Bokeh
In photography, bokeh is the aesthetic quality of the out-of-focus, blurred area, mainly the appearance of the blurred spots of light visible in the background. It comes up in the analysis of UFO/UAP photos and videos because a light source recorded out of focus often looks like a "strange object."
Bokeh (from the Japanese word "boke," meaning blurriness) describes, in photography and videography, the appearance and aesthetic quality of the out-of-focus parts of an image that fall outside the plane of focus. In everyday usage, it is often also applied to the blurred spots of light themselves that are visible in the background or foreground.
##How it forms
If a point-like light source (for example a light bulb, a star, an aircraft light or sunlit foliage) is out of focus, the camera records it not as a point but as a smaller or larger spot. The shape and edge of the spot are influenced mainly by the lens aperture and the design of the lens elements. With the aperture wide open, the spots are usually round. With a stopped-down aperture, they can become polygonal depending on the number of aperture blades, and at the edges of the image they are often "cat's-eye" shaped. Mirror lenses also produce ring- or donut-shaped spots. The spots can be evenly bright or have ringed edges, and concentric rings or dots, caused by the structure of the lens elements or by dust on them, can also appear inside them.
##Why it matters in the UFO/UAP field
Bokeh often features in the analysis of photos and videos. Analysts of such footage point out that some of the unusual "disc," "ring" or polygon-shaped light objects seen in nighttime or twilight footage are actually the bokeh of an out-of-focus, distant light source. Such a light source can be, for example, an aircraft, a drone, a planet or a streetlight. Since with a point-like light source the shape of the spot is determined by the optics of the lens, the shape of the apparent "object" is not necessarily the shape of the real object.
##Some characteristic effects can be especially misleading:
Change in size: the size of the spot depends on the degree of defocus, not on the size of the object. If the autofocus is "hunting," the spot appears to grow and shrink, which can look like approaching, receding or pulsating. Apparent structure: the rings and dots appearing inside the spot can create the impression of an "object with structure." Phone footage: in the dark, phone autofocus often does not set itself to infinity, so even a distant planet, star or aircraft can turn into a blurred disc.
This explanation is a common consideration when evaluating such footage, but it cannot be applied to every case. To judge individual recordings, it is worth also taking into account the circumstances of the recording, the device settings and several independent data sources.