Digital Photographer's Software Guide - Bertemes - Net

7
RAW Converters
The first step in the digital photographic workflow is to convert the RAW file. Many
photographers rely on their camera to perform this conversion, a procedure that manufacturers
believed would be almost universal when DSLRs were first introduced. It was
not long, however, before photographers started noticing that RAW files devoted four
extra bits to the image. What happened, they wondered, to all the extra information
encoded by those four bits?
When people gained practical experience with RAW files, it soon become apparent that
additional information could indeed be extracted from them, given sufficient processing
time. Desperate in particular for greater dynamic range, photographers jumped at
the opportunity of using RAW instead of in-camera JPEGs, sometimes shooting in dual
mode to gain the best of both worlds. The only problem, apart from speed, was the fact
that RAW was not (and still is not) a single format but a multiplicity of proprietary formats.
Canon has CRW, Nikon has NEF, Olympus has ORF, Pentax has PTX, and so
on. No one publishes the source code, obliging users to accept the official conversion
package usually but not always supplied with the camera.
To be effective, RAW conversion needs to do a lot more than “de-mosaic” the image,
although this is an important step that has a fundamental impact on resolution. The
mosaic of red, green, and blue filters, with two green pixel sites for every red or blue one,
becomes converted into complete RGB pixels by a process that guesses the missing values.
The predominance of green provides a convenient luminance channel that serves as
a guide for the overall light level. Once converted, the image is inevitably slightly soft,
because of the overlapping values. It may also be noisy or have an incorrect color cast.
Through no fault of the user it can have lens errors such as barrel distortion, vignetting,
or chromatic aberration. All of these imperfections can and should be corrected at the
earliest possible stage, in a processing package shortly after basic RAW conversion. It is
for this reason that RAW converters, like Topsy, just seem to grow.