Digital Raw

A raw file is a record of the raw sensor data from the camera, accompanied by some camera generated metadata.

Digital Raw

There is no single digital camera raw file format like there is with jpg.

Different cameras encode the raw data in different ways. For example:

Canon *.CRW Nikon *.NEF Minolta *.MRW Olympus *.ORF

These can all be read by the Adobe Camera Raw plugin.

How digital raw works

To understand how digital raw works, it helps to understand how digital cameras record an image and what the sensor in a digital camera actually captures.

The image sensor

Nearly all cameras use a two dimensional array to collect the photons that are the recorded image.

The array is made up of rows and columns of photosensitive detectors, using either CCD or CMOS technology, to form the image.

Photosensor

Taking a picture

When you take a picture, the camera’s shutter opens briefly and each photosite on the image sensor records the brightness of the light that falls on it by accumulating photons. The more light that hits a photosite, the more photons it records. Photosites capturing light from highlights in the scene will have many photons. Those capturing light from shadows will have few.

Taking a picture

The more light that hits a photosite, the more photons it records. Photosites capturing light from highlights in the scene will have many photons. Those capturing light from shadows will have few.

Taking a picture

After the shutter closes to end the exposure, the photons from each photosite are counted and converted into a digital number – 0 to 255

Taking a picture

This numbers represents the color of a single pixel and it, along with pixels captured by all of the other photosites on the sensor are used to reconstruct the image by setting the color and brightness of matching pixels on the screen or printed page.

Taking a picture

Because each pixel on the sensor has a color filter that only lets through one color, a captured image records the brightness of the red, green, and blue pixels separately. (There are usually twice as many photosites with green filters because the human eye is more sensitive to that color so green color accuracy is more important.)

It may be surprising, but pixels on an image sensor only capture brightness, not colour. They record the gray scale—a series of tones ranging from pure white (255) to pure Black (0)

Bayer Colour filter array

Greyscale Image

The sensors in the array, whether CCD or CMOS, just count photons – they produce a charge proportional to the amount of light they receive – without recording any color information.

The raw file is a greyscale image. It doesn’t contain anything that humans can interpret (see) as color.

Greyscale Image

In a greyscale image (such as a jpg) each pixel is represented by 256 possible tonal values from 0 (black) to 255 (white) with 254 intermediate shades of grey.

The color information is produced by color filters that are applied over the individual elements in the array in a process known as striping.

Bayer Pattern color filter array

Most cameras use what is called a Bayer Pattern color filter array, alternating red, green and blue filters.

Each photosensor is filtered so it captures only one color of light.

Obtaining color information is the job of a raw converter such as Camera Raw.

RGB Color Space

This is why digital cameras produce images in the RGB color space.

Red, Green and Blue are the additive colors that occur in nature.

Other colors result from a mixture of these colors.

RGB Color Space

Many other devices also use the RGB color space.

Scanners, multimedia projectors, computer monitors and TVs, digital video and some printers.

Photoshop also supports other color spaces, such as LAB color and CMYK

CMYK and LAB Color

CMYK is used in the printing industry as it offers a cyan, magenta yellow and black color channels.

LAB offers 3 channels, the L is for lightness/contrast with the a and b channels being the color (greyscale) channels.

It is easy to switch color channels in Photoshop to make adjustments.

What the raw converter does.

The raw converter interpolates the missing color information for each pixel from its neighbors, a process known as demosaicing.

White balance. Colorimetric interpretation. Gamma correction. Noise reduction, antialiasing and sharpening.

Why shoot RAW? Post editing in camera RAW is non destructive. No

data – or very little -is lost, unlike what occurs when jpgs are edited.

Greater control over the interpretation of the image. White balance, colorimetric rendering, tonal response, sharpening, noise reduction, exposure compensation can all be manipulated.

Most cameras capture at least 12 bits per channel per pixel for a possible 4,096 levels in each channel, giving you greater editing headroom. JPG’s are limited to 8 bits per channel, so when a camera converts to JPG, this data is ‘thrown away’. Note: All cameras shoot RAW and then convert to JPG. Not all cameras allow you to recover the RAW file.

Limitations of RAW

Processing time, including final conversion to JPG, TIFF or whatever.

File size. RAW file are bigger, but not as big as uncompressed TIFFs.

Longevity. Will the different brands of RAW files still be supported and readable in the future.

File formats used in digital imaging.

Tiff, EPS, DCS, Gif, PDF, PNG, PICT, BMP and Scitex CT are all file formats used in digital imaging.

Tiff (Tagged-Image File Format), JPEG (Joint Photographic Experts Group) (or JPG) and GIF are compression formats.

Compression allows for smaller file sizes but can lead to loss of image quality.

File compression

For example, in its RAW format, the example photo is 5.27 Mb.

As a High Quality jpg it is 2.87 Mb, Medium quality 410 Kb, Lowest quality 125 Kb.

As a tiff, the uncompressed file is 36 Mb, and using LZW (Lempel-Zif-Welch) compression is 44.7 Mb.

Most digital cameras use JPG compression .