Why is the viewfinder of a SLR camera rectangular? The lens is round?

Our common photo specification is 3:2, which is different from the imaging of the lens.

This is the circular picture imaged by the lens, and it is also the specification for recording photos.

If the exposure range is forcibly expanded, dark corners will appear in the photos.

The dark corner of a photo is often caused by the lack of field of view of the lens.

Why are lenses round instead of rectangular? This is because the lens is composed of a lens with a truncated part on the spherical surface, and the central point is used as the coordinate origin to symmetrically converge light for imaging.

Although the light path diagram we saw in the book is sauce purple.

But the actual lens is sauce purple.

If the lens is not symmetrical at the origin, but symmetrical on a horizontal line, then the lens will become a part of the truncated cylinder, and what you see is not a clear image, but a one-dimensional information like a bar code.

Semi-cylindrical lens

Lose the image of the y axis

The lens is the cross section of a sphere, so it is naturally round. At the same time, the processing and use of lens barrel, cutting or rotating zoom and circle are all good choices. And there is a very important reason. In many previous cameras, the lens was fixed in a spiral, and there was no accurate direction. The angle of the picture cannot be guaranteed by using square or other special-shaped lenses cut like spectacles.

Then why is the photo square? This is because, like other carriers of traditional civilization, film and photographic paper are cut out, which has caused historical problems. In the silver salt photography era of film/photographic paper, both film and photographic paper were cut from large pieces of material. The most reasonable plane cutting method is regular quadrangle. If it is cut into a circle, it will produce a lot of scraps, which is not conducive to collection and binding.

This is the rest of the circular cut.

In fact, if the content of the picture is extremely important, far higher than the recording cost, the photo can also be round, and the content can be shot to the maximum with the lens. Such as space exploration or photomicrography. Only when it is displayed on paper or a monitor, a rectangular box is put on it to facilitate our management and browsing.

Microscopy, the effective picture is round.

After historical evolution and elimination, this phenomenon we have seen has appeared. The lens is round and the photo is square.