Can someone please help me to understand why Adobe Lightbox converts a 7.23MB CR2 file from my EOS 1D Mark II to a 5.04MB JPG file, but also converts a 19.5MB CR2 file from my EOS 50D to only a 5.72MB JPG file? Same program and compression setting, but the JPG files are very close in size compared to the huge difference in he CR2 file sizes. Thanks.
Posted by David Bostedo on November 8, 2008 at 11:24 PM
JPG compression doesn't work equally on all files. The smoother the tones, and the less transistions there are in groups of pixels, the more compression will be achieved. A very monochrome image will compress very well, and one with many changes in color and tone throughout will not compress as well. Along the same lines, a file that is noisy will not compress as well as one that's noiseless, because of all the small transitions the noisy pixels create. All of this assumes that you're saving at the same quality level (e.g. the same level of compression).
As far as what's happening in your specific case, I'd assume that the 50D file was more uniform throughout. Also, are you sure you're using the same JPG quality number for saving? Also, is it possible that you're saving compressed RAW files on the 1DMKII and uncompressed on the 50D?
Posted by Robert Fisher on November 9, 2008 at 8:10 AM
All RAW files are compressed. There's no way to change the compression factor with RAW, only JPEG, as far as I know.
Other than the higher resolution, your 50D files also use 14 bits of data per pixel vs. the 12 bits of the 1D. Both use 16-bit files, but the 1D will have a lot more zeroes in it, which can be compressed better. Note that raw files use lossless compression so all data must be retained, limiting the amount of compression possible.
JPEG on the other hand uses lossy compression which means, as David describes, it will average out close values rather than retain the original data. It also uses just 8 bits of data per pixel so a lot of color information is discarded to achieve better compression.
Discussion
3 repliesAs far as what's happening in your specific case, I'd assume that the 50D file was more uniform throughout. Also, are you sure you're using the same JPG quality number for saving? Also, is it possible that you're saving compressed RAW files on the 1DMKII and uncompressed on the 50D?
JPEG on the other hand uses lossy compression which means, as David describes, it will average out close values rather than retain the original data. It also uses just 8 bits of data per pixel so a lot of color information is discarded to achieve better compression.
Guy