Exposure & Metering
For the most part, this explanation of metering and exposure can be applied to any photographic discipline. However nearer the end, I give some tips as to how I meter for portraiture. I think most people tend to stay away from manually metering and exposing a scene because they find it intimidating at the start. So the goal of this tutorial is to demystify what really is a pretty simple idea. I hope some of you find this useful.
So where do we start? Well first off, metering and exposure is pretty easy. Yes! most of our cameras can do one or both of these automatically for us, but to get the most out of your shots, it’s really important to understand what your camera is doing when it’s metering a scene and why it makes the decisions it does.
To begin with, we really need to discuss the term 18% GREY. If we have a surface which 100 particles of light fall on to and 18 are reflected off, then that surface will be a grey colour, referred to as 18% grey or sometimes middle grey. We’ll talk about this further on, but it is pretty misleading to call it middle grey, because exposure wise it isn’t really in the middle of black (zero light) and white (completely blown).
So we know what middle grey is.. but who cares? Well we care because the light meters in our camera live in a middle grey world. Here’s where we need to branch off a little because there are two types of light meter. The first is a reflective light meter and the second is an incident light meter. Both measure light in order to help a photographer expose for a scene, but both have very different methods of metering light and as such both are used differently.
Reflective light meters are what we have installed in our cameras. They measure the light reflected back from a surface/object. I mentioned already that our cameras reflective light meters live in a middle grey world, now is the time to expand on this. If we have a pure white wall, evenly lit and we use our cameras meter, the shot will come out GREY. If we take a pure black wall, again evenly lit (or evenly unlit) our meter will again convince our camera that it’s living in a middle grey world and again the shot will come out GREY. The same would happen if we shot a grey wall. Regardless of the colour, as long as there is one tone and we expose using the reflective meter in our camera, all the shots will come out the same colour – GREY! Anything you point it at – GREY. Anything! To summarise, the reflective meter in our camera tells us how to make the thing it’s pointed at to record as an 18% grey value.
Now we know what middle grey is and we know that our cameras meters try to average a scene out as middle grey. Why on earth is this useful?. The reason is because it’s a constant – our meters behave in the same consistent way, regardless of lighting conditions. If we point our meter at a bride in a really white dress, lit by powerful strobes against a white backgroud and we point our meter at the dress… what will we get?? Yes, we get GREY (good to see you’re listening ). If we take the same bride, in the same dress and we change the lighting conditions and location, we can still meter off the dress and still get the correct GREY exposure. If you’re asking yourself “If our meters make a white dress GREY, then how do we make it white?” then you’ve completely understood what’s been said so far. If you aren’t, time to read back over it. The answer to that question comes later.
Before we move off reflective meters, there are somethings we should be aware of. Because the meters in our cameras measure reflected light, they can be fooled sometimes depending on the surface they’re being pointed at. If you’re metering a dark scene (black cat for instance) the black absorbs more light than it reflects and it can fool your camera into compensating by overexposing. Similarly white, or reflective surfaces (an ocean, water etc) can also give misleading reflective meter readings. Often wedding photographers have an extremely difficult task because they have to meter for a white dress as well as a black grooms suit in a scene – so the camera meter can be thrown off completely. One tip would be to buy and use a grey card. These are essential square pieces of card, coloured 18% grey. Basically the grey card is put in the same light as the subject we’re shooting – we then meter off the card and lock in the exposure values for the correct middle grey reading, remove the grey card and take our picture.
So, reflective meters measure reflected light and they turn what ever we’re pointing them at to middle grey. Incident light meters are subtly different. Incident light meters measure the light falling on the surface of our subject from the light source NOT the reflected light. Unlike the grey world our reflective meters live in, incident metering render any object as its true tone, so black records as black, grey records as grey and white records as white. To take the example of the bride and groom, if we use an incident light meter with the bride as the subject but then replace her with the groom, the groom will be properly exposed. Why? Well because the bride was exposed in a way thay she was rendered as her own true tone, so why not the groom in his black tux? One point I want to hammer home is that when using incident light meters, you have to point the light meter at the light that is lighting the part of the subject you want to properly exposed – NOT AT THE CAMERA
So…. Reflective meters make whatever you point them at record on film as 18% grey, regardless of what they are in real life. Incident meters make black record as black, gray record as gray, and white record as white.
So now we know what metering is, the next thing to understand is exposure. An exposure value (EV) is the total amount of light available, where as exposure (in our cameras) is the way we use that light. To explain exposure, I’ll use a common analogy of a bucket and a water tap, where water is an analogy for light. For a correctly exposed scene, we need to fill the bucket to the brim. If the bucket is not quite full of water, the scene/picture is underexposed. If we over fill the bucket, water spills away and detail is lost, which is equivalent to over exposing a scene, commonly referred to in digital as blown highlights.
