Thursday, August 4, 2011

Understanding Nikon’s Three Light Metering Systems

The basis for a Nikon DSLR’s exposure meter is an RGB sensor that meters a wide area of the frame. When used with a G or D Nikkor CPU lens, the camera can set exposure based on the distribution of brightness, color, distance, and composition. Most people leave their cameras set to Matrix metering and enjoy excellent results. Others use the Center-weighted meter, or the Spot meter. Let's look more closely at each of the Nikon exposure meters.

3D Color Matrix II Meter

Nikons use a 3D Color Matrix II metering system that is one of the most powerful and accurate automatic exposure meters in any camera today. It uses the symbol shown in Figure 1. Look in your manual to see how to set the camera to Matrix metering. This is the default setting from the factory.

Figure 1 – Matrix metering symbol in a D300S Control Panel. This symbol will also be found on the monitor when you press the Info button and look for the metering symbol.

There are characteristics for many thousands of images stored in the camera. These characteristics are used—along with proprietary Nikon software and complex evaluative computations—to analyze the image that appears in your Viewfinder. The meter is then set to provide very accurate exposures for the majority of your images.

A simple example of this might be a picture where the horizon runs through the middle of the image. The sky above is bright and the earth below is much dimmer. By evaluating this image and comparing it to hundreds of similar images in the camera's database, an exposure setting is automatically input for you.

The Matrix meter examines four critical areas of each picture. It compares the levels of brightness in various parts of the scene to determine the total range of exposure values. It then notices the color of the subject and its surroundings. If you are using a G or D CPU lens, it also determines how far away your lens is focused so that it can figure the distance to your subject. Finally, it looks at the compositional elements of the subject.

Once it has all that information, it compares your image to tens of thousands of image characteristics in its image database, makes complex evaluations, and comes up with a meter value that is usually right on the money, even in complex lighting situations.

Center-Weighted Meter

If you were raised on a classic center-weighted meter and still prefer that type, your Nikon's exposure meter can be transformed into a flexible center-weighted meter with a variable-sized weighting that you can control. Examine the user's manual for instructions on setting the camera to Center-weighted metering.

Figure 2 – Center-weighted metering symbol in a D300S Control Panel. Look for this symbol on the monitor too, when you have selected Center-weighted metering.

The Center-weighted meter examines the entire frame, but concentrates most of the metering in an small circle in the middle of the frame. If you'd like, you can make the circle as small as 6mm or as large as 13mm (may vary with some Nikons). Let’s examine the Center-weighted meter more closely.

Using the Custom setting b Metering/exposure called something similar to Center-weighted area, you can change the size of the circle where the camera concentrates the meter reading. If you'd like, you can even completely eliminate the circle and use the entire Viewfinder frame as a basic averaging meter.

As mentioned previously, the circle in your Viewfinder is normally 8mm. However, by using the Custom setting b - Center-weighted area, you can adjust this size to one of the following (may vary with different Nikons):

  • 6mm (.24 inch) 
  • 8mm (.32 inch)
  • 10mm (.39 inch)
  • 13mm (.51 inch)
  • Avg – Entire Frame

The Center-weighted meter is a pretty simple concept. The part of your subject that's in the center of your camera's Viewfinder influences the meter more than the parts closer to the edges of the frame.

Where's the Circle?

You can't see any indication of a circle in the Viewfinder, so you'll have to imagine one.

Figure 3 – Series of imaginary red circles in the viewfinder and averaging full frame 

Here's how (see figure 3): Locate your current AF point in the middle of your Viewfinder. The length of the little rectangle you see is about 2 or 3mm (.10–.12 inch) in size. If you imagine about three of these little rectangles side-by-side, that's about the same size as the default 8mm circle, which at .32 inches is about 1/3 of an inch. The 13mm maximum size circle, at .51 inches, is about 1/2 inch wide.

Primarily, just remember that the center area of the Viewfinder provides the most important metering area and you'll do fine. For information on fine-tuning Center-weighted metering, refer to the section titled “Fine Tune Optimal Exposure – Custom Setting b6” in chapter 4, Custom Setting Menu.

What about the Averaging Meter?

If you set your meter to Avg in Custom setting b Metering/exposure > Center-weighted area (full averaging), the light values of the entire Viewfinder are averaged to arrive at an exposure value. No particular area of the frame is assigned any greater importance (figure 3, image 5).

This is a little bit like Matrix metering, but without the extra smarts. In fact, on several test subjects, I got remarkably similar meter readings from Avg and Matrix. Matrix should do better in difficult lighting situations, since it has a database of image characteristics to compare with your current image, and it looks at color, distance, and where your subject is located in the frame.

Spot Meter

Sometimes no other meter but a spot meter will do. In situations where you must get an accurate exposure for a very small section of the frame, or must get several meter readings from different small areas, the camera can, once again, be adjusted to fit your needs. Look into the user's manual for instructions on setting the camera to Spot metering.

Figure 4A – Spot metering symbol in a D300S Control Panel. Also check the monitor after setting the camera to Spot metering mode.

The Spot meter consists of a 3mm circle surrounding the currently active AF point (figure 4B). The Spot meter evaluates only 2 or 3 percent of the frame, so it is indeed a "spot" meter. Since the spot surrounds the currently active AF point, you can move the Spot meter around the Viewfinder within the AF points in your camera’s viewfinder.

FIG 4B – Viewfinder view of the 3mm spot in a Nikon D300S
How big is the 2 or 3mm spot? Well, the Spot meter barely surrounds the little AF point rectangle in your Viewfinder. It is rather small! When your camera is in Spot meter mode, and you move the AF point to some small section of your subject, you can rest assured that you're getting a true spot reading.

In fact, you can use your Spot meter to determine an approximate range of light values in the entire image. You can do this by metering the lightest spot in the frame and the darkest spot. If this value exceeds 5 or 6 stops difference in light level, you've got to decide which part of your subject is most important to you and meter only for that part. Something is going to blow out.

On an overcast day, you can usually get by with no compensation since the range of light values is often within the recording capability of the sensor. On a bright sunny day, the range of light exceeds what your sensor can record by as much as two times. This range can often be as large as 12 stops total, while your sensor can only record a maximum of 6 or 7 stops!

Don't let the numbers make you nervous. Just remember that spot metering is often a trade-off. You trade the ability of the camera's multiple "averaging" skills to generally get the correct exposure throughout the frame, for the highly specific ability to ensure a certain portion of an image is "spot-on". The choice is yours, depending on the shooting situation.

If you spot-meter the face of a person standing in the sun, the shadows around that person will contain little or no data. The shadows will often come out as solid black in the final image. If you spot-meter for the shadows instead, the person's face is likely to blow out to solid white. We'll discuss this in more detail in a later section of this chapter when we explore the Histogram.

Use your Spot meter to get specific meter readings of small areas on and around your subject, make some exposure decisions yourself, and your subject should be well exposed. Just remember that the Spot meter evaluates only for the small area that it sees, so it cannot adjust the camera for anything except that one tiny area. Spot metering requires some practice to learn how to use it well, but it is a very powerful tool to balance exposure values in your images.

My Conclusions

Most people use Matrix metering most of the time. In my experience, few people use the Center-weighted meter. However, for those raised on that type of meter, Nikon gives you a choice. Spot metering is very useful to take careful control of the exposure when you need detailed control.

Learn to use all three meter types and you can make an intelligent choice when the time comes to change to a different style of meter.

Keep on capturing time…
Darrell Young

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