This is one of the tricks that I use most in order to get high contrast black and white photographs when I’m walking the streets. To understand and dominate how our camera is reading the light in front of us is the key element for us to be able to get the picture we are looking for.
If you are not familiar with the metering modes, you should read this article about them first.
How does it work?
When you are framing with your camera, you will see how it is continuously calculating the correct exposure for the image; this means the speed, aperture and ISO are being updated in real time depending on the light in front of us (of course, this will depend on the shooting mode we have selected).
So, how do we use it?
Combining the AEL button with the Spot Metering mode allows us to read the light in very specific areas of the scene, allowing us to re-frame keeping these shooting parameters (Speed, F and ISO).
For example, in the picture below, the light was measured in the stairs area lighted by the lamps, and as the contrast was so high, the other zones came completely dark.
The only thing you need to do is to aim the bright area with the center of your viewfinder and hold the AEL button. You will see how, even you move or frame to another brighter or darker place, the camera will keep the speed, aperture and ISO parameters unchanged.
Is something not clear?
I will be happy to help you, just ask!