RAW image format

The most common image format among digital cameras is JPEG, it's a format which produces relatively small files from large amounts of image data by discarding certain information, however JPEG uses a "lossy compression algorithm". The only common lossless alternative is TIFF, this produces an uncompressed 24-bit per pixel image often in the multiple megabytes, for a 4 megapixel camera in excess of 11 MB per image, not really practical.

RAW is simply pixel data as it comes directly off the CCD, no in-camera processing is performed. Typically this data is 8, 10 or 12 bits per pixel. The advantage being that file sizes are considerably smaller (2272 x 1704 x 10 bits = 4,839,360 bytes), the image has not been processed or white balanced which means you can correct the image, and it's a better representation of the "digital negative" captured. The disadvantage is you can't open these image files with a normal photo package without using an "acquire module" (a plugin, typically TWAIN, which can open / process such images).

A little background: each pixel of a CCD can only see one colour, depending on the CFA (colour filter array) placed over the CCD this is either Red/Green/Blue or Cyan/Magenta/Green/Yellow. The cameras internal image processing engine then interpolates colours from the value of neighbouring pixels to calculate a full 24-bit colour for each pixel.

Supplied Software

The G2 is supplied with two methods for converting RAW files back to standard files (or load directly into a photo application):

  • TWAIN RAW acquire module which allows you to open RAW (or G2 JPEG) files in any TWAIN compatible photo package (Photoshop, Paint Shop Pro, etc. etc.). These files can come either directly from the camera (via. USB) or your local hard disk.
  • RAW Image Converter is an application which 'associates' itself with .CRW files, double click on a single or multiple files in Windows Explorer and you can convert the selected RAW files directly into TIFF (8/16-bit) or BMP format.

TWAIN RAW acquire module

The acquire module main window has a row of control buttons along the top (camera settings, preferences, rotate, set raw param.) as well as action buttons down the left side. You can choose to connect directly to the camera (via. USB) or browse files on your local hard disk (clicking on 'My Computer'). Select one or more files and click on 'Acquire' to convert the files and pass them back to the host application.

Camera Settings Window

Clicking on the 'Camera Settings' button displays the window above, here you can set the owners name (which is subsequently recorded against all images), date & time and format the current CF card.


The Preferences dialog allows you to set your acquire options. The most interesting settings here are the choice to acquire at 8 or 16 bits per channel and also to disable the 'False Color Filter' which can lead to some moiré artifacts but also speeds up acquire quite a bit.

Set RAW Param. Window

Clicking on an image (or group of images) and then clicking on "Set RAW Param." displays the parameters window which allows the modification of parameters such as white balance, sharpening, contrast and colour saturation which can be left as default (camera settings), set individually or for the group of images.

White Balance can even be manually "picked" from a white area on the image. New to the G2 is the ability to acquire as a 16-bit file and also perform linear acquire.

Canon still haven't done anything new with the acquire module, I'd like to have seen more contrast / sharpness / saturation settings (like +/- two levels instead of just one) as well as an exposure compensation slider which would allow you to get the most out of the 10-bits of data stored in the RAW file. The acquire module is also still pretty slow (even with 'false colour removal' disabled).

File Properties

From the main window you can also extract shooting information from an image (RAW or JPEG):

File Name CRW_0483.CRW
Camera Model Name Canon PowerShot G2
Shooting Date/Time 08/04/2001 11:18:28 AM
Shooting Mode Aperture-Priority AE
Tv( Shutter Speed ) 1/20
Av( Aperture Value ) 4.0
Metering Mode Evaluative
Exposure Compen. 0
ISO Speed 50
Lens 7.0 - 21.0mm
Focal Length 16.8 mm
Digital Zoom None
Image Size 2272x1704
Image Quality RAW
Flash Off
White Balance Daylight
AF Mode Single
Active AF Points [ Center ]
Parameters Contrast Normal
Sharpness Normal
Color saturation Normal
File Size 3199KB
Serial Number 0-0240
Drive Mode Single-frame shooting
Macro On
Owners name  

Acquire Examples: Linear Acquire

Linear mode allows you to acquire a 16-bit image with all 10-bits of information maintained, this can sometimes allow you to eek out a little more dynamic range from a contrasty image. Linear files have had no processing applied, no sharpening, contrast or tone which is why they appear very dark. It is possible to adjust linear images later though it's not for the faint hearted.

Linear Acquire
(resaved as 8-bit JPEG)
Linear Acquire 'fixed'* Normal Acquire

* Photoshop 6: Curve applied, Saturation increased, Unsharpen mask applied

Acquire Examples: Parameters

Below we've provided a few samples of the same RAW file acquired with different "RAW parameters" to try to give an impression of why RAW is useful and allows for flexibility. All images were acquired into Photoshop then resaved, the Contrast and Saturation samples were resized to 1024 x 768 (to conserve bandwidth).


White Balance: Auto, Sharpness: Normal

Contrast: Low Contrast: Normal Contrast: High


White Balance: Auto, Contrast: Normal

Saturation: Low Saturation: Normal Saturation: High


White Balance: Auto, Contrast: Normal, Saturation: Normal

Sharpness: Low Sharpness: Normal Sharpness: High

RAW Image Converter

Also supplied is the RAW Image Converter, this program associates itself with .CRW files. Double-clicking on a single or multiple CRW files in Windows Explorer will display the window shown on the left, you can then set the conversion settings, where you'd like the images saved and in what format (TIFF 8/16-bit, BMP). This utility allows you to convert RAW files directly into a standard image format without running through a TWAIN compatible photo application first.

I'd really like to have seen the option to output as JPEG.