A round camera lens itself doesn't directly produce a rectangular picture. It's the combination of the lens and the camera sensor that determines the shape of the final image. This phenomenon is a result of how cameras have evolved to capture and present images in a practical and visually appealing manner.

While a camera lens is indeed circular, the image sensor inside the camera is typically rectangular. The lens gathers light from the scene and focuses it onto the image sensor. The sensor consists of millions of light-sensitive elements or pixels that are arranged in a grid pattern. These pixels collectively capture the intensity of light that falls on them.

The camera lens projects a circular image onto the rectangular image sensor. Light rays from various points in the scene pass through the lens and converge onto the sensor. However, the circular projection of the lens is constrained by the rectangular boundary of the sensor.

The central portion of the circular lens projection is the most accurate and focused, while the outer edges tend to exhibit distortion and reduced clarity. To capture the sharpest and most accurate representation of the scene, photographers use the central part of the lens's projection, which naturally fits within the rectangular sensor area.

The choice of a rectangular image sensor is closely tied to the aspect ratio of the final photograph. An aspect ratio is the ratio of an image's width to its height. Common aspect ratios include 3:2 and 4:3. These ratios closely resemble the dimensions of traditional print sizes, such as 4x6 inches or 8x10 inches. The rectangular sensor's aspect ratio matches these print sizes, which allows easy and efficient printing without cropping.

Rectangular images are more visually pleasing to the human eye and align with how we perceive the world. Horizontal and vertical lines are dominant in most scenes, and rectangular images naturally accommodate these lines. Additionally, the rectangular format allows for composition techniques such as the rule of thirds, which guides photographers to place key elements along imaginary lines that divide the frame into thirds both horizontally and vertically.

The transition from film to digital photography retained the rectangular format due to familiarity and practicality. Film cameras also had rectangular film frames, and digital cameras inherited this design to ensure a smooth transition for photographers.

While the initial projection from the lens is circular, the rectangular frame of the sensor crops this circular projection. Photographers can further adjust the composition by cropping the captured image during post-processing. This ability to crop while maintaining high resolution and detail is another advantage of the rectangular image sensor.