incident meter, your eye and your firm posture in front of the camera are the tools with which to understand how light is playing in the physical world.

Today I want to discuss this in a more practical manner by introducing the way I try to think about exposure and the light in the scene, and how a thought process like this can lead to valuable conclusions about how to light and expose.

The diagram below is a conceptual illustration of exposure in terms of the chain of elements which create the image value for a given pixel, in the simplified case of matte subjects.

Obviously there’s no single correct way of picturing something, and this is just my way of breaking it down in my head. But what’s important is that I have a way, and every DP must have a clear picture in their head of how the physics of light and exposure come together to create the backbone of our craft.

Let’s say we have a white mug with black spots as our subject, as in the diagram. The footcandle level* expresses the amount of light falling on the subject. Let’s start with the simplified case that our subject is evenly lit. In this case, the same amount of footcandles falls on every part of the subject.

The subject has a range of different reflectivities, the main two being the white of the outside of the mug and the black of the spots. These two are vastly different in value. The footlambert level* expresses how bright a certain part of the object looks to the eye or to the camera, and it is a function of the footcandle level falling on that part as well as the reflectivity of that part. Although the same footcandle level falls everywhere on our mug, the white part of it has a much higher reflectivity and therefore creates a much higher footlambert level than the black part does, which causes the white part to look brighter than the black part.

Of course, it may be the case that there is a more complex light falling on the mug. Let’s say we have flagged our light source off half of the mug so that the flagged half has a much lower footcandle level falling on it. Even though the white of the mug has the same reflectivity everywhere, half of the white has a much higher footcandle level falling on it, and so generates a much higher footlambert level. The flagged part of the black spots generates a really tiny footlambert level, and hence looks very dark.

Sound easy? Okay then, answer this: we’re in a living room with a nice view into the garden, which is lit by a scorching hot sun. There’s a black shirt out to dry outside, and our friend inside is wearing a white shirt. We can all picture from experience that no matter how dark it is inside, the white shirt inside will still look much whiter than the black shirt outside. The black shirt, despite being of low reflectivity, is under such a high footcandle level that it is giving off a higher footlambert level than the white shirt inside. So how is it possible that the white shirt still looks brighter than the black shirt?

The answer is that the human visual system has what we might call spatially adaptive light sensitivity, and determines the brightness of objects by comparing their footlambert levels to what’s immediately surrounding them. So the whole of the area visible outside the window is ‘reduced’ in exposure by our visual system while what’s inside is amplified. This is what allows us our visual system to have such extraordinary dynamic range. And, as you might have suspected, cameras don’t do this. If you expose for the white shirt, and the black shirt outside is indeed emitting more footlambert, the black shirt will look whiter than the white shirt in our footage.

So, to summarise what happens in front of the lens: every part of the scene in front of us is giving off a footlambert level which determines how bright that part of the scene looks. That footlambert level is a function of the footcandle level falling on that part of the scene and the reflectivity of that part of the scene. This is actually only the case for matte subjects. Glossy and mirror-like subjects and self-illuminating subjects are a slightly different story, but they still ultimately present a footlambert level.

So what happens after that? The footlambert ray representing a given part of the scene passes through the lens and is affected by the T stop set on that lens before being focused onto the sensor or film. There is no term in common usage to describe the ray of light after it has been affected by the T stop, so I coined the term final ray. Understanding this bit is key to understanding ISO and where to rate cameras, but that’s for another article - let’s get back to the scene in front of the lens.

Consider that a camera sees only footlambert levels. It does not know anything about footcandles, and it does not know anything about reflectivities. It cannot distinguish whether the footlambert level it sees is coming from a black shirt under bright sunlight or a white shirt in the shade. Ansel Adams’ zone system, which is well worth studying**, comes out of this basic truth, and is all about measuring footlambert levels using a spot meter and using our taste and pre-visualisation skills to decide how bright the part of the subject that gave rise to that footlambert level should appear in the final image.

