Why are Lenses Round, and Images Rectangular?
There are a lot of crazy lenses out there. From film to digital, from ancient lenses uncovered by archeologists to fresh-off-the-shelf precisely engineered metal and glass, With few exceptions, lenses all share one common design characteristic: They are barrell shaped.
Lenses are round - and they generally1 don’t care which way is up. A circle of light is going through the lens to the image sensor, which only pays attention to a rectangular part in the center. The image sensor might care which way is up.
Film negatives are rectangular because images are rectangular. Images are rectangular because photo primts are rectangular. Prints are made out of paper, and it’s easy to cut paper along right angles. It is possible to have round prints. Think about old portraits that have heavy vignetting - those dark corners come from the circular design of the lens.
[[photo of such a portrait]]
Image sensors are square because they are designed to emulate film. But also, because pixels and screens are square! Every round image you see on an LCD screen is made up of a bunch of small squares. Further, image formats store images in 2 dimensional arrays (like a grid, or a spreadsheet). These, of course, are rectangular.
Basically, design restraints. In a hypothetical world, it would be possible to make a lens rectangular - like an eyeglass maker shapes lenses to be more-or-less rectangular. It would be a whole lot of trouble for no benefit. There is no practical reason to make square camera lenses!
Let’s not forget that our controls on the lens, the focus and zoom rings, will only work on barrel shaped lenses. They can rotate different pieces of glass and - like a screw - move it forward and backward precisely. This clever bit of engineering wouldn’t work with rectangular lenses! Why fix what ain’t broken?
If somebody ever asks you why lenses are circular and not rectangular, The correct answer is “Just Because” or “Don’t worry about it”.
1 The exceptions being: Tilt-shift lenses, lenses with abnormal aperture shapes, and old lenses that have been turned into coffee mugs.