Digital Outback Photo
- Photography using Digital SLRs


The Art of RAW Conversion #001

"RAW Converter Essentials"

note by Uwe Steinmueller



Over the next coming months we will have a look at some more new and updated RAW converters (PhaseOne CaptureOne, Nikon Capture 3.5, Kodak Photo Desk 3.0, ...).

We want to have a framework how to judge the quality of the diverse RAW converters. That we want to summarize what a RAW converter needs to do.

  • Some features are essential and need to be supported by the RAW converter
  • Other features can also be done later in Photoshop or some plug-in
  • Finally there are some functional requirements a RAW converter has to meet to be useful
The RAW Workflow
Here is a diagram of the whole principle RAW file workflow:

Principle Workflow Steps
The rest of this article will discuss which of these steps are the responsibilty of a RAW converte.r
RAW Converter Responsibilities
This section lists the responsibilities and we later discuss them in some detail
Where required?
Bayer Pattern interpolation
Apply Tone Curve
Converter (*)
Apply Camera Profile
Converter (*)
White Balance (WB)
Exposure Compensation (EV)
Rotate 90 (CW,CCW) and 180 degrees Converter, Browser, Photoshop
Noise removal Converter of Photoshop
Artifact Removal (e.g. noise) Converter or Photoshop
Contrast and Brightness (Levels, Curves)
Converter or Photoshop
Saturation Converter or Photoshop
Sharpening Converter or Photoshop
Color Corrections Converter or Photoshop
(*) Can be done in Photoshop if the so called linear conversion is used (see later)
RAW converter Features
  • User interface experience
  • Support for Color Management (Monitor profile, Camera profiles, Output Profiles)
  • Histogram or equivalent aid for EV correction
  • Reasonable large preview
  • Realtime WB, EV and other correction preview
  • Batch capabilities
  • Good workflow integration
  • Integration with RAW image browser
  • Saving and recalling settings or setting groups (like WB, Sharpening, ...)
  • Saving files at 16bit
  • Tag saved files with profiles
  • Record RAW conversion settings

There are actually two extreme schools of RAW conversion strategies and all RAW converters fall somewhere in between:

  • Linear conversion: Performs only the interpolation, EV and WB in the RAW converter and the rest in Photoshop using actions, plug-ins or automation.
  • Full Service Conversion: Does all the jobs in the RAW converter. Photoshop is only used for final retouching (dust removal, minor corrections)
Linear Conversion
As said the linear conversion only performs the RAW interpolation (for more detail read here), EV and WB. The result is a dangerously dark image.

Linear converted file

This looks to us like a very bad exposed file. This is not the case as all(!) interpolated RAW files look like this if only the interpolation had been done. To get this image to life a so called "tone curve" is applied.

Tone Curve

Also this is not really the kind of curve one would use every day. In most cases this tone curve is actually part of a generic camera profile.

Resulting image after profile and tone curve have been applied

RAW Converter Essentials

This is the key responsibility of the RAW converter. There are many differences in the quality of RAW interpolation:

  • How many details are resolved (especially in the shadows)?
  • Does the noise get amplified by pulling out too much detail?
  • Stair casing in fine lines?

If readers know more criteria it would be very helpful to send me their feedback.

Tone Curve and Camera Profiles
A RAW converter should come with good generic camera profiles for all cameras supported
White Balance (WB)

Good white balance support is crucial for any useful RAW converter. It should be clear that WB correction without support for monitor profiles does not make too much sense. There are two kinds of WB correction techniques:

  • Gray balance by clicking on a neutral spot in the photo
  • WB correction controls (mostly in terms of presets and/or color temperature)

Good white balance support is not a trivial task for any RAW converter.

Exposure (EV)
Also good exposure compensation is essential. The tools should be aided by a histogram or equivalent (under- and overexposure) indicators.
Noise Removal (NR)

Generally it is a good idea to remove the noise as early as possible before it gets amplified in the next steps. Also the noise removal routine can make use of metadata like ISO and camera model.We pretty sure that we will see also some improvements on noise removal over the next months (Currently the best noise removal tools seem to exist as external tools - especially NeatImage gets high marks - or Photoshop plug-ins, Nik Multimedia announced a tool called Dfine).

Noise removal normally also lowers the sharpness of the image and that is why NR and sharpening are very much correlated.

Artifact Removal (Moire)

Sometimes noise and artifacts are treated as the same. Here is our view:

  • Noise is a result of the image capturing process especially in the shadows
  • Artifacts are the result of the Bayer pattern limitations and the interpolation process

Moire removal is especially important for cameras with a weak AA (Anti Aliasing) filter (e.g. Fuji S2, Canon 1D, Kodak 760 without AA filter).

Removing moire can be nasty or even impossible sometimes.

Contrast, Brightness, Saturation, Color Corrections and Sharpening
These steps are usually bet done in Photoshop. But if the RAW converter delivers these tools in a good quality then this can speed up the RAW file workflow significantly.
We hope you find this summary helpful and we now have a framework for all our future reviews on new and updated RAW converters.

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