The issue is this: the camera does not know a black shirt from a white one. A reflective meter – such as a spot meter – also doesn’t. If you read a footlambert level with a spot meter and translate the reading directly onto the lens, you’ll get an image in which the part of the subject which gave rise to the footlambert level you measured is rendered as middle grey, that is, in the middle of the dynamic range. The problem is that neither a black shirt nor a white shirt should be middle grey. The first way to fix this is essentially the zone system: read the footlambert level, judge how many stops above middle grey the part of the subject you just measured should be rendered, adjust the reading by that amount (so you’d overexpose the reading a few stops for a white shirt, and underexpose it a few stops for a black one) and put the adjusted reading on the lens. Caucasian skin tone, for example, is typically exposed around one stop over its spot reading.

But there is another method, in which you don’t have to guess how many stops above or below middle grey you want a particular object to be rendered: the incident meter. Our incident meters are actually measuring footcandles – the amount of light falling on a subject***. That’s why we use an incident meter at the subject. The white dome on your meter takes an average of the footcandle levels falling on it over a solid angle of 180degrees. Pointing the dome at the camera gives you a useful average footcandle reading of the light falling on the front side of your subject. Reading to the source rather than the camera tells you how many footcandles are coming from that source.

If we take an incident reading of a given footcandle level and put the T-stop readout on the lens, we will expose such that a middle grey reflectivity under that footcandle level is rendered in the middle of the dynamic range, and all reflectivities above and below, provided that they are lit by the same footcandle level, will be rendered above and below in the correct proportion. This way the guesswork is taken out of it – we don’t need to decide where to place individual reflectivities, but only what source to pick as our key. Obviously no real scene is lit by a single equal footcandle level, so we have to decide what footcandle level to treat as ‘normal’, or in jargon, as key. The key footcandle level is the footcandle level under which a middle grey reflectance falls in the middle of the dynamic range, and it is set by reading what you’ve decided is that level and setting the resulting T stop on the lens.

From here onwards, it’s a question of personal preference of exposure methods, and as usual, the important thing is not what your method is, but the fact you have one. I personally read to the source, not to the camera, and then decide how many stops above or below key to place that source. I think of a light source being at, above or below key.

A cloudy sky might be at key (so I read the light and place that stop on the lens) whereas sunlight be one to two stops above key (so I read directly to the sun and overexpose the reading by one to two stops). How do I decide where to place a given source? The sun needs to feel bright so needs to be above key – that’s an easy one. But how about a scene consisting of two areas lit by two different sources of different brightness? How about a wide shot of a subject at a table lit by a lamp with darkness all around? We’ll see later that the most important thing is actually the difference between readings of various sources. It depends on your taste and judgement. This is the art of exposure, and it’s inextricably bound with the art of lighting. What do we want to draw attention to, what do we want to keep in the shade, where do we want to direct the eye? What feeling do we want to evoke? What do we want it to look like? These are the important questions, and everything we’ve said above is only useful insofar as it helps us get what is in our heads consistently onto the screen. 

I’m going to present a few frames from my own work as specific examples of interesting situations in terms of how to read the light and set an exposure. All images are copyright of their respective owners - the first two are from TryLife: Jacob's Story and the third from Tea for Two.

Apart from the funny anecdote that I used a mirror to reflect in the light from a unit outside which we didn’t have a high enough stand for, this is an interesting shot in terms of exposure because nothing is at key. The shafts of light on his eye and on the bed are over two stops above key, and anything on the face which isn’t hit by those shafts is below key. Let’s review what I mean by that: reading those shafts to the source reveals a number that is more than two stops closed from the shooting stop, and reading any part of the face not hit by those shafts reveals a number that is further open than the lens is. How would I actually decide on the stop? What actually happened on the day is that this was the last shot in the scene, and my shooting stop was established earlier in the scene, on a shot roughly like this:

I metered and lit this with the idea that there should be bright light coming from the window but that the room should still be dark. At the character’s face position, the window reading should be a couple of stops over, and the reading away from the window should be severely under. So the process might be:

(1)  Set lights outside window and position of blinds to visual taste

(2)  Read to window at face position

(3)  Set shooting stop to be 1-2 stops closed from this reading

(4)  Read away from window and adjust fill or negative fill until this reading is sufficiently under the shooting stop

Although in practice, it’s likely that once I had the shooting stop, I set the fill (negative, in this case) by eye and just quickly checked that the reading was far enough under key.

