October 11, 2009

Metering by Eye

Light is the basic element of photography, just as sound is that of music. A good photographer should be as familiar with light as the good musician is with notes and scales. Photography is, at its core, based on a very simple principle. An image of the world is captured by allowing a certain amount of light to fall on a piece of photosensitive material. Whether the photosensitive material in question is a silicon chip, silver film, glass plate or salted paper, this elegant little concept holds. Whether or not the light is shaped by the latest cutting edge in glass and coating technology, an old brass lens or indeed a humble pinhole, the same principles apply. The crucial questions of how much light reaches the photosensitive material and in what ways that amount may be controlled go to the very essence of photographic technique – the determination of exposure.

“Reading the light” or “metering by eye” can be easily mastered with a little practice, yet most photographers leave this central photographic decision almost entirely to their camera. Of course, camera meters work, but it helps to know precisely how a camera sets exposure and what a particular process' limitations are – in other words it is useful to know when to disagree with the camera and to take control of exposure decisions. But many new photographers think metering is some unfathomable mystical art best left to eccentric geniuses and pieces of silicon. Camera companies have done their bit towards obfuscating the issue by bragging about the wonders of “35 area evaluated metering,” “3D matrix metering,” “40 segment multi pattern metering” and such. These modes can indeed be powerful and useful tools for the right job but they don't yet supplant the intelligent photographer's brain. So let's try to lay out a practical approach to metering by eye, or being able to set the correct exposure without the help of a light meter. I shall assume that the reader is familiar with the basics of photographic exposure and can use a light meter efficiently (although I'll deal with those basic issues in a later article).

LV and EV: Ways to Think About Light
For the purposes of figuring out incident light levels we will try to think in something called LV or “light values.” LVs are a sequence of numbers representing intensity of light. Each number is double the previous one and half the next one – i.e. they are on a logarithmic scale. Thus LV 11 denotes twice the amount of light than LV 10 and only half the amount of light compared to LV 12. This might remind you of the f-stop scale and that is exactly what we are going to think of the LV scale as. Henceforth we will think of LV values in terms of “stops of light” - thus we could say LV 10 is two stops below LV 12 and so forth.

To avoid confusion, we might briefly mention EV here (although you won't need it for our purposes of metering by eye). EV, or exposure value, which you will see on many light meters, refers to particular exposure levels which can be achieved by certain aperture shutter combinations. For example EV 15 can be 1/100 at f16 which is the equivalent of 1/200 at f11 or 1/400 at f8 and so forth. All of these shutter-aperture combinations result in the same level of exposure (although the depth of field will differ, of course) and are designated by EV 15. If you notice we have not yet taken account of the sensitivity of the film or sensor – i.e. the ISO value which is the other major factor that influences exposure. So naturally as ISO varies, the same EV shutter aperture combination will produce different levels of final exposure. So while for bright sunlight EV 15 will be the correct exposure for ISO 100, the same exposure will be produced by EV 16 for ISO 200 or EV 13 for ISO 25 etc. Key point: EV numbers change with changing ISO.

LV numbers, on the other hand refer simply to the level of light and don't take into account the specifics of photographic exposure. Thus, they provide a better and more uniform way to think about light levels. If you familiarize yourself with the LV scale you can see a certain amount of light and immediately think of its LV value whether or not you have ISO 1600 or ISO 100 set on your camera. This separation of light levels and specific exposure factors like ISO-aperture-shutter has its advantages.

How do LV and EV numbers relate (or, another technicality you might want to ignore)? Well, they are exactly the same for ISO 100. But if the ISO changes to anything else, EV numbers naturally change while LV remains constant. If you figure the ISO in terms of an offset from 100 in number of stops then you can get EV by adding that number to the LV value: EV=LV+offset. A couple of examples will make this clear. Let's say you have ISO 400 film – that's two stops more sensitive than 100. So the EV value would be LV+2. If it's bright sun, the LV value is 15 and the EV value is 17. For ISO 50, the offset will be -1. So the corresponding EV value would be 15+(-1)=14. If this seems too complicated, ignore it – in fact, totally ignore EV for the moment and think of light only in terms of LV.

