The steps you take to correct for it also affect the bokeh of an image. Spherical aberration is present in all photographic lenses. Shot wide open at f/5.6 with a consumer zoom lens, the bokeh is less pronounced here.
At narrower apertures, the shape of the iris is more defined, whether it be a circle or a polygon, and the circles of confusion will be smaller. A wider aperture will produce bigger, rounder bokeh. The aperture of the lens also affects bokeh. For example, a lens with seven aperture blades produces heptagons, while a lens with nine (or more) produces more rounded bokeh. Those with fewer aperture blades will render more polygonal circles of confusion. The first is the number of aperture blades in the lens. Several lens design elements affect how bokeh appears. So, that’s also what determines the bokeh quality. How they actually appear is determined by the design and construction of the lens. Point light sources are only theoretically rendered as circles. However, like everything to do with optics, there’s a little more nuance to it than that. This phenomenon is called a “ circle of confusion.” It’s most apparent with point light sources, which is why lights and other specular highlights are so visible when they’re out of focus. When something falls outside the depth of field, instead of being reproduced exactly on the camera sensor, it’s reproduced as a blurry circle. What matters for bokeh isn’t so much that images have out-of-focus areas, but rather, how they’re rendered.
The depth of field is affected by the focal length of the lens, the aperture to which the lens is set, the distance the subject is from the camera, and the size of the camera sensor. In an image with a large depth of field, like the photo on the right above, everything is in focus. You’ll notice that even the model’s ears are slightly blurred. In an image with a shallow depth of field, like the portrait on the left above, only a small part (in this case, just a few millimeters) of the focal plane is in focus. It’s what determines what is in or out of focus in an image. The depth of field is the amount of the focal plane that’s acceptably sharp to the viewer.
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