Optical systems are studied under two assumptions
object point does not lie far away from the axis of the optical system.
rays taking part in image formation make a small angle with the axis of the optical system.
The domain of optics where above two assumptions are valid is called as Paraxial optics. Paraxial systems are highly idealized and in reality do not perfectly represent the situation. The consequential errors in image reconstruction are known as aberrations.
The paraxial assumption can be represented by truncating at the first term of the polynomial expansion of the sin function by the Maclaurin series. More…
Stationary state and squirrels !
Okay so the squirrel isn’t moving around so much, giving a fuzzy image. We can call the instant during which the squirrel wasn’t moving akin to a “gross” stationary state.
But why is the railing of window fuzzy? It isn’t moving !
Due to diffraction enough light is coming from the other side of the iron bar, to enable us to see whats there. That quadrangle is diffracting the light. But why its fuzzy?
Diffraction is by its own very nature even though purely classical in many sense, fuzzy towards energy if time window is small. [due to energy vs time uncertainty]
That is, full energy cycle isn’t available, because a small time window is chosen. If you time-lapse the photograph, diffraction will become quite insignificant. Although it might still be there depending on details. More…