Spot Metering vs. Matrix Metering
Today's blog entry comes from a question we received from one of our readers regarding what's going on inside the camera (Nikon) when you select spot metering versus matrix metering. Specifically, the reader asks if anything is happening other than exposure calculation? Can a spot metered photo be made to look like a matrix metered photo in post? Keep in mind that we're not going to discuss what is the appropriate time to use each of these settings (that's for another post) but just what is happening in the camera. So, here we go.
Nikon says that when spot metering is selected exposure calculations are based on a "circle 3mm (0.12in) in diameter (approximately 2.0% of the frame) [This] circle is centered on the current focus area, making it possible to meter off-center subjects" (Nikon D200 Manual). In matrix metering mode, exposure is based on a variety of information from all areas of the frame. This suggests that the weighting selected is ONLY used for exposure calculation, but lets confirm with an example.
In the pictures below, the camera is set for spot metering in the left image and matrix metering in the right image. The focus area is set to the very center of the image. The D200 (in aperture priority mode) chose an exposure of 1/500s at f/8 for the spot metered image, and 1/250s at f/8 for the matrix metered image.
In post-processing (Adobe Lightroom) - I bumped the spot metered image (left) up approximately 1 stop. As you can see, they look fairly close. This provides some confirmation that metering selection only affects the calculation of exposure and does not introduce any other artifacts into the image data.
Thanks for a great question, hope that this helps clear things up a little. If you have a question, please submit it to firstname.lastname@example.org and we'll address the questions of interest to the most people. (Or of interest to me) Have a great day.