What is focal length?

Focal length is defined as the distance between the lens and the image sensor when the subject is in focus. This measurement typically determines how much of a scene can be captured and how zoomed-in or zoomed-out the image appears. A lens with a shorter focal length captures a wider field of view, making it suitable for landscapes and architecture, while a longer focal length results in a narrower field of view, ideal for portraits or distant subjects. Understanding focal length is crucial for photographers to control perspective and composition effectively.

The other options refer to different concepts in photography. The distance from the subject to the camera pertains to composition and framing rather than focal length. The size of the aperture opening relates to exposure and depth of field, not focal length. The duration the shutter remains open relates to the measurement of time for which light is allowed to hit the sensor, commonly referred to as shutter speed, and does not define focal length either.