rick.lang wrote:I often wondered why BMD never confirmed any of my posts when I’d say from day one that the BMPCC4K photosites were 4.63 microns. You know always nice to have some courteous confirmation. But it would appear now that they never publicly confirmed that because possibly I was wrong all along.
It's probably partly due to being proprietary business information. It might not be quite the same sensor that everyone has been saying it is; it might be a newer one, and it might also be a semi-custom design (BMD could for example have specified the pattern and dyes for the color filter array).
Now I’m just speculating because I don’t really know what’s happening behind the scenes, but I can’t help but wonder if this is why the camera sensor has dual native ISO. If four photosites are ‘added’ together, it could result in an effective ISO 3200 with more noise; if four photosites are ‘averaged’ together, it could result in ISO 400 with a cleaner image.
My guess is that it's a semi-custom Sony sensor using back-side illumination with a BMD specified color filter array pattern and dyes. I'm guessing that it's using a approach similar to what Panasonic uses with its Varicam sensor, where it has a dedicated analog gain stage for each of the two native ISOs.
But that's just a guess based on the common theory that the Pocket4K is based on a Sony sensor, the fact that Sony does that sort of custom work for other customers (Phase One, Hasselblad, Fujifilm for example), and the fact that Sony has migrated BSI all the way to its medium format sensors now, so it's obviously able to do BSI on pretty much any sensor it wants to at this stage.
BSI improves a sensor's light gathering ability quite a bit, so it allows for higher dynamic range with smaller photosites. In turn that also means that one option is to keep the photosites at the same time, but use more saturated dyes, since more saturated dyes also compromise light gathering ability. The up side is that the more saturated dyes improve the color separation between adjacent photo sites, which makes the recorded color cleaner. Since color quality is obviously more important to BMD than resolution (which is IMO a good choice), it would be a reasonable tradeoff for BMD.
But that's just a theory.