Lag time, also commonly referred to as shutter lag, is the delay that occurs between the moment the photographer presses the shutter release button and the moment the camera actually captures the image. Rather than responding instantaneously to the photographer's input, virtually all cameras require a brief interval to complete a series of internal processes before the exposure is made, and it is the duration of this interval that constitutes the lag time.
Several factors contribute to lag time in modern cameras. Autofocus acquisition is typically the most significant, as the camera must first analyse the scene, calculate the correct focus distance, and drive the lens to the appropriate position before allowing the shutter to fire. Exposure calculation, white balance assessment, image stabilisation activation, and the charging or readying of the flash system can all add further increments of delay. In cameras that have already been focused and metered - for example when the shutter button is half pressed to lock focus and exposure before the full press - the residual lag time is reduced considerably, representing only the mechanical and electronic delay of the shutter mechanism itself.
Lag time is a particularly important consideration in action, sports, wildlife, and street photography, where split second timing is critical and even a delay of a fraction of a second can mean the difference between capturing a decisive moment and missing it entirely. Professional digital SLR cameras and mirrorless cameras are generally engineered to minimise lag time as much as possible, while compact cameras and smartphones, with their more complex autofocus and processing demands, can exhibit more noticeable delays that require the photographer to anticipate the moment and release the shutter slightly in advance of the desired instant.
Photographers can adopt several practical strategies to minimise the impact of lag time, including pre-focusing on the anticipated area of action using the half press technique, using continuous autofocus modes that track moving subjects, and familiarising themselves with the specific lag characteristics of their camera to develop an intuitive sense of when to press the shutter in order to capture the intended moment.