March 1, 2017 Leave a comment
Back in 2012 when D800 was released, Nikon did a bit tweaking on its antialiasing filter which led to the higher resolution D800E. A pair of birefringent crystal is organized in the parallel 180 degree to cancel out the effect. But were they worth it? As we had disassembled more camera, I decided to write a post on how these filter stack is organized.
ChipMod sent me a pair of filters on the Nikon D600. The IMX128 was scraped during monochromatic mod.
Filters from D600: UV-IR, CMOS Cover Glass, Color Correction Stack
Back on D7000, I had shown the filter set consists of an antialiasing layer with UV-IR coating and an ICF stack sandwiched from a wave plate, a color correction glass and an other AA layer. Upon receiving the filter, I initially suspect the same. After closer examination, I found the color correction glass was actually just a single thin layer. No wave plate was glued to it. On a micrometer, it registered 0.484mm thick.
Without a wave plate, it’s impossible to spread a point into four, since the two light rays are in orthogonal polarized directions. I thought a workaround was to cut the AA filter at 45 degree instead of 0 or 90. (Here I refer to the orientation to the direction where two light rays separate. The AA filter is always cut perpendicular to the optical axis, or Z-axis, of the birefringent crystal) As such, the blue color could be mixed with red. However, upon inspection under a microscope, this was again rebutted. It turned out, the first UV-IR layer is only blurring on the vertical direction, leaving moiré as is in the horizontal direction.
Stage setup with micrometer ruler in the vertical direction
The spread from this filter is around 5 micron and wider than that in D7k. This corresponds to a thicker crystal at 0.8mm. Now we know for sure D600 only blurs vertically. This gives the advantage to gain a bit higher resolution in the horizontal direction. The DPreview had an excellent resolution test confirming the case. D600 resolve horizontally well beyond 36, albeit accompanying color moiré. But it blurs out at around 34 in vertical directions.
Any other cameras also do this? It turns out that many other cameras follow this trend. To name a few: Sony A7Rii, Nikon D5100, and possibly other low end DSLRs all had a single AA glued to a color correction filter. One possibility is to suppress the already strong false color during video live view rising from row skipping. However, I would still argue the effect of this is minimal given the spread distance close to pixel pitch.
The material for AA filter and wave plate is usually crystalline quartz glass. Many website cites lithium niobate and that is incorrect. An argument floats around that quartz has too small a birefringent value and it requires a thick slice. This is true during the early days of digital imaging where pixel pitch were huge! (>10um) Once a proper calculation is done, the above 0.8mm thick material happens to give a close to 5um displacement. Should lithium niobate be used, it would be way too thin to manufacture. Another interesting property with quartz, or fused silica, is its UV transparent property. Based on the above transmission spectrum scan, the AA substrate material permits UV to pass when measured at corner. Lithium niobate would absorb strongly in UV just like those ICFs. Notice that without any coating, the glass itself reflects 10% of light. Again, for emission nebula imaging, you could keep the UV-IR filter.