Sometimes no meter other than a spot meter will do. In situations where you must get an accurate exposure for a very small section of the frame, or you must get several meter readings from different small areas, the D700 can, once again, be adjusted to fit your needs (Figure 2-5).
The D700’s Spot meter consists of a 4 mm circle surrounding the currently active auto-focus (AF) point. That’s evaluating less than 1.5% of the frame, so it is indeed a “spot” meter. Since the spot is surrounding the currently active AF point, you can move the Spot meter around the viewfinder within the 51 AF points (Figure 2-6).
How big is the 4 mm spot? Well, the Spot meter barely surrounds the little AF point rectangle in your viewfinder. It is rather small at .16 inch. When your D700 is in Spot metering 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 exposure value (EV) range of light values in the entire image. You can do this by metering both the lightest and the darkest spots in the frame. If this value exceeds a four or five stop difference, you’ve got to decide which part of your subject is most important to you and meter only for it. Something is going to “blow out”.
On an overcast day, you can usually get by with no compensation since the range of light values are 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 much as 12 stops total, while your sensor can only record a maximum of six or seven stops!
Don’t let the numbers make you nervous. Just remember that spot metering is often a trade-off. You trade the highly specific ability to ensure a certain portion of an image is “spot-on” for the ability of the camera’s multiple “averaging” skills to generally get the correct exposure throughout the frame. The choice is yours, depending on the shooting situation.
If you spot-meter a person’s face while they are 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. On the other hand, 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 later in 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, but it is a powerful tool to balance exposure values in your images.
For information on fine-tuning spot metering, see the section Exposure or Flash Compensation in Chapter 6.