The AEL functionality is one that I use a lot during the long Street Photography walks. It is one of the most useful tricks to use along with the spot or center-weighted metering modes.
If you are using, for example, the Aperture (A) mode and have a specific aperture, you camera will continuously measure the light and adjust the shutter speed accordingly.
If you frame a very bright scene, your camera will set a very high shutter speed so the photograph gets correctly exposed.
The AEL allows us to have a little bit more control over this dynamic auto-exposure, allowing us to block a certain Aperture/Shutter/ISO calculation.
Following the previous example, if you are framing a very bright area and while you do so you hit the AEL button, hold it pressed, and reframe to a darker zone, your A/S/ISO parameters will be blocked, this is, they will no longer be recalculated.
As you expect, your re-framed photograph will be over exposed, because the A/S/ISO values that the camera calculated where for the bright scene, not the dark one.
How and when to use AEL
When we are looking for high contrast photographs, usually the matrix metering does not work very well, as the images we get are normally quite balanced (as the light is measured all over the sensor, the camera tries to expose all of it properly).
However, in some situations, due to the such a high difference between the lights and the shadows this is not possible.
In this scenario, we will have to decide if we want to sacrifice the dark or the bright side of the picture, and the AEL allows us to do so in a controlled way.
If you choose the spot metering, remember that the light will be read in a small spot. Then, if you target your camera to a very bright subject, the automatic exposure algorithm will set the parameters so this small area is properly exposed.
Now, if you press and hold the AEL at this point, the A/S/ISO parameters will be locked, and you will be able to re-frame, so even the bright subject is not in the center it will still be well exposed, as the parameters will not change.
The consequence of this is that the shadows of the photograph will be under-exposed, but that’s fine, as we are looking for a high contrast picture.
An example of AEL usage
To shoot this picture I used this exact technique. I set the spot metering mode and aimed to the bright spot on the floor, using the AEL over there. Then, I re-framed and waited for a while until some interesting subject crossed under the light.
The illuminated building o the left was perfect to compensate the composition and give some context to the skater.
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