General

RAW conversion matters

Every camera stores sensor image data in a file for further use. The sensor image data are just grey values of different intensities in every pixel. A single pixel doesn’t show color. Our sensor image data contain a black and white world. How sad.

How do we get color images with a sensor that maps only grey values between white and black  ?

All the colors we know can be derived from a combination of a specific value of Red,  a specific value of Green and a specific value of Blue. If we know the shares of Red, Green and Blue in a color we know the color.

Our camera sensor measures the shares of Red, Green and Blue with small lenses in front of each pixel on the sensor that act as filters for Red, Green or Blue. The complete sample of these filter lenses is called a color filter array or „CFA“. The distribution of Red, Green and Blue filter lenses on the sensor is completely known only to the manufacturer.

The file that contains the sensor image data is called a RAW file. Beside the sensor image data at the beginning of a RAW file are informations about the complete file content. I found a nice Wikipedia article pointing this out. A RAW files contains also exposure data and information about the camera lens.

A software converting the sensor image data of a RAW file into pleasing colorful images is called a RAW converter. A well known and popular RAW converter is Adobe Camera RAW (ACR). A Phase One Raw file can be straight forward converted by ACR to a 16-bit TIFF. No further postproduction has been applied (see left image below).

A RAW conversion of the same file with the Phase One manufacturer’s own RAW converter Capture One to a 16-bit TIFF shows different color and brightness (see right image below). There was no setting of temperature, hue and saturation to make the images identical.

Still. Conversion ACR. Courtesy P. Kleiber
Still. Conversion Capture One. Courtesy P. Kleiber

One RAW file was converted to two different TIFF. The difference between the TIFF can easily be obtained in Photoshop with the difference layer mode. The difference is an image in itself. With a sensitive gradient applied the following image is obtained:

Difference image of RAW conversions using Capture One and ACR © Julian Köpke

The difference TIFF (Capture One) – TIFF (ACR) is derived pixel by pixel. The converted TIFF using Capture One contains more green, more magenta and more yellow in the darker values. The impact on the final result is unclear at the moment.

I like to make things visible the naked eye isn't able to see. That's part of my profession as a radiologist, too.

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