Was thinking of ways to solve the auto focus problem perfectly even for surfaces without any contrast and came up with a few proposals which I do not know are practical or not. Light that is converged by the lens we all know forms a 3D cone between the point from where it takes off and the surface of the lens towards the point. This cone will be right circular only for points on the optical axis. The lens only sees the 2D projection of the light points existing in the 3D space around it and so there can only be one point on the optical axis that the lens will be able to see at any instant of time. This is the point that I am trying to FOCUS on. I need to somehow come up with a technique to detect the rays emerging from a point on the optical axis.
One difference between a point on the optical axis and any other is that, rays emanating from the former point meet a circle of radius R drawn from the center of the lens, at the same angle.
But any point on the lens would receive light from all points visible around it. So at any point on the lens light rays will be converging from every possible angle, which leaves us with no way to pinpoint the ray that started from the optical axis.There are many more problems with this very way of thinking to solve the problem. From the perspective of the lens we never know where the real point is located on the optical axis. Different points from the surrounding space can create the same effect as though there was a real point at a different location on the optical axis. This indeed can happen continuously all along the axis! Assuming that the frequency of light reflected from a real point will almost be the same when it meets the circle and the probability of such a thing happening for a virtual point zero, the problem could be solved. But if you recall, the very reason why I started to think about this, was to get a solution to cases where there is zero contrast.
After a while I came across a theory called QED that solved a lot of these problems but kept the hardware required to achieve it out of our current technology’s reach. According to QED, a photon represents the “particle” of light, and its instantaneous phase the “wave” counterpart. This phase depends on the frequency of the light under consideration. A lens focuses light because the probability that the photons reach the focus point with the same phase is high and zero anywhere else. For more details refer to the book “QED: The Strange Theory of Light and Matter”. Putting the same theory into action for our current scenario, this would hold good only for a real particle. Since phase is something that repeats as the photon travels through space, the random points that form the virtual particle should be present at exact locations (again that can repeat in space) to meet the point “a”, all with the same phase!, which is highly improbable in a practical scenario. Now this should work for ZERO contrast!