We have two primary ways to fill the bucket. We can turn on the tap to give us a very slow trickle of water (narrow aperture/high f-number) and leave it running for a very long time (slow shutter speed). An alternative way would be to turn the tap on wide open (wide aperture/low f-number) and leave it running for a much shorter duration (faster shutter speed). It’s really important to note that both methods end up with a full bucket/correctly exposed scene. It’s way beyond the scope of this article to discuss the implications of choosing aperture and shutter speeds as these often have different artistic affects and implications. But what is important is that we ensure we always fill the bucket, regardless of whether we’re using a huge gush of water or a drip at a time.
EXPOSURE & METERING
So how do we tie metering and exposure together? Well we’ll concentrate on reflective metering as this is what most of us will use in our cameras. Imagine we had a wall that was painted 1/3 white, 1/3 black and 1/3 grey. If we pointed our camera at the 18% grey part of the wall, the meter would give use the correct exposure value to fill the bucket/expose the scene correctly. If we pointed it at the white part, the meter would think you had a much smaller bucket (more light reflected) and would call for a smaller aperture or a shorter shutter speed. If however we pointed it at the black section, less light would be reflected, so the bucket would seem bigger and a longer shutter speed and/or wider aperture is required. All 3 surfaces try to render the subject as 18% grey.
When we look through our cameras and look at the meter. By varying the shutter speed and aperture, we are exposing to make the scene go middle grey assuming that our meter falls on the center value.
MATRIX METERING Vs. SPOT METERING
One final thing we need to discuss before getting into the practicality of how to put all this to use, is to discuss the different metering systems on cameras. The two main ones I’m going to talk about are Matrix metering (or evaluative for Canon users) and spot metering. Simply put (and it is simply put), matrix metering looks at the range of light in a scene from bright all the way through to dark. It will try and average the light in a scene so as it all becomes middle grey. Obviously if there is a huge range of tones (dynamic range) the camera will struggle, leading to whites being blown or losing detail in the shadows.
Spot metering is far more precise. It measures the reflected light at the specific point your are pointing to in the scene. This allows YOU to decide which exposure values to use in a scene. A classic example to demonstrate the differences between both systems is to put a person standing in front of a window with very strong back lighting. If we are trying to expose the scene for the person, we need to understand how each system works. The matrix system takes into account the very strong back lighting and will tone down the exposure in order to bring the whites towards middle grey. The upside of this is that you will not end up with blown highlights. However the down side is that the subject, which in reality is all we care about, is now going to be underexposed.
However, with spot metering, we can point the camera (usually the focus point) at our subject and meter specifically for the subject. Now our camera tries to bring our subject towards middle grey, leading to a correctly exposed subject, but also blows the highlights. I’d rather make sure my subject is correctly exposed though.. wouldn’t you?
MAKING WHITE WHITE AND BLACK BLACK & METERING FOR PORTRAITS
Ok, so our cameras try to change the world to be a neutral/middle grey. The problem is, like DAZ we want our whites to be white.. NOT GREY. Ansel Adams “invented” an exposure system which is called the Zone VII system. It’s WELL worth doing a google on this and reading up on it in more detail. But I’ll summarise the useful bits here.
We already know that we can record a white surface as being grey using our cameras internal reflective light meter. Two stops up from middle grey is white, without losing detail. After two stops, we have pure white, which is equivalent to over filling the bucket, losing detail or blowing highlights. Pure black is about 4 stops below middle grey, which means no water at all in the bucket and no detail recorded. Now we’re ready to take our correctly exposed portraits. So, put our camera into manual exposure mode and change your meter to spot metering. My personal preference for portraits is to shoot WIDE open because I love to make sure the eyes are in sharp focus but throw everything else out of focus, making for very simple, uncluttered portraits. Using the camera, look around the scene, literally moving your camera/focal point, looking for the brightest important part on your subject.
Usually this is forheads, noses, cheeks.. or areas of tight skin. In the cases of backlit portraits, it’s often the hair. Once you decide on the brightest part of the scene in which you want to record detail, using your cameras meter, alter your shutter speed (my aperture is generally locked wide open) so as the meter is now at 2 stops above middle grey. I have now locked in the correct exposure for the scene. As I’m in full manual mode I can recompose the scene as the aperture and shutter speed will not change. Then focus and CLICK
Job done. A well exposed portrait