Having established a shooting stop for the scene, when I came to the close-up of the character lying down in bed, I maintained that stop and it was a matter of controlling the brightness of those beams to make them look right for the shooting stop, and I did that by eye. I’m quite comfortable doing things like that by eye, and using the security blanket of the monitor to make sure I’m not blowing anything. On film, a spot meter would come out quickly to make sure those beams were in range. On digital, it’s very easy to use a monitor or waveform to check we’re not clipping. And using a monitor in that way is not a problem – it’s using it to its strengths.

Once we have this contact with the reality of the light in the scene, we can use the monitor to it’s strengths and still be confident that we understand what’s happening with the light in the scene. We know what’s going on, we know that it’s unimportant exactly where the beams fall exposure wise, as long as they’re bright and as long as they don’t clip. So you use your eye and your taste, and you check the monitor for clipping.

This is another example of nothing really at key. The front of our character’s face is under key, and if we were to read to her ‘kicker’, the light that’s falling on her left ear and shoulder, the reading would be slightly above key. And pretty much everything behind her reads above that. To light and expose this, you’d start with the controlling idea that her face should be below key and that the background should be quite a lot brighter than that, decide shooting stop using a reading of your choice – eg ‘her face should be 1.5 below key’ and then check other readings against your shooting stop and tweak lights to get those readings where you want them. In the case of this specific shot it would actually be easier to tweak the face lighting, but whatever you’re tweaking, it should be apparent by now that what really matters is the contrast – the difference in reading between the various elements. It’s pretty easy to print up or down a little in the grade. It’s much harder to make changes to scene contrast in a pleasing way, and pretty much impossible in most cases to isolate the effect of a specific source and globally ‘brighten’ or ‘darken’ that source with respect to the rest of the scene.

The only reason why ‘key’ and ‘shooting stop’ are important is that they are a benchmark against which to measure differences in reading. We’re talking about one reading being a little below and one being above as a way of talking about the difference between two readings. Face -1 and background +2 or face at key and background +3? The choice between those two is not as important as the decision that the difference between the two is 3 stops.

This concept also applies to colour temperature – it’s not too hard to warm up and cool down the overall shot slightly in post – but it’s nigh on impossible to change the warmth or coolness of a specific source relative to another. The difference in colour temperature between two sources working in one shot is much more important than where we set the white balance.

So what I’m really asking myself when I light, once I’ve decided what something should look like, is: what is the brightness relationship between the different sources playing in this scene? What is the colour relationship between them? And once that’s clear in my head, where to set the white balance, and where to set the shooting stop, is mostly a matter of what to show everyone in the edit and in dailies. The key decision is about relationships, contrast, differences.

I’m not in Burgundy anymore. It’s taken me so long to write this that I’m in a café in Bristol, and yet we’ve only dealt with the tip of the iceberg that is the physicality of light in space. It’s the beginning of a mindset that leads us to think with the camera “behind us”. We’re not thinking about what’s on the monitor, we’re thinking in terms of footcandles, keys, reflectivity, footlamberts. We no longer have to light from behind the camera and the monitor, but we can use those tools to our advantage when we judge that it’s useful. We can light with our eyes and our understanding of how the light is playing in the scene, and then walk back to the viewfinder confident.

L.

Burgundy and Bristol, May 2015

*The footcandle level is more formally known as the illuminance, and is measured in footcandles. The footlambert level is formally known as the luminance, and is measured in footlamberts.

**Read The Negative and The Print by Ansel Adams.

 ***They give you a T stop once you tell them what ISO and frame rate you’re shooting at, but what they’re actually measuring is footcandles, and the ones specialised for cinematography can read out directly in footcandles.