When we lightly press the shutter release button, the AF system starts its auto focus process. The time between pressing the shutter release button lightly to activate the AF system and the camera reporting the subjects being in focus is referred to as the auto focus lag, or simply AF lag. As discussed on the Auto Focus page, the AF lag is a variable of many factors such as illumination level, contrast, and so on. Most cameras will also determine exposure and white balance within this AF lag; however, the time required to perform these calculations is insignificant compared with the mechanical based focusing activities. Since the AF lag involves so many different environmental parameters, it is understandable that AF lag data posted on the web vary dramatically from site to site. This is because different site has a different measurement procedure in an environment with different illumination, contrast and so on.
When the subjects are in focus, the time between fully pressing the shutter release button and the camera starting to take a picture is referred to as the shutter lag. This lag will include steps such as preparing to close down the aperture, activating the shutter, initializing image processing components, and so on. If flashes is turned on, the flash sensor will also be activated. All of these activities usually take constant time. As a result, shutter lag usually does not vary significantly.
Note that some authors define shutter lag to be the time between fully pressing the shutter release button (with An auto-focus lock) and the camera taking a picture. In this way, it is the sum of AF lag and shutter lag. This definition is not used here because, as mentioned earlier, it involves too many variables and could vary too much to lose its actual meaning.