Note: The following is beyond my pay grade in optical sophistication. It almost certainly contains oversimplification and misunderstanding.
Film cameras rely on external filters of various types to adjust light for exposure conditions and desired special effects. Some external filters have utility on digital cameras as well. But before buying any filters, it is useful to understand that digital cameras internally provide IR and UV filtering and red-channel color correction.
The sensor of a Nikon DSLR is fronted by its own 4-layer filter stack, variously called the anti-aliasing (AA) or blurring filter, the infrared cut filter (ICF), or the optical low pass filter (OLPF). The AA function is enabled by filter layers containing a strata of birefringent material, whose dual refractive characteristic is used to split light of different polarization into two slightly different rays during transit.
The stack consists of:
- dust removal filter, combined with the initial AA stage through an included birefringent layer, further coated to provide (IR) and ultraviolet (UV) channel attenuation
- wave plate, similar in function to a circular polarizing filter, changes the light’s wave phase
- color correction filter to remove most photons from the deep red and IR channel, designed to match the red response of the human eye
- final AA stage, includes a birefringent layer
Layers 1, 2, and 4 comprise the AA function of the stack. Layer 1 splits each incident ray horizontally into two. Layer 2 changes the wave phase 90 degrees in preparation for the second AA layer. Layer 4 again splits each of the doubled light rays into two, now in the vertical direction due to polarization of layer 2. The combined effect of the AA layers is the blurring of each original incident light ray into a closely associated set of 4 rays prior to interacting with the sensor.
This blurring seeks to avoid optical artifacts that can arise when any fine geometric pattern, with the approximate spatial resolution of the sensor’s pixel density, is detected from the exposed subject. Light interference (moiré) then beomes a possibility during the demosaicing function of the common Bayer-type arrays responsible for sensor color detection. The thickness of the first and last birefringent strata tunes the AA response to the specific sensor pixel density.
Moiré becomes less problematic for cameras with sensors having high pixel density. Thus newer cameras are omitting the AA filter, to once again transmit the full acuity of the lens. The filter stack remains similar without AA filtering, perhaps in part for reasons of economy. Layer 1 splits a dot into two dots vertically, the layer 2 wave plate is replaced by optical glass, and layer 4 is oriented to undo the splitting done by layer 1. The stack is efficient, admitting 95% of incident visible light to the sensor, while largely preventing IR and UV wavelengths from reaching the sensor.