Exposure Metering

Exposure metering modes are similar to focusing modes. In fact, in some modes they are tied together.

Lets look at the path of light in a typical DSLR again.
 
As you can see the metering function is physically separate from everything else. It is also relevant that not all of the light that will hit the image sensor is used entirely for any other function. Some of the light is allowed to pass to the focusing sensor (blue lines) and only some of the light is allowed to pass to the metering sensor (orange line), and a large portion is allowed to pass thru the eyepiece (red line) so that you can see what you are doing.

It is also interesting to note that, while the exposure meter is physically separate from the focusing sensor, they are tied together in most autofocus modes (see Focusing).


The metering Sensor

The metering sensor on modern cameras is a RGB sensor which is sensitive to both color (Red, Green, Blue) as well as luminance. The sensor sites are then usually grouped into "zones" and a zone is "averaged" in order to determine the exposure. To give an example of the variability in sensors, the Canon 60D has 2016 pixels divided into 63 zones while the Nikon D4 isn't divided at all and works at the pixel level of 91,000 zones. The more pixels the metering sensor has the more accurate it should be. However, the number of zones is not really relevant to basic exposure metering. The zones are more related to advanced functions such as scene recognition, face recognition, focus tracking, etc.


Metering Modes

With metering you have three basic options, Matrix/Evaluative metering, Spot metering, or Center weighted metering.


The Nikon D4 metering mode selector button, used with command wheel.


The Nikon D3 metering mode selector (l-r spot, matrix, center weighted)


The G10 metering mode selector, used with command dial.


Matrix metering uses the information from the entire sensor.


This image shows that the entire metering sensor is being used with an off center focus point selected (shown in red). The black lines show how a very coarse "zone configuration" might look.

Matrix metering is best used with any of the fully automatic and scene modes. But matrix metering is also VERY smart. For instance, the current Nikon matrix metering system (2013) compares the scene in the viewfinder against a database of over 30,000 images in it's database in order to determine the correct exposure. I can guarantee you that somewhere in that database is an image with a backlit subject and a bride in a white wedding dress... and probably anything else you might want to photograph.


Spot metering uses only the metering zone(s) that the selected focus point is on. But spot metering is not as small as just one focus point. It is generally ~ 4mm in diameter which is roughly the area within the 9 focus points surrounding the active focus point in this image.


This image shows spot metering with an off center focus point selected and a metering sensor with many more zones.

Spot metering is best used for a difficult subject or when precise metering of a specific subject within a scene is needed.


Center Weighted metering is kind of like a cross between spot metering and matrix metering. With center weighted metering the entire metering sensor is used, but more "weight" is given to the central portion of the image when determining the exposure. This area is generally ~ 8mm in diameter, and unlike spot metering it does not move with the active focus point.

Often the size of the central spot, and therefore it's "weight", can be set in the menus.

Center weighted metering is often best used in action photography when you have a small subject where it is hard to keep the metering point on.


More on Spot Metering

Spot metering is a very useful tool in getting the proper exposure. I use spot metering 90+ % of the time. Usually there is one part of an image for which it is critical it has the proper exposure; "the subject". By using spot metering and placing "the spot" at the correct location you can ensure proper exposure. 
This is particularly useful in situations where the camera is often "fooled". Situations where the subject or scene is largely dark or largely light. The camera wants an image that is "average" it will expose to an average luminance level (12-18% gray). Because of this both a very light scene and a very dark scene will both be exposed as gray. The light scene will be underexposed and the dark scene will be overexposed.

Here the spot metering is on the subject resulting in a proper exposure.


Here the spot metering is on a dark point in the image resulting in overexposure.


Here the spot metering is on a light point in the image resulting in underexposure.


Remember, spot metering occurs at the selected focus point. For this reason it may be preferable to separate the functions in order to allow metering and focusing to be done independently. This can be accomplished several ways, i.e. by using exposure lock (AE-L) or "back button" focus.... RTM


Manual Metering

In difficult situations "manual metering" may be the easiest method to use. To do this you first meter the scene, or an "average spot" within the scene, and make note of the exposure. You then place your camera into manual mode with those settings. This works well in situations where the subject is rapidly changing but the lighting is not.

Some will use an incident meter instead of the built in camera meter. An incident meter measures the light levels the location the meter is held. The camera measures the light reflected off of a subject. It can make a difference but I don't find it useful unless it's for "studio type" lighting setups.

 

Exposure Compensation

Exposure compensation (EC) is a useful tool for when the camera metering system will be fooled. It is similar to spot metering but instead of moving the metering spot within the scene you tell the camera that the area being metered is either dark (-EC) or light (+EC).
It is also often used to offset the camera's tendency to expose slightly off. For example, most nikon users will have a positive EC set, and most Canon users will have a negative EC set. 
A third example of when to use EC is when you want to underexpose slightly to saturate colors such as for a sunset.

Regardless of what metering and exposure mode you are in the exposure compensation setting will affect the actual exposure recorded (except for full manual control). Exposure compensation is set differently on different cameras.


EC button on a Nikon D4, used in conjunction with a control dial.

EC dial on a Canon G10

Different cameras will be have different limits to the control of EC. Nikon DSLR's generally have +/- 5 stops, Canon DSLR's generally have +/- 3 stops, and my Canon G10 has +/- 2 stops. 

One way to think of it is in terms of the histogram. The histogram is broken into 5 sections with the middle being #3, and this is where the meter wants to put everything. So if the subject is "bright white" it should be in section 5 and therefore a +2 EC would be a good starting point. And if the subject is "dark black" -2 EC would be a good starting point... but this is only approrpriate if you want them to be "bright white" or "dark black," and you might not want that.


 


ASSIGNMENT:

Part One- Take several images of the same scene while using the various metering modes available on your camera. The differences will be most apparent if the scene has both very light and very dark areas within it.

Part Two- using spot metering, meter a particularly light or dark subject and use exposure compensation to get a correct exposure. With practice you will be able to judge how much exposure compensation a particular subject will require. 



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