I wanted to make this shot to show the edge-diffraction phenomenon. The image is made of two photos of the same flag with the Moon behind the pole. In the left photo the Moon is correctly exposed and the pole is clearly visible, while in the right photo the Moon is overexposed and the pole is visibly thinned near the lunar disk.
Let’s see what phenomenon this is: diffraction is a physical phenomenon associated with wave propagation whose effects are relevant when a wave meets an obstacle or a slit whose dimensions are comparable to, or smaller than, its wavelength. According to the Huygens–Fresnel principle, after light has passed through a slit, each point of the slit behaves as if it were in turn a source of circular waves. Diffraction is in fact the interference between the different parts of the wave that occurs after the wave itself has met an obstacle. Since the diffraction phenomenon is generated by the encounter between a light beam and an obstacle, the shape of the latter will influence the pattern formed by the fringes on a distant screen (from https://cispio.diet.uniroma1.it/).
The effects change depending on whether the slit is linear or circular. In the case of an edge we can consider the edge itself as a linear slit.
Generally a point light source produces apparently sharp shadows whatever their shape, but by observing the shadows at high magnification one can make out narrow interference bands inside the shadow, typical of diffraction.
In the particular case in which the light source is hidden behind a sharp silhouette, the observer can see a dark object outlined by a brightly illuminated boundary line and without interference bands. The light that illuminates the edge represents the light source diffracted only by the edge. This is the case of “Edge diffraction”, i.e. diffraction at the edge (Edge diffraction according to the Schlieren optical technique).
In this photo the edge diffraction, i.e. the edge lit up as if it were itself the light source, can be seen on the pole of the flag on the right. The bright photo is the overexposed Moon. In the left photo the effect is not visible because the photographic exposure on the lunar disk was insufficient to record the phenomenon.
Shooting data:
Scene with overexposed lunar disk: Nikon Z9, 10,000 ISO, Nikon Z400 f/4.5 at f/20, exposure 1/8s.
Scene with correctly exposed lunar disk: Nikon Z9, 8,000 ISO, Nikon Z400 f/4.5 at f/20, exposure 1/1250s (the flag was later overexposed with Photoshop).
Another example of edge diffraction:
In the photo by Katiuscia Pederneschi, edge diffraction makes the edges of the lawn and the people appear bright. For more details see the GrAG Optic Picture of Day page: https://www.grag.org/edge-diffraction/
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