I am a beginner in the area of computer vision. I have a basic doubt in 2D perspective projection. While analyzing 3D-2D transform we are taking the focal length as the distance to image plane. ( In almost all the reference). Focal length of a lens is actually the distance between center point of the lens to the point on the optical axis where the parallel lines will converge. So if we are placing the image plane at this point how we can get the image? If any clarification on my question please mention, I will elaborate. Hope valuable reply, it will help me to improve my basic knowledge in the area.
I'm not sure of having understood your question properly. Are you looking for the lens equation? It relates the focal length (f), the object distance (d_o) and the image distance or distance from lens to sensor in the case of a camera (d_i). The equation is the following:
1/d_o + 1/d_i = 1/f.
the distance is close to the focus f. Usually it is thought to be the same value.
nice question, that's rights that you have to know focal length first before use it. That name is camera callibration. try to search some method of callibration camera.
Btw, in modern camera there is note in front of it. so you can use it.
Manikandan,
I don't know if I get your question, but if you are questioning the set up of CV system with the image plane (virtual?) containing the camera focal point is an idea that makes sense. But be aware that if you are focusing in an specific object, the background will be blured, unfocused. And that, depending in you application, may or may not be an issue. If you are just analysing the object on focus, you are good to go. But if you want to analyse the whole image (ex: searching patterns) then you maybe want to set your camera for infinite focus, and all the captured image will be focused and ok for CV analysis.
In the pin-hole camera model, which is an approximation for real optical devices mainly used for perspective geometry, 3D reconstruction, etc, the distance between the focus and the image plane is f, that is, the image is formed in the plane Z = f.
For real optical devices the aperture is non-zero (and is usually approximated by a circle), the image of a point (X, Y,Z) is not formed in the plane Z = f but on the plane Z = t > f, with t defined by Gauss’ Formula
1/t = 1/f - 1/ Z
I clearly understand Mr. Manikandan's question as, many literatures say that the focal point is the point where the image plane is situated and the image is formed as all the rays converge there. here there are only two parameters, and they are the object distance, and the focal length = image distance from the lens. But other part of the story in internet says, that the image is formed beyond the focal point. In this case, a new term image distance, focal length and object distance emerges.
I am still confused! any help would be highly appreciated.
br,
Ragu
See the book "Optical Metrology, 3rd edition", John Wiley & Sons, Chichester 2002.
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