What is a Digital Raw File?
I have written several posts regarding digital Raw files produced by digital cameras. There is also much discussion online as to whether one should save photos in JPEG or Raw files. As with any file format, Raw files have both advantages and disadvantages.
In this post I will look at what exactly a digital Raw file is, what is needed produce a digital photo from a Raw file, and finally the advantages and disadvantages.
A Raw file
In my post titled How Digital Cameras Work I explain how a digital camera captures a scene and processes the information to produce a photo. Also in that post I talk about the camera’s sensor and how a colour filter is used to add the colours to the photos. Understanding camera sensors will help to understand a Raw file since this type of digital format is simply the sensor data saved to a file.
Although there are a few different colour filter arrangements available in digital cameras, a Raw file is simply a grayscale representation of the photo because it records the luminance values of each pixel in the photo. The Raw file does, however, contain colour information from the colour filter, but doesn’t contain anything readable by a human. The job of interpreting the colour is the responsibility of the Raw converter.
The Raw Converter
The colour from a Raw image is obtained by a program called a Raw Converter. For cameras that are able to take Raw pictures, a Raw converter software is included on one of the CDs that came with the camera. There are other converters available such as Capture One and Adobe Camera Raw.
Besides obtaining the colour from a Raw file, the converter does other tasks as well:
- White Balance – For JPEG files the white balance is set and applied to the digital photo in the camera. Since a Raw file is record of the sensor data, the white balance hasn’t been applied to a Raw file. The white balance setting is simply saved as metadata in the Raw file and can be applied in the converter. You can also change the white balance in the converter if you wish.
- Interpreting Colour – A Raw file contains luminance values for red, green, and blue values. The converter must then assign a colour value to each pixel based on the luminance values.
- Gamma Correction – There is complex theory behind gamma, but I won’t go into the details. A Raw file has linear gamma (gamma 1.0), which is much different than the human eye. The job of a converter is to apply gamma correction on the image to redistribute the tonal values to match more closely they a human eye sees light and shade.
- Sharpening – Similar to white balance, sharpening is usually performed in-camera for JPEG files. For Raw files, no sharpening is applied so this can be done within the converter. For most, however, sharpening the image should be done as the very last step in a digital workflow.
Finer Control Over Corrections
As I mentioned in the previous section, you can perform such things as white balance and sharpening after the photo is taken. This allows you to more finely tune the white balance, and perform more targeted sharpening of the image.
More Bits of Colour
For JPEG files, the most bits you can have is 24bits, or 8bits for each red, green, and blue. Raw files, however, can have many more bits. Each file can have 10bits or more for each red, green, and blue value which provides more overhead for editing the photo.
Unlike JPEGs, you can’t download and print files directly from the camera. These files first need to be converted to a more usable file by a Raw converter, and then saved as another file format, such as JPEG or TIFF before it can be printed.
A Raw file can be considerably larger than a JPEG file. This is because a JPEG file is compressed while a Raw file may not be. Even if it were compressed, it would still be larger than a JPEG.
Each camera manufacturer, and even model for that matter, have a different format Raw file. This is no common format use by all cameras, which means that for newer cameras you would need to wait for one of the third party converters to be updated to allow editing of the Raw files. In an effort to standardize the Raw format, Adobe has created the Digital Negative (DNG) file format, but it has yet to catch on with digital cameras.
Lack of Future Support
The proprietary format of Raw files may not ba around in 10 years time, which can make it an obsolete format. This is obviously a concern for archiving the photos. For those that are worried, just remember that the format won’t be gone in one day, but will probably take years to disappear, and by then you can easily convert the images to another format.
Camera digital Raw files are proprietary formats that record information directly from the sensor of a digital camera. All processing, such as white balance, gamma correction, and sharpening, is performed in a piece of software called a Raw converter. As with all image formats, the Raw format also has several advantages and disadvantages that should be known before choosing to shoot in Raw.