- Why add an another anti alias filter to cancel instead of simply removing it?
- How does the dual filter compare against a camera that simply doesn't have any filters in terms of picture quality etc?
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.
Artifacts such as a moiré (more-ay) are repeating maze-like patterns, with or without color. Artifacts are a fact of life because the digital camera uses an array of primary colors to fracture the projected image from the lens into three juxtaposed images. This technique allows color photography by recording three images simultaneously. Image inaccuracy results when the subject has texture or pattern with an interval that fools the camera’s demosaicing software (algorithm). The result is a jumble pattern.
To help avoid the moiré, an optical low-pass or anti-aliasing filter hovers over the image sensor. Such a filter works by slightly blurring the optically projected image. This blurring is restricted to image detail smaller than the spacing of the photo sites of the array. In other words, such a filter only marginally tempers detail.
The camera maker might omit such a filter when cutting-edge resolution is deemed more important than reducing artifacts (scientific imagining etc.)