Being adequately full of coffee and other caffeinated substances, I feel inspired to start a brief mini-series on a subject I’ve been interested in lately: metering.

Basic Matrix Metering

Let’s start with the metering mode most of us are likely to be using: matrix. Now, in matrix metering what your camera does is divide a scene into a grid (the number of segments in a grid depends on your brand, my E-3 Olympus digital camera has a 49 area matrix, the newest pro Nikon digital SLR cameras use an advanced 1,005 pixel sensor). No matter how it’s divided, at heart the basic idea is this:

1. The metering reads how much light is in each area.
2. It then decides how exposure needs to change to average the scene out to middle gray.

Let’s demonastrate with a very simple 6 area matrix (similar to what was used in cameras in the earl-mid 90s). Let’s say the six zones average out to the six shades below. Grey on the right is what the camera wants this scene to average to.

In this case, it doesn’t, it’s actually a little too dark. So, the camera would adjust the exposure accordingly, and the final light metering in the exposure would go more like this:

You can see how the whole exposure is lightened. This is an oversimplified version, but it helps show what matrix metering is trying to do. The more zones it meters the more accurately it can decide how to balance the light and dark areas. Keep in mind that changing exposure does change every square, the camera can’t increase the light hitting only certain areas of the sensor.

Modern cameras have very sophisticated matrix meters going far beyond this basic example, and I’ll talk about some more advanced tricks in matrix metering next time.