What is RAW?
     The normal way digital cameras present images is in the JPEG format. The images produced benfit from a relatively small file size but this is at the expense of some image information. The image is quite heavily manipulated within the camera before being saved onto the camera's memory storage medium. The JPEG format itself is described as 'lossy' in nature and has other features which make it less than ideal for use with high quality images.

Many of the higher quality digital cameras, particularly the SLRs, offer other storage alternatives. Some offer TIFF as a storage format. This differs from JPEG in being non-destructive or 'lossless'. There is less destruction of the image in the camera but the file produced is enormous. Relatively few cameras offer this now as RAW has massive advantages over TIFF.

A little about digital cameras

Any digital camera can simplistically be described as having five components:

  1. The light-sensitive chip: CCD or CMOS;
  2. The noise reduction chip: eg. Canon's DIGIC processor range;
  3. Image processing software;
  4. Storage: normally a memory card;
  5. The optical elements: lens and shutter.

All CCD/CMOS chips comprise a high number of light-sensitive cells. These are arranged in a rectangle which may be several thousand pixels wide and high.

The brightness of the light falling on a cell controls its output with no reference to the colour of the light. A sensor is inherently a monochromatic device and only creates a colour image by the use of colour filters placed individually over each and every cell.

These filters are typically arranged in a 'Bayer Pattern'. Every other filter is green and the remaining filters are alternately red and blue. The green filters are twice as frequent as the others and the resulting green portion of the image shows twice the resolution and is used, not only to give the green element of the image, but also to determine much of its sharpness.

Figure 1 shows a typical flow through a digital camera. The CCD data is processed to reduce the image noise which is largely caused by electrical interference between the cells. The data is then sent through the image processing software where an automatic determination is made of colour balance, brightness, contrast and gamma. The image is then converted into its output format, normally JPEG, and sent to the memory card.

Figure 1 also shows the alternative route offered by saving a RAW image. The image is not processed, nor is it converted into an image format. The RAW, unprocessed, data is sent straight to the memory card.

This achieves two objectives;

  1. The image is not degraded by image processing and can be stored in as perfect a state as it can be, and;
  2. More comprehensive image processing, with more user control, can be applied in a computer that can ever be applied in the restricted space in the camera.
Furthermore a third benefit accrues because of the way the RAW data is stored. Although the file will be considerably larger than after JPEG conversion, the RAW file will be between three and six times smaller than the TIFF or Photoshop file derived from it. It is thus quite an economical way of archiving images.

flow/gif
Figure 1. An example flowchart showing the point of divergence of a RAW workflow from normal.

RAW Converters

A RAW converter has the same basic functionality as the image processing software within the camera itself. However, as the speed and memory of a desktop computer are so much greater than those of a digital camera, the software can be much more sophisticated. This, together with the ability to store the image unmanipulated, is the core benefit of a RAW workflow.

A comprehensive feature list for a RAW converter programme includes the following:

  1. The ability to convert a RAW data stream into a range of visible image formats which it can either save to disc or send directly to an image manipulation package;
  2. The ability to modify the RAW data prior to conversion with reference to the settings sent with the image from the camera and with manual over-ride. Modifications being to the following;
    1. The white balance (or colour temperature);
    2. The exposure;
    3. The dynamic range;
    4. The contrast and brightness;
    5. The saturation.
  3. The ability to output in a range of sizes and resolutions;
  4. The ability to output in a choice of bit-depths;
  5. The ability to associate a choice of colour spaces with the image, determined automatically but with manual over-ride;
  6. The ability to apply sharpening;
  7. The ability to do all the above without the need for user input;
  8. The ability to batch process a range of images using the same or different processing parameters;
  9. The ability to rotate an image both at a right-angle and arbitrarily;
  10. The ability to cancel all user-input when it all goes wrong. (It will...);
  11. The ability to store the editing options applied to an image for future reference.

There are numerous RAW converters available. They are generally from one of three sources:

  • From the camera manufacturer;
    Software from this source is generally renowned for being bad. This is an over-simplification and this software should not be ignored out of hand (it comes free, after all). Try it and see...
  • With image editing software;
    The big example is Adobe's Camera RAW. Well featured and comprehensive, this is many peoples' choice. It is only available with Adobe's editing software though...
  • From 3rd party suppliers;
    There are currently quite a few independent converters available. Normally these will be as good as what one pays and can cost hundreds of dollars. Some producers produce 'Light' versions which cost less but offer reduced facilities. Some produce freeware versions of their software which are well worth a look.

In conclusion

Current thinking asserts that as much image editing should be decided prior to initial conversion as possible, hence in the RAW converter software rather than the image editor. This should minimise the need to make image-wide changes after conversion and hence minimise the degradation of the image from excessive editing.

However, a RAW workflow is not for the faint-hearted. Most RAW converters offer adjustments to a bewildering array of parameters. For new users, items 7 and 10 above, the get-out-of-gaol options, can assume great importance.

Any user should have the confidence to try things out by being aware that no RAW conversion software makes any changes to the original RAW file itself. This means that, whatever nasty, unrecoverable results are produced, one can dump it all and start anew.

The advantages gained are worth the effort of getting to know the general methodology, which is similar to much general image editing, and the specific features of the RAW converter chosen.

copyright © tony cropper 2006