We have already looked at what bokeh is, but bokeh exhibits different characteristics, too.
One of the more interesting aspects of using freelensing and other off-axis techniques is the type of bokeh that is possible. Often, the effect is oval shaped or bright on one side and soft on the other. This can be altered by the angle at which a lens is held, offset, or skewed from the axis.
Oval-shaped bokeh can also happen within the lens axis. It is a sought-after effect by some contemporary users and was a prominent characteristic of many older lenses, especially those used in collodion wet-plate photography. Many old portrait lenses exhibited this type of bokeh effect.
Usually associated with standard and wide-angle lenses used at wide apertures, the effect is due to the relationship of image cones entering the lens from extreme angles as they pass through a wide aperture, which is elongated by the shape of the aperture circle from a sharp angle.
Cat’s eye bokeh
Cat’s eye bokeh is a variant of oval bokeh that happens when light subjects are rendered toward the edges of an image. This is a result of optical vignetting; the bokeh effect narrows from the centre of the image toward the corners. Consequently, the bokeh seems to swirl in a rotational pattern around the edges of the frame, as flattened disks of light.
A slightly different cat’s eye bokeh effect can also happen if the inside edges of the mirror/sensor chamber clip one side of the bokeh disk. This can occur when the lens is not matched to the camera, so the lens aperture is too big (too fast) for the size of the mirror/sensor chamber.The edges of the smaller chamber cut off the edges of the light exiting the lens before it hits the sensor. This generally affects only the bokeh closer to the edges of the frame.
Many older lenses produce what is referred to as spiral bokeh. If you look closely, you will notice that the centre bokeh appears normal, but as you get closer to the outer edges of the image circle, the bokeh disks appear flattened.
This is due to the curvature of the lens. As the outer edge light rays hit the first surface of the lens, they strike at an angle and, as they pass through the glass, the light is flattened into an oval shape.
Rays closer to the centre are less affected. They are rounder in shape due to their closer proximity to the axis of the lens, which allows them to travel in a more or less straight line through the aperture.
Focus in Photography is John Neel’s exploration of advanced focus that will greatly enhance your skill as a photographer. These pages will show you everything, from high-end techniques to create infinite focus, through to using it as an artistic tool, directing your viewer’s eye around your photographic composition.