Parallax is the difference in perspective between a cameras optical viewfinder and its taking lens. This discrepancy becomes more noticeable as the photographer moves closer to the subject, causing the composition seen in the viewfinder to differ from the actual captured image.
Understanding parallax is important in photography, especially with rangefinder and viewfinder cameras, macro photography, or close-up work. Techniques such as using frame lines, adjusting the camera angle, or relying on the lens itself for framing help compensate for parallax and ensure accurate composition.