The principle of stereo photography

Let's use human vision to understand the technical principle of stereoscopic photography. The human eye is like a fully automatic digital camera with a zoom lens and an iris diaphragm, which can convert light signals into electrical signals that can be recognized by the brain. Essentially, what one eye sees is only a two-dimensional image of an object. We can tell its height and width, but we can't tell its depth. In two-dimensional images, we can judge the order of these objects in front of the background by using visual clues such as size and overlap provided by them, but we can't know how far they are.

Fortunately, we humans have two eyes, both of which are arranged horizontally on the front face, with an interval of about 60 mm When we look at the same object, the images seen by our two eyes are slightly different because of the slightly different angles. Our brain combines these two images to form a depth vision. This comprehensive processing process is very rapid and seamless, creating a visual feeling, just like we are looking at things through a big eye.

More importantly, the brain can judge the depth and distance of the same object according to the potential difference between the two received images. The farther away from the eyes, the smaller the potential difference, and vice versa. The schematic diagram shows how this process works: (when we look at the scenery at infinity) the position of the distant sun in the two images is (almost) the same, but the position of the nearby trees is different by 1/4 inches.

This whole process is called stereoscopic vision, which comes from the Greek stereos, meaning a shape or solid with a three-dimensional structure. Stereoscopy refers to the method of putting two photos of the same object with slightly different angles together (with the left eye and the right eye respectively) to obtain a sense of depth, thus generating conventional stereo vision. Then (the most basic) stereoscopic photography is to shoot the same object from two observation points with slightly different angles, and then show the two photos in the same way, so that the left and right eyes can watch the photos with left vision and right vision respectively (to obtain stereoscopic reproduction). The distance between these two observation points is called stereoscopic distance, which is usually 63mm on average, but it is not very strict, because our brain can compensate automatically, but if it is too large, it will cause an image that cannot be synthesized (if it is too small, it will lose stereoscopic effect). In stereo shooting, the camera needs to aim at the same object in front of the same background. A simple orientation to ensure the horizontal height of two shots is to pay attention to the more distinctive objects in the background scene in the first shot, and then arrange them in the same position in the picture in the second shot.

In order to generate the necessary stereo interval, it is necessary to adjust the camera interval according to the distance of the subject. When shooting an object at close range, the interval is small, and vice versa. Of course, our brains can automatically correct this shooting interval of different sizes (thus producing different degrees of stereoscopic impression). This also explains why distant objects lose stereoscopic vision (because of our actual shooting conditions, the stereoscopic interval cannot be large enough). However, when looking at an object at close range, human eyes can't (at the same time) focus on the distant object, thus losing stereoscopic vision of the distant object. Therefore, in order to obtain the ideal stereoscopic effect, we need to set the stereoscopic interval very large when shooting landscapes, and the stereoscopic interval may be reduced to several tenths of an inch when shooting close-range and macro photography. There are no strict and quick rules for shooting these extreme occasions, so we can only find out the interval suitable for the subject and camera configuration according to experience. There are also different opinions on the practical guidance of stereo photography. For example, john hedgecoe suggested that the distance between the camera and the subject should be 1/50 when shooting a distant object, and 1/4 when shooting a short distance, but some people suggested that the distance should be 1/30. Dennis Brown used a different set of rules to determine the best stereo interval. He believes that the stereo interval should change every 1/4 inch (about 6mm) with the distance between the foreground and the subject, while the background distance remains the same. According to different camera distances and focal lengths, he calculated the number of steps to be changed and made a conversion table.

Another manifestation of stereo photo is grating stereo photo, which combines digital technology with traditional printing technology and can show different special effects on special films. Show the lifelike three-dimensional world on the plane, the smooth animation fragments like movies, and the incredible illusion effect. Click here to add a picture to show that the grating is a thin sheet composed of strip lenses. When we look at one side of the lens, we will see a thin line on the other side of the picture, and the position of this line is determined by the observation angle. If we print these images on different lines on the back of the grating in turn, corresponding to the width of each lens and observe them through the lens from different angles, we will see different images.