A Canon patent application in Japan outlines the technology and optical formulas to achieve Defocus Smoothing using electrochromic elements (EC). With EC elements, a lens with variable apodization is possible. Before looking at the patent literature, it’s worthwhile to look at Canon’s existing Defocus Smoothing technology.
In 2019, Canon released the RF 85mm F1.2L USM DS lens. The Defocus Smoothing (DS) technology works to create softer bokeh with smoother outlines. The result is achieved by controlling the light that passes through the center to the periphery of the lens. In the case of the RF 85mm F1.2L USM DS lens, two lens elements have a new vapor-deposited coating, which effectively acts like a radial graduated neutral density coating that feathers toward the outermost edge of the lens element. You can see the technology explained in a Canon video below.
The DS technology is similar to the apodization technology incorporated into the Sony FE 100mm F2.8 STF GM OSS and the apodized version of the Fujifilm’s XF 56mm F1.2 R.
Screenshot from Canon’s technology video above
In an article, B&H describes an apodization filter as affecting the transmission of light through a lens, but not impacting the aperture. B&H writes, ‘What exactly is better bokeh? Great question, though I think if you went with smoother and rounder, most photographers would agree. Standard optics can fall victim to undesired flaws in the bokeh they render. Cat-eye bokeh, onion rings, and polygonal bokeh are good examples. Apodization filters can correct for many of these issues when combined with the proper optics. As the filter darkens as you move toward the edges, it will soften the edges of the bokeh. In many instances, this results in the bokeh ‘balls’ slowly fading out and, since the filter is circular, it can help ensure that the bokeh remains circular.’
Canon Watch published an excerpt from the patent literature, which you can read below:
An electrochromic (hereinafter sometimes referred to as ‘EC’) element is an element having a pair of electrodes and an EC layer disposed between the electrodes. It is an optical element that adjusts the hue and amount of light in the visible light band by applying a voltage between them to oxidize or reduce the compounds in the EC layer.
EC elements have already been applied to products such as variable transmittance windows for aircraft and anti-glare mirrors for automobiles. Attempts have been made to apply it to the apodization filter is an optical element that smoothes the outline of a blurred image, and has a transmittance distribution in which the transmittance decreases with increasing distance from the optical axis.
Image from Canon’s patent application – 2022124678 (Japan)
In the conventional EC device described above, a desirable transmittance distribution is achieved by defining the resistance range of the electrodes, but the setting of the constituent requirements was not sufficient. That is, the transmittance distribution of the solution-type EC element depends on the resistance ratio between the electrode resistance and the solution resistance per unit width, depending on the resistivity of the electrochromic layer (solution). Therefore, it was necessary to define these in order to achieve a desirable transmittance distribution.
An object of the present invention is to provide an EC element capable of realizing a suitable transmittance distribution in a solution-type EC element in which power is supplied from the outer periphery of the dimming area, and furthermore, a suitable transmittance by following the aperture diameter of the mechanical aperture of the lens. It is an object of the present invention to provide an EC device capable of forming an index distribution. Another object of the present invention is to provide a lens unit and an image pickup apparatus having excellent optical characteristics using such an EC element.
The patent application outlines an EC element influenced by precise chemical composition and voltage. Instead of an apodization element having a specific, unchanging amount of darkening toward the outermost edges, with an EC element, it could be possible to control the darkening using electronic signals. Or, perhaps, if you didn’t want the DS effect applied, you could disable it entirely.