Recognizing LV values
Bright sunlight – that is, LV 15 - and that is our first cornerstone. Silly though it might seem, familiarize yourself with bright sunlit conditions - how intense is it, what kind of shadows does it cast? We will learn to think of other light levels in terms of how they differ in intensity from bright sun. And once you have mastered LV 15, think of everything else in terms of how many stops of light darker (in very few cases brighter) it is compared to bright sun. The best way to master this is to walk around for a couple of days with a light meter or even a camera with a meter. Whenever you see a new light level – take a guess how many stops below bright sun it would be and cross-check against your meter. Cloudy? That's maybe 2 stops below (LV 13). You will be surprised how fast this can be picked up.

Once you know your bright sun, there is another light level that I recommend familiarizing yourself with thoroughly – that of a moderately well-lit indoor setting. This is definitely harder to define in any concrete manner, so a light meter will definitely help. Look for LV 5 – or 10 stops below bright sun. It should be a about the light level of a moderate sized room lit with a 100 watt bulb. Once you find it, familiarize yourself with it fully, because it is much easier to think of night exposures as offsets from this instead of from bright sunlight. A brightly lit hall? A stop above – so EV 6. A lamplit street? Two stops below – so EV 3.

Converting LV Values to Exposure Settings
You may know the “sunny 16 rule” which says that in bright sun the correct exposure is 1/ISO value. So if you have ISO 250 film you'd set f16 at 1/250. What happens when your light is, say, four stops below bright sun – or EV 11. You can simply take the f16 at 1/250 combination and “open up” four stops. This can mean opening up the aperture or slowing down the shutter or a combination of both. So open up to f4 and keep 1/250; or alternatively keep f16 and slow your shutter to 1/15; or open aperture two stops to f8 and shutter two stops to 1/60. The combinations don't matter – as long as you “open up” the requisite number of stops. So, always start with the ISO value of your film/sensor for your shutter speed and f16 and then calculate how much you want to open up each. This implies of course that you are totally familiar with the aperture and shutter speed scales. That f2.8 is five stops from f16 or that 1/8 is 5 stops from 1/250 should be second nature – as much a reflex as scales under a pianist's finger. Fortunately, you'll find mastering this set of apertures and shutter speeds - if you don't already know them well - is much easier than learning to play the piano! Once this is accomplished, setting exposures for any LV will not be a problem. Let's say you're shooting in a relatively well lit restaurant that you think is LV 6. You have ISO 500 set on your digital camera and a fast f1.4 lens. However, you would like as much DOF as possible but don't want to handhold below 1/30. You need a total of 9 stops below bright sun. So set shutter to 1/30. That's 4 stops. The other 5 stops you'll need to get from aperture – so set it to f2.8 and presto!

In the age of advanced camera meters this might seem tedious and way too complicated at first. But just like synth-loops haven't put good drummers out of a job yet, the good photographer will know exactly when the meter can handle a scene and when to take control. I'll end with an ordinary example. This shot was made very hastily indeed. I saw these cool looking guys walking unconsciously in formation lost in the latest football news and had only a few quick seconds to frame, focus, set exposure and shoot. This is exactly the kind of scene where you want the quick automation of a modern intelligent camera meter, right? Wrong! Not because the meter isn't competent – but because it simply can't read my mind. There's more than one way to shoot this scene, surely. What if I wanted to have the street perfectly exposed for bright sun and have the guys looking like cool silhouettes? Most old camera meters would be “fooled” by the bright background and do just that. But it's not a question of getting fooled – it's about knowing the intention of the photographer. No matter what the camera companies tell us – there is not just one "correct exposure" of a scene. The exposure depends on the photographer's intended interpretation – just like this scene would work with at least three or four different exposure levels and produce four different interpretations. So unless they make a camera that plugs into a USB jack behind my ear to pick up what my brain is thinking, the camera meter simply doesn't know what effect I am going for. So even if you have a fancy meter, knowing the ins and outs of exposure will let you tell it just what to do (by setting the amount of compensation you want, for example). For me in this scene it involved a couple of quick and simple steps. I wanted to expose for the guys' faces but not totally blow out the background. I know that in bright sun, full shade is almost four stops less bright. So I decided to go for 3 stops below LV 15 (underexpose their faces by a stop at most but hold as much detail as possible in the background). So with ISO 250 film, I set my aperture to f8 and shutter to 1/125 – three stops – click!