weekly camera fact! #1 | cmos white balance
Did you knew that all digital cameras are daylight balanced and that we (gafpa Gear) use ‘ 80a’ correction filters on most night shoots? Most cmos sensors are D60,(some d55 or even d50). This means that blue and red in the bayer mosaic are balanced to have similar output in equivalent kelvin (for d60 6000 kelvin).
All these daylight balanced sensor achieve their incandescent tungsten white balance by amplifying the blue channel over two full stops relative to the red channel. Now comes the fun part, because the raw religious people believe this is done on a sensor level by amplifying the iso only on blue pixels, but this is NOT TRUE.
White balance is archieved by demoisaicing and then change the xyz white balance before the color matrix. Thus digital gaining the blue level in the case of tungsten (3200 kelvin) white balance.
This adds a great deal of noise to the blue channel, because blue is likely getting 2 stops more underexposed than red or green.
It also makes it very easy to blow the red channel because it relatively more dominant in highlights, especially when shooting red dominated subjects (human skin, cosmetics, and fall colors near dusk and dawn when the light is warm).
So how to fix this? Of course we could opt for a camera with a dynamic range with 2 more stops so the blues are less likely to get underexposed, but still we end up with an relatively unbalanced image! So what we do at gafpa gear is to shoot night scenes around 5600-6000 kelvin and use a 80a correction filter instead. Yes it will eat the orange red tones by 2 stops, but leaves the blue tones relatively okay. Depending on how you rate it you will loose around 1 -2 stops of light. But with a camera like MAVO Edge6k and its 5120 iso clean mode you still have at least 1250/2500 iso performance!
YOur blues will be way better especially in the shadows and your reds wont clip as fast so incadecent bulbs and their hallows will look less blown out, which is a great win.
Its funny how concerned dops were during the film days (the more intellectual and experimantal days), and this topic still is exactly the same. For instance When I met Chivo lubezki back in 2009 and worked with him on a nike commercial I was amazed he used tungsten stock (500t two stops pushed) under hmi and daylight. He refused to use a correction filter and thus left skin tones (red tones etc) underexposed, they corrected in post, but he loved that more because film doesnt have a lineair response to light (white balance is executed on digital sensors lineairy before log curve etc). and by not correcting tungsten stock under daylight he left the skintones slightly more pastel and shadows (which are always more blue than direct light under daylight conditions) very underexposed in the red tones, ending up with a look he loved and is praised for (tree of life got the same treatment). So this is the otherway around; making the blue layer in a filmstock way more sensative to light than red tones, which leads to a different color pallette even after correction, because film has a very wide gamut and non lineair response much more complicated and interesting than doing the same thing on a digital camera.