New Canon Raw Format

[Mike Farley]

For those who like to understand what is going inside their cameras, there is an interesting article on DPReview. Canon's recently introduced M50 has a new Raw file format, CR3. It also has the option to save images in C-Raw format, which are around 30-40% smaller. Until now, Canon has always reduced the size of the Raw files its cameras produce by using lossless compression. That is necessary as the file sizes would be much larger without it. While the article does not implicitly state what is happening with C-Raw, the implication is that the smaller file sizes are achieved by using a lossy compression algorithm. In other words, some of the information captured by the camera's sensor is discarded.

In theory, that is a bad idea but DPReview wanted to know whether it had any practical implication in real world use. What they found was a slight reduction in dynamic range where the image was underexposed by 4 stops or more. Otherwise, there was no discernible difference in image quality. Their conclusion is that users can safely utilise it to save space both on memory cards and disk space. That is an impressive achievement on Canon's part.

https://www.dpreview.com/articles/04833 ... raw-format

This is not the first tome lossy compression has been used for images. The JPEG format has relied on it for years to optimise file sizes. In that case, it only becomes an issue if the image is going to be used as the basis for further processing, particularly if the manipulation involves some extreme adjustments. At least some of Sony's cameras use lossy compression by default for Raw files.* After an outcry, a firmware update to the A7 II R added an option to save files which are uncompressed and end up around twice as large.

Some people have published results of tests to demonstrate that Sony's lossy compression can create artefacts in some circumstances. A while back, I asked Joe Cornish who is a Sony Ambassador whether he found it to be an issue in practice. He said not. Of course he would say that, some might claim. While Sony does provide Joe with free gear, it does not pay him to use their cameras. My impression is that image quality is more important to him than the sponsorship and he would not use Sony cameras if he believed they compromised his photography.

* The issue arose in respect of Sony's A7 range of cameras and I have not been able to verify whether other models are affected. To my knowledge, Sony does not currently offer an option for lossless Raw file compression on any of its cameras.