I was looking at some test footage from several raw video cameras the other day and it became very obvious that some of the cameras were better than others and one or two had some real problems with skin tones. You would think that once you bypass all the cameras internal image processing that you should be able to get whatever colorimetry that you want from a raw camera. After all, your dealing with raw camera data. With traditional video cameras a lot of the “look” is created by the cameras color matrix, gamma curves and other internal processing, but a raw camera bypasses all of this outputting the raw sensor data. With an almost infinite amount of adjustment available in post production why is it that not all raw cameras are created equal?
For a start there are differences in sensor sensitivity and noise. This will depend on the size of the sensor, the number of pixels and the effectiveness of the on-sensor noise reduction. Many new sensors employ noise reduction at both analog and digital levels and this can be very effective at producing a cleaner image. So, clearly there are differences in the underlying electronics of different sensors but in addition there is also the quality of the color filters applied over the top of the pixels.
On a single chip camera a color filter array (CFA) is applied to the surface of the sensor. The properties of this filter array are crucial to the performance of the camera. If the filters are not selective enough there will be leakage of red light on to the green sensor pixels, green into blue etc. Designing and manufacturing such a microscopically small filter array is not easy. The filters need to be effective at blocking undesired wavelengths while still passing enough light so as not to compromise the sensitivity of the sensor. The dyes and materials used must not age or fade and must be resistant to the heat generated by the sensor. One of the reasons why Sony’s new PMW-F55 camera is so much more expensive than the F5 is because the F55’s sensor has a higher quality color filter array that gives a larger color gamut (range) than the F5’s more conventional filter array.
The quality of the color filter array will affect the quality of the final image. If there is too much leakage between the red, green and blue channels there will be a loss of subtle color textures. Faces, skin tones and those mid range image nuances that make a great image great will suffer and no amount of post production processing will make up for the loss of verisimilitude. This is what I believe I was seeing in the comparison raw footage where a couple of the cameras just didn’t have good looking skin tones. So, just because a camera can output raw data from it’s sensor, this is not a guarantee of a superior image. It might well be raw, but because of sensor differences not all raw cameras are created equal.