RAW

...is an Image Format.

A raw image file contains minimally processed data from the image sensor of a digital camera or image scanner. Raw files are so named because they are not yet processed and ready to be used with a bitmap graphics editor or printed. Normally, the image will be processed by a raw converter in a wide-gamut internal colorspace where precise adjustments can be made before conversion to an RGB file format such as TIFF or JPEG for storage, printing, or further manipulation.

There is no single RAW format: different manufacturers use their own proprietary formats, which are collectively known as RAW format. RAW files contain pixel data from the image sensor, usually at 12 or 14 bits per sensor bucket. The sensor is almost invariably overlaid with a so-called Bayer filter, consisting of a mosaic of red, blue and green filters in alternating rows of RG and GB. Given that three colors fit uncomfortably in a rectangular grid, green was chosen to be doubly present, since the human eye is more sensitive to it. Green also often serves as the luminance channel, and as the dominant channel for in-camera B&W conversions. To retrieve an image from a RAW file, this mosaic of data must be converted into a full RGB image. This is known as demosaicing, but is referred to by many manufacturers as digital development.

The contents of RAW files are often considered to be of higher quality than the final converted results, usually JPG. Each pixel in a RAW file has a greater bit-depth, and can thus store more subtle variations in color, and has more detail specifically in the shadow areas. Hence, large transformations of the data, such as increasing the exposure of a dramatically under-exposed photo, result in less visible artifacts. This leaves more scope for both corrections and artistic manipulations, without resulting in images with visible flaws such as posterization.

One variation on the Bayer scheme is the RGBE sensor of the Sony DSC-F828, which experimented with exchanging the green in the RG rows with Emerald (cyan). Other sensors, such as the Foveon X3 sensor capture information directly in RGB form, having three pixel sensors in each location, one for each colour component.

Benefits

Nearly all digital cameras can process the image from the sensor into a JPEG file using settings for white balance, color saturation, contrast, and sharpness that are either selected automatically or entered by the photographer before taking the picture. Cameras that support RAW files save these settings in the file, but defer the processing. This results in an extra step for the photographer, so RAW is normally only used when additional computer processing is intended. However, RAW permits much greater control than JPEG for several reasons:

* Finer control is easier for the settings when a mouse and keyboard are available to set them. For example, the white point can be set to any value, not just discrete values like "daylight" or "incandescent".

* The settings can be previewed and tweaked to obtain the best quality image or desired effect. (With in-camera processing, the values must be set before the exposure). This is especially pertinent to the white balance setting since color casts can be difficult to correct after the conversion to RGB is done.

* Camera raw files have 12 or 14 bits of brightness information. But you cannot compare this number alone to other methods. JPEG stores a brightness gradient in an 8-bit number every 4 or 8 pixels and stores color values even more infrequently depending on the parameters used. Because of this JPEG loses fine details and is ill-suited for major color or brightness changes. By comparison the mosaicing used by the Bayer filter in raw files changes colors every 3-4 pixels and brightness every 1-2 pixels producing much finer resolution detail in a same size image. And because it is 12-bit each of these values are far more precise.

* The working color space can be set to whatever is desired.

* Different demosaicing algorithms can be used, not just the one coded into the camera.

Drawbacks

Camera raw files are typically 2-6 times larger than JPEG files. Some raw formats do not use compression, some implement lossless data compression to reduce the size of the files without affecting image quality and others use lossy data compression where quantization and filtering is performed on the image data. While use of raw formats avoids or reduces the compression artifacts inherent in JPEG, fewer images can fit on a given memory card. It also takes longer for the camera to write raw images to the card, so fewer pictures can be taken in quick succession (affecting the ability to take, for example, a sports sequence).

There is still no widely-accepted standard RAW format; Adobe's DNG format has been put forward as a standard, but is not adopted by major camera companies. Numerous different RAW formats are currently in use; new RAW formats keep being developed, and others orphaned.

More specific software may be required to open RAW files on some systems, as opposed to standard formats like JPEG or TIFF.

The time taken in the image workflow is an important factor for choosing between RAW and ready-to-use image formats.