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In the book Learning OpenCV, there is one figure that show the image plane always is focal image. enter image description here The description of the figure from the book is as follow.

We begin by looking at the simplest model of a camera, the pinhole camera model. In this simple model, light is envisioned as entering from the scene or a distant object, but only a single ray enters from any particular point. In a physical pinhole camera, this point is then “projected” onto an imaging surface. As a result, the image on this image plane(also called the projective plane) is always in focus, and the size of the image relative to the distant object is given by a single parameter of the camera: its focal length. For our idealized pinhole camera, the distance from the pinhole aperture to the screen is precisely the focal length. Th is is shown in Figure 11-1, where fis the focal length of the camera, Zis the distance from the camera to the object, Xis the length of the object, and xis the object’s image on the imaging plane. In the figure, we can see by similar triangles that –x/f = X/Z,

From this link https://en.wikipedia.org/wiki/Lens_(optics) enter image description here

There is thin lens equation. Only object in infinity can be imaged in focal image. Which one is correct?

Jogging Song
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Both are correct but they're stating different subjects. The mentioned Wikipedia's page is not talking about pinhole camera model. In pinhole camera you can put the image plane in any distance that you want (limited to the size of the image plane) and the image would be formed on that anyway. However in the second model that you brought from Wikipedia, the image plane has to be somewhere that is matched with the formula because of the existence of the lens, and that's the reason you have blur image when you take a picture out of the camera's focus.

In other word with a specific lens, there is a predefined focal length, so you have to set your image plane in a distance that gives you the clear image (Of course in digital camera, this distance is constant but there are multiple lenses and the camera changes the focal length by moving the lenses). In pinhole model there is no lens so there is no predefined focal length and the image will be caught correctly wherever you put the catcher plane. (There are many things that we have to consider but I came up with the simplest sketch.)

MeiH
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