Aliasing and Moire issues are heavily discussed topics for DSLR HD video, however the performance evaluations remain very subjective and are typically just based on random Youtube or Vimeo videos. Adding to this, camera manufactures hardly disclose any details about the related actual sensor level pixel processing.
This article is an attempt to present a simple test method to bring quantified measurements of the aliasing and Moire properties.
The test is demonstrated in the video:
The test is basically 2 fine line patterns (stacks of 600 lines), one vertical and one horizontal. Bear in mind that this line pattern is designed to be a worst case scenario for provoking Moire patterns out of any image sensor, even high end sensors. By making a video where there is zooming in and out, it is possible to scan for line densities displaying critical interferences vs. line frequencies. A subsequent analyse the video will readily provide several critical image properties for line skipping, effective pixels resolution and pixel fill factor and possible interactions of the color mask coloring and optical low pass filter (OPLF).
The line patterns can be printed from this document: 600 lines – line width 0.5.pdf . If needed, two alternative line widths are available here: 600 lines – line width 0.2.pdf , 600 lines – line width 1.0.pdf
The presented video test is performed on the Nikon d5100 and a kit lens 18-55mm at f/5.6, at a distance of about 2 meters. The line patterns are placed perfectly flat and in vertical/horizontal direction.
To conclude on the result from the test for the Nikon d5100 camera:
- Horizontal lines are prone to heavy aliasing and Moire problems. This is caused by a horizontal line skipping, which skips 2 out of 3 horizontal pixel lines corresponding to a fill factor of 33% for red and blue colors, and 60% for the green color.
- Effective vertical resolution is 924 px (ideally this should have been 1080 px!)
- Vertical lines (horizontal direction), are generally well sampled with a fill factor of 100%, with an effective pixel resolution in horizontal direction is minimum 2624p px (well above 1920px)
- However clear of colorfull Moire was observed for vertical lines at 1/2 of the native sensor.
- The test shows serious coloring of the Moire pattern (red, blue and green), mainly generated in the line skipping processes, but also for vertical lines, probably due to some kind of interaction between sub pixel sampling of the Bayer color mask and the optical low pass filter (OLPF).
- The test pattern shows an extreme amount of Moire, however, it should also be noted that just a tiny amount of defocus completely removes all Moire patterns and coloring.
- Video sampling algorithms appear to always use an integer fraction of the total pixels.
All in all, it is definitely concluded that this camera will have great image detail- and color improvements by applying an optical anti aliasing filter.