When designing wide-angle zoom lenses, it’s often difficult to take steps to avoid ghosting. RF15-35mm f/2.8L IS USM was no exception, and we struggled to prevent this imaging issue.
As a countermeasure, two types of anti-reflective coatings are available. One is SWC, which is especially anti-reflective for light that enters the lens at a low angle of incidence. Another is ASC, which is especially anti-reflective for light that enters the lens straight-on. In optical design, knowing the ideal coating to use for each surface was difficult, because the coatings can be combined in so many ways. To move ahead in design, we would run simulations on a very high-speed computer on a Friday evening, for example, to get all the results when we returned to the office on Monday. After an array of simulations, we found that the short back focus of RF lenses makes it more effective to use certain anti-reflective coatings on the lens group near the image sensor –the rear lens group – to reduce ghosting. This is because the rear lens group is so close to the sensor that any ghosting by that group would be immediately picked up by the sensor. There would be no room for the ghosting to dissipate, even if we modified the curvature of the lens slightly. As a result, although EF16-35mm f/2.8L III USM features both SWC and ASC coatings on the front lens group, RF15-35mm f/2.8 L IS USM has an optimal arrangement of SWC coating in the front and ASC at the rear.
Still, even if we know which surface to coat with SWC or ASC, some ghosting may occur. We reduce it by controlling lens curvature of each surface in the optics. We reduce some ghosting by applying SWC or ASC and other ghosting by allowing it to dissipate away from the sensor. This is the kind of delicate optical design work the lens required.