The Digital Sensor: Part 2

How a Digital Image File is Created

In Part 1 of The Digital Sensor the basics were presented of how the digital image data is captured with an analog sensor using a Bayer array. As mentioned in the first part, the mosaics of the three colors are reassembled into an image in a process called demosaicing. Of course, that is only the starting point for creating a usable image from a capture of color bits with an analog sensor.

The process of demosaicing includes interpreting Bayer array data and converting it into an image with three colors for each pixel. During this step the camera also applies white balance - to make the image color appear "correct" under the lighting used when the image was captured.

Digital sensors "see" differences in light linearly, so that twice the light intensity produces twice the response in the sensor. Our eyes, on the other hand, see light logarithmically. So when light intensity quadruples the eye perceives this as a doubling of light. The camera takes this into account by applying tone curves to the image in the conversion process.

In the above example we have gone from the pixilated Bayer array with a typical green tint to a dark reconstructed color image to a brighter finished image. Contrast, color saturation, and sharpening may also be applied in this step, depending on what is selected in the camera. Finally the high bit-depth RAW image is then converted into 8-bits per channel and compressed into a JPEG based on the compression settings chosen or used by default in your camera. All of the processing up to this point has likely taken place with the image in the camera buffer.

One of the problems with traditional photographers moving to digital is that they really don't understand that a digital camera is little more than an image processing computer with an optic attached that spits out a compressed standard format JPEG image file. With that understanding it is easy to see where RAW images might be an advantage in some situations.

All that goes on in the computer (camera) takes control away from the photographer. Those that prefer total control over their final image, including tricks learned over many years in the darkroom, often prefer to control the conversion process themselves. The argument goes that image control is part of what makes a person a photographer, and that digital is like a trained monkey pointing and clicking. Other photographers who sing the praises of digital see it as liberating, freeing them to concentrate on their unique photographic "vision".

Most serious cameras provide users a choice. The great majority of users, and virtually all casual users of DSLRs, shoot and use the JPEG images processed by the camera they are using. They choose to trust the camera manufacturer to make useful decisions in the conversion process. In fairness it should also be pointed out that most serious cameras allow immense control over the choices made for in-camera processing. Those who want control can often customize and fine-tune the in-camera processing to deliver the kind of image they choose.

Those who want full control over the conversion process may choose to shoot RAW and make their own decisions about what software to use, the conversion "work-flow", and the parameters important to them in the finished image. This photographer is more often a professional, but with the cheap cost of computer processing power these days, it is also a real option for photo amateurs and hobbyists.