Why is a camera lens a circle, but the picture comes out as a rectangle?

It's the fault of those physicists. They can't make rectangle lenses.

Blame those industrialists too. They can't make printers that can handle round paper.

Blame the environmentalists! They refuse to produce round paper.

Finally, blame the consumers for continuously patronizing these people by buying round lenses and paying for rectangular photos.

The projected image of a lens is round but the digital sensor inside the camera is rectangular. Only the parts of the image that falls on the sensor get to make the picture. Then there is printing. Although you can actually make round pictures, it is wasteful since you have to cut out the circle from a rectangular piece of paper.

Put a round pizza on a square dish. Now, cut the parts of the pizza hanging off of the dish. You now have a square pizza even though you started off with a round one.

The images are not round because the sensor is rectangular and not round. If the sensor was round, only then would the images be round too.

The camera lens projects an image of the outside world onto the surface of a film or digital image sensor which is located at the rear of the camera. The shape of the image-capturing medium has traditionally been a square or rectangle. This shape predates photography. Most art images are applied to a canvas whose dimensions are akin to what is called the “golden rectangle”. Photographers naturally gravitated to making images with this shape. You should take the time and surf the internet on this subject.

Now the camera lens projects a circular image. This image is quite large, but only the central portion is suitable for photography. The rest is too dim and too blurred to be useful. The useful central portion is called “the circle of good definition”.

Camera makers and lens makers desire to use only the central portion of the image circle. The camera body has internal baffles, painted flat black. These exclude all but the central portion. Hovering just over the film or digital chip is a rectangular opening called a “mask”. The mask makes certain that the shape of the image area matches the format dimensions. For the full frame camera, this will be 24mm wide by 36mm long. For the compact digital, this will be 16mm wide by 24mm long. Other smaller sizes are common. The fact remains that the public favors a rectangular picture size. Further, this rectangle matches closely the proportions of the mythical golden rectangle.

The shutter opening, and image sensor are rectangular.

It gets cropped. If you saw the full output of the lens - it would be a circle. The camera is black inside and the shape of the film gate crops it to a rectangle.

Good question...because it limits the picture to the screen you are viewing it on.