Bokeh signature. amazing character of bokeh. wikipedia describes it as, “In photography, bokeh ( /ˈboʊkə/ boh-kə, Japanese: [boke]) is the blur, or the aesthetic quality of the blur, in out-of-focus areas of an image, or “the way the lens renders out-of-focus points of light.” Differences in lens aberrations and aperture shape cause some lens designs to blur the image in a way that is pleasing to the eye, while others produce blurring that is unpleasant or distracting—”good” and “bad” bokeh, respectively. Bokeh occurs for parts of the scene that lie outside the depth of field. Photographers sometimes deliberately use a shallow focus technique to create images with prominent out-of-focus regions.
Bokeh is often most visible around small background highlights, such as specular reflections and light sources, which is why it is often associated with such areas. However, bokeh is not limited to highlights; blur occurs in all out-of-focus regions of the image.”
the key point is in the lens aberrations and large aperture with shallow depth of field. a key point, bokeh’d highlights can occur in the foreground when focus is at or near infinity. try it ! see below image.
the extreme shallow depth of the Carl Zeiss Jena 75mm f/1.5 Biotar, though difficult to focus, has some interesting lens issues that create a very unique bokeh signature when used at f/1.5.