If you have not already read the previous article about why we actually need light metering and the challenges we face, I strongly suggest you do to have a firm and concrete understanding of the basic light metering problems and their solutions.
Now, I assume you have understood the difference between incident and reflective light metering. We will be discussing TTL (Through The Lens) light metering (i.e. reflective light metering) in this article.
So, let’s get started!
The principle of TTL light metering is pretty simple. It is exactly what it sounds like. In this light metering technique, we measure ONLY the amount of light that gets reflected off the subject and comes in through our lens. TTL light meters come built-in, in almost all modern cameras and DSLRs. It is through this tiny device that the camera can actually “see” and “perceive” the amount of light that is available and makes changes accordingly to get a well-exposed image.
Needs for many modes:
Now that you know about types of light and various form of light metering, you might be thinking why you need more than one light metering mode? The answer is pretty simple. You see, light metering is most required and necessary when the scene you are shooting is challenging and the entire scene is NOT evenly lit.
As a matter of fact, the human eye can see about sixteen (16) stops of light. That means our eyes are extremely good at picking up details from a scene which has both bright and dark areas. In other words, our eyes have a huge dynamic range. Our cameras, on the other hand, are not so lucky. They cannot “see” so much dynamic range at the same time. Most modern cameras can only pick up and record up to eight (8) stops of light. So, it can be easily inferred that even if you can actually see details in dark and bright areas of a scene you are trying to photograph, your camera may not be able to see all of that at all.
The solution?….Light metering!
As our cameras can record only about half the dynamic range our naked eyes can perceive; it is imperative that we make the most of the eight stops of dynamic range the camera has to offer and put it to its BEST use. We can do this by instructing and guiding our camera to prioritize what is really important in the scene and care about that part the most/only.
Let’s take an example: Take a good look at the image below.
In the image above, most of the part of the frame is dark. The image is of a man looking through a window. Now the question you have to ask yourself is, what is the prime focus of the image? The answer in this case is of course the man, more precisely the man’s face. Now, to have a look like this; the frame actually has to be dark in most of the regions while the man’s face is perfectly exposed; the perfect situation for the light metering to kick in and help you out.
What you can do in this scenario is just set the camera to meter the light on the man’s face ONLY, and let other parts of the image be dark. The camera will then care for only the man’s face’s exposure and ignore the rest of the scene. Had you not done this, the camera would have tried to expose the entire frame properly and that would have given rise to two huge problems:
- Firstly, in trying to expose the parts of the scene which are far from the window and get really less light, the camera would have most likely overexposed the man. Moreover, in this case, the man is wearing white from head to toe; that would only accentuate the problem.
- Secondly, properly exposing the entire frame in this particular picture would kill the mood and feel of it. It would defeat the purpose of the image totally.
Light Metering Modes:
While cycling through the various light metering modes available on our camera, what we do in essence is tell the camera the amount of area and the part of the frame that it should prioritize the most. The camera literally collects data from the instructed parts and based on that calculates its exposure settings. Now there is a small problem here that camera manufacturers have kindly gifted to us. The light metering modes are not only named differently by different manufacturers but sometimes it varies from model to model of the same manufacturer. A mind blowing tactic to confuse us. So, kindly consult your manual on this.
The different light metering modes described below are same in principle; they may just be named differently on your camera. In this case, I am using Nikon’s terminology (just because I have a Nikon):
Spot light metering is used when we want to use just a little spot (3% to 5%) of the frame (usually in the center) and expose that part properly. Although this light metering mode is the fastest among all (since all it needs to do is meter just a spot), it is very unforgiving. The camera would only look at that spot and that spot alone and based on that, it would calculate the exposure setting. So if that spot is not properly placed on the desired subject or it moves around during the shoot; there is a high chance that the exposure may be off.
Example: If you are shooting portraits of a person, you always want the skin to be exposed correctly; so that the color representation in the final image is good. However, if you use spot metering in this scenario and while shooting if the chosen spot happens to fall where the person’s hair is, the camera is going to think “…dark black subject, reflecting little light back….I need more light to expose properly”, the entire image hence maybe overexposed. If you use it correctly though and place the spot right on the skin, it should turn out just fine. What I am trying to explain is that the margin of error using spot light metering is very low.
Spot Light metering is widely used in bird photography; since birds are nearly always against a bright sky and what we want to expose properly is the bird and ignore the bright sky as the background.
Partial light metering is almost the same as spot metering. The only difference is, in partial metering, the camera meters light from not just a single spot but an area little larger than a spot; about 8% to 10% of the frame. The area from where the light is metered generally is located at the center.
This type of light metering is optimal for portraits where the face of a person is of utmost importance. The partial metering in most cameras does a pretty good job at covering and exposing the face well. In the above example of the man by the window, I would have used partial light metering.
Partial light metering is a little forgiving than spot metering as the exposure is not dependent on only a single spot but on an area (you can think of it as a bunch of spots grouped together). So, even if the light metering is not perfectly used; the exposure most of the time turns out pretty much alright.
Center Weighted Average Metering:
Camera manufacturers came up with the Center Weighted Average Light Metering when the previous light metering technique called the Average Light Metering failed to expose important subjects in the frame properly. An everyday user of a camera tends to shoot subjects like friends, family and structures by placing them right in the middle of the frame. Center Weighted Metering takes advantage of this exact behavior of humans. It places more importance on the meter reading of the subjects in the middle of the frame. In other words, the camera takes a light meter reading of the entire frame but the meter reading from the center of the frames is given much more importance than the reading from edge of the frame.
So, if the photographer is trying to take an image of a group of people and the sun is behind them at the corner of the frame. The camera will place more importance on the light from center-frame (where the group is) and ignore the offsetting effects of the sun in the corner; thereby exposing the group image perfectly.
Matrix Light Metering is one of the most advanced and accurate light metering we have at our disposal. Our cameras are all digital now and all of them have substantial computing power. Matrix Light Metering harnesses the computing power of our camera’s processors and uses it to configure the exact light metering for a particular scene.
The basic working of Matrix Light Metering is as follows:
The camera divides the entire scene into a few zones. Each zone is then carefully metered and the readings are then used as inputs for the camera’s internal algorithm. Although each camera manufacturer has different algorithms to follow, overall these algorithms are extremely intelligent. They can identify key subjects in the frame no matter where they are placed in the scene and then suggest a specific setting of the camera to attain that perfect exposure.
The algorithm uses a host of other data like color, autofocus points, distance of the subject from the camera, light type and intensity, etc so that the camera is as accurate as possible. Many of the Nikon cameras even use an on-camera image database; the stored images are used as references and for comparison with the present scene. This makes the process of Matrix Light Metering quick and extremely accurate.
If you are just starting out, I would highly recommend you to use Matrix Light Metering as it is the easiest and most accurate of all. If you are shooting regular subjects, Matrix Metering will give you fantastic results nearly always. Canon calls the same metering mode as ‘Evaluative Metering’.
Believe it or not you are more intelligent than your camera (in the field of exposure) if you understand how your camera determines its own exposure settings and what could potentially mislead it. The Matrix Metering Mode for all its advanced algorithms still seeks the holy grail of exposure that is 18% grey. There are many scenes (possible both indoors or outdoors) that may completely fool the camera’s internal light metering and ruin potentially good shots.
It is important that you as a photographer know about these possible situations and take care so that you have perfectly exposed, nice images ALL THE TIME.
This is exactly what we discuss in the next article.
Join me there now. Come on!
Also published on Medium.