Nestled quietly on the back of most cameras is a button that is often ignored or misunderstood, particularly by those who have recently upgraded to a DSLR or Mirrorless camera. It’s the Exposure Lock button, and it can be a tremendous asset to photographers who are looking for ways to get more control over their cameras and ultimately their photography in general.
To understand what this button does it’s helpful to know a bit about how your camera calculates the exposure using aperture, shutter speed, and ISO sensitivity.
In Full Automatic mode these are all handled by the camera itself, producing images that are often pretty nice but can sometimes turn out too dark, too bright, or otherwise not exactly how you intended. In Manual mode you are required to set all three parameters yourself, which can sound intimidating at first, but is quite easy once you get the hang of it. However if you shoot in one of the semi-automatic modes like Aperture Priority, Shutter Priority, or Program Auto you have a high degree of control while still letting your camera figure out a few things too.
In Aperture Priority you set the aperture and ISO, while your camera figures out the shutter speed to use for a properly exposed photo. But, in Shutter Priority you set the shutter speed and ISO to use, while letting your camera set the aperture. In these so-called Creative modes the automatic parameters (whichever settings are handled by your camera, not you) are constantly changing depending on what you point your camera at in order to get a nice, even overall exposure.
This is where the Exposure Lock button comes in; it freezes whichever exposure parameter was set by your camera until you take a picture. At first this might sound like a silly thing to do but makes a bit more sense when you understand how it can be used.