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What is Sensor RAW?


Digital Photography Tutorial - RAW Usage

Sensor RAW is a special image recording option that is only available on digital SLRs and a few other high-quality semi-professional digital cameras. If your camera has this option, you have access to much higher image quality than the standard JPEG file format. In this article I’ll explain what you need to unlock that potential.

First, the name. I’m really not sure why RAW is always capitalised, and neither is anyone else I’ve asked about it. It isn’t an acronym, so really it should be written ‘raw’, but for some reason it’s always written in capitals –‘RAW’. However inexplicably silly it may be, I’ll continue to use the conventional presentation for this article.

Essentially, RAW is just what it sounds like. It’s the raw data straight from the camera’s sensor.

In a digital camera the photographic image is, as I’m sure you’re aware, captured by an electronic image sensor. This sensor has millions of tiny photocells that produce a charge when they are exposed to light; the brighter the light, the higher the charge.

Digital camera sensors don’t record the colour of the light hitting them, just the brightness, so a special mosaic of coloured filters is placed in front of the sensor, called a Bayer filter.

Naturally, the image generated by the sensor and filter wouldn’t make much sense to the unaided eye, so the signal from the sensor feeds into the camera’s image processor, a combination of computer electronics and software which turns the brightness data from the sensor, adjusted for the colours in the Bayer filter, into a full colour digital image that you can view and print.

Part of that process includes reducing the bit-depth of the basic image data (the amount of 1s and 0s used by a computer to describe each pixel), usually from 36-bit (12 bits per channel) or even 48-bit (16 bits per channel) to 24-bit (8 bits per channel), which makes a smaller and more easily processed file, but loses some colour depth because of the smaller palette that can be represented. Other adjustments to the colour balance, such as unusual white balance settings, can also reduce image quality, as will heavy noise reduction.

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