In the following discussion I take certain liberties with the precise facts in the interest of fostering understanding of the mechanism involved in diffraction.
The mechanism of diffraction can be better understood if we note that when the wavefront of a light "beam" passes through an aperture, in effect at each infinitesimal point across the aperture the luminous flux that arrives there in effect powers a tiny "point source". (The wave is in effect "reconstructed" at such a place.) The luminous flux that emerges from each of these point sources spreads out in sort of a cone.
Next, imagine a very tiny aperture. Across it there are just a very few of these infinitesimal point sources, and the "cones" of light they each emit essentially completely overlap in their "wave" natures. They therefore collaborate in their effect, and thus the overall "output" of the aperture is a cone. When (after being converged by the following lens elements) that falls on the film or a sensor, it illuminates a rather large diameter spot. This is diffraction at its "worst".
Next imagine a quite large aperture. Across it there are a large number of these infinitesimal point sources, each emitting a tiny amount of luminous flux in a cone.
In these little cones, flux is of course in the form of a wave. If we consider the parts of these cones that are propagating "straight ahead", their wave natures remain "in phase", and so they cooperate in making a "straight ahead" aspect of the whole beam.
But the, if we consider the parts of all these cones that are propagating at a slight angle to "straight ahead, their wave natures are not quite "in phase". The reason is that the in at any given place in their travel they have traveled different distances from the plane of the aperture, their place of birth (like two runners who start off together at positions along a slanted starting line). Thus those parts of the respective cones "cancel out" to some extent, reducing their collective potency.
As we consider this effect for all the little cones, at each angle from the axis, we find that overall, the result of this is that the preponderance of the luminous flux traveling at an any angle to the axis is canceled out, while the preponderance of the flux traveling "straight ahead" works "in concert".
This almost totally straight-ahead only "beam", after passing through the remaining lens elements, creates almost a point on the film or sensor. This is diffraction in a negligible degree.
The larger the aperture, the more does this happen (the less is the spot on the film "spread" by diffraction). And the smaller the aperture, the closer is the operation to my first scenario, that of an infinitesimal-sized aperture, in which the spot on the film is greatly spread by diffraction.
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I close by calling attention to a phenomenon that is not an exact parallel but which nevertheless my be of some value to those trying to understand diffraction.
Consider a "dish-type" microwave radio transmitting antenna. The larger its diameter (in fact spoken of as the diameter of its "aperture"), the narrower will be its "beamwidth". A very large diameter antenna will create a beam so narrow that its "footprint" on a distant surface will be very small in diameter. An antenna of lesser diameter will create a wider beam such that its "footprint" on a distant surface will be much larger diameter.
This all happens by the way that the radio waves reflected from different parts of the dish cancel out with regard to their effect at an angle, but "cooperate" with respect to their effect "straight ahead". The larger the diameter of the dish, the more prominent is this phenomenon.
Time now for breakfast.
Best regards,
Doug