At the most basic level, one can surmise that Canon chose to include a low pass filter in the EOS 5Ds while also offering the EOS 5Ds R without¹ a low pass filter because they felt there was enough demand for both options in the marketplace to generate a worthwhile return on the additional investment during product developments as well as the additional expense of having two models instead of one during production and distribution.

¹ Though it is not actually without a low pass filter. Rather, it has a low pass filter that reverses its own effect, as your question accurately reflects.

Just like the Nikon D800E, the EOS 5Ds R actually does have an "anti-aliasing" low pass filter in which the second layer is oriented at 180° from the first. Orienting at 180° instead of the typical 90° largely reverses the effect of the first layer. This makes two different versions of the same model much more feasible from a production cost standpoint:

• All of the spacing in the filter stack in front of the surface of the sensor can be the same.
• The resulting filter stack will have the same effective "cover glass" thickness for both models. Cover glass thickness is important because it determines certain lens design parameters to take into account regarding the way light is refracted as it goes through the filter stack immediately in front of the sensor. Remove the cover glass, or change its thickness, and the same lenses will perform differently at the same lens to sensor distance.

Roger Cicala, the founder and chief lens guru at lensrentals.com, wrote a series of blogs on the subject of cover glass thickness after he discovered not using a cover glass for one of his testing methods on a lab bench were giving very unexpected results. These explain how varying the cover glass thickness can affect lens performance.

• The Glass in the Path: Sensor Stacks and Adapted Lenses
• Sensor Stack Thickness: When Does It Matter?
• Sensor Stack Thickness Part III: The Summary