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It is known that an image is in focus (and it results sharp and clear) only at the focus, in which all rays intersect themselves. Why do we see sharp images only at the point in which the rays emitted of an object meet themselves?

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Light rays converge to the points on the focal plane, and the diverge as they go past. Only at the focal plane are they concentrated to a sharp point; elsewhere they would look blurred out.

enter image description here (From this answer to a related question)

If you put your sensor at the focus distance, all the yellow lines hit it at one point. Elsewhere, they would hit at many points, making a larger blurry yellow spot.

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  • $\begingroup$ Also, note that the picture in Bob's answer implies that rays with the same color come from a single point on the object, assumed far-away since these rays are parallel. The rays converging off-axis are from a different point. The image of an extended object is formed by light from all visible points on the object. $\endgroup$ – amateurAstro May 26 at 23:20
  • $\begingroup$ Ok, but why our eye, or a sensor, is able to see a sharp image only at the intersect point? If I put it in the middle between the lens and the focus, it will receive anyway the rays, but the image is blur. Which is the physical explanation? $\endgroup$ – Kinka-Byo May 27 at 7:31
  • $\begingroup$ If you put your sensor at the focus distance, all the yellow lines hit it at one point. Elsewhere, they hit at many points, making a larger blurry yellow spot. $\endgroup$ – Bob Jacobsen May 27 at 13:49

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