The soft focus lens has designed-in spherical aberration which provides excellent out of focus highlights (bokeh). The diffusion disks are thin metal pieces that look like small metal bottlecaps. They are fitted one at a time only, though are not required to use the lens. They have a larger central hole with smaller holes surrounding. The size of the central hole and the amount of light that passes the disk in total changes the apparent aperture for exposure calculation. While the smaller apparent base-aperture given by the disks does not appreciably increase depth of field, it does require compensation. I wonder if the DoF is increased but is not apparent due to the increased softness of focus.
The disks are, #1 (ƒ5.0), #2 (ƒ5.6) and #3 (ƒ6.3). They are supplied in a clear plastic “filter case” style carrier with a fitted, molded soft plastic piece to hold them. To use, unscrew the front lens cell (group of lenses held together, screwed into the shutter,) and simply slip the diffusion disk on to the back end of the cell, then return the cell to the shutter.
Using the the lens’s built in aperture, the diffusion of the disks is apparent between ƒ4.0 and ƒ6.3. Apertures ƒ8.0 and smaller shoot through the center of the lens and center hole of any of the disks, reducing or eliminating both the spherical aberration and diffusion from a diffusion disk if used, showing this to be one of the sharpest lenses built for the RB system.
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