I’m wondering if there is a rule-of-thumb that one can use to predict the approximate size when exporting a raw file in Lr to a jpeg file. For instance, would an average 25 Mb raw file convert to about 5 Mb jpeg with the Lr export quality set to 100? To about 3.5 Mb with a quality of 80? …
I know the size will vary, but when I am exporting a collection of raw files and I have set the max file size to a certain amount, Lr sometimes won’t convert all the images because I have limited the file size too small. I’d like to be able to get a handle on some sort of quality/file size relationship before I start the export process.
There are many variables that determine raw to JPEG conversion efficiency. These include using loss-less raw format (i.e. most cameras) versus lossy raw format (i.e. Nikon), amount of detail in the picture subject, and lens resolving capability (i.e. sharpness). It is not uncommon to see a 5 to 1 variation in JPEG file size for the same Quality setting, camera, lens, and Fstop setting. In fact this is a good way to determine the sharpness of a lens or lens Fstop setting. The JPEG file size will increase in proportion to the amount of detail captured in the raw image file.
The more important question is just how much JPEG compression artifacting can you tolerate. Typically a LR Quality setting of 80-90 will show no visible artifacts, but lower quality settings may. It is dependent on subject, usage and your tolerance. If you intend on doing additional processing of the exported JPEG file I would suggest using a setting of 90, since repeated JPEG 'File Saves' cause cumulative image degradation. For low resolution screen images destined for Web posting and no further processing a Quality setting of 50-70 (or even less) should produce acceptable image quality.
The best way to determine this is to run your own tests with your equipment and typical pictures you shoot.
I recommend to *never* use file-size limiter, unless it's an absolute requirement of the end-point consumer.
Why? Some images don't need anywhere near as many bytes to look good as others do.
So, if your goal is to limit total/average size, instead of max size of any one image, you can get better overall quality for the same total bytes by setting jpeg quality instead of max file size.
Also, Lightroom's file-size limiter is buggy (i.e. it should *never* fail to export, unless file-size was set ridiculously low, yet sometimes - it does fail: takes all the fun out of it, for me...)
Sorry I haven't been more help, but the file-size depends so much on content and processing that I don't try to predict anymore...
PS - you can reduce the time required for a test export by using PreviewExporter.
If you absolutely must limit individual file-size, then forget everything I just said, and good luck: please let us know how it goes...
Another option: Export tiffs from Lightroom, then use a 3rd party jpeg converter with file-size limiter that actually works.
I totally agree with Rob. JPEG file size limiters should not be used unless you absolutely don't care about image quality degradation. I can't imagine what application where that would be acceptable.
JPEG compression efficiency at any given Quality setting varies typically in the range of 5:1. Using a file size limit means the images with the least detail will be saved at very high quality setting (i.e. 90-100), and images with the most detail will be saved at much lower Quality settings (0-50). This is the exact opposite of what you want to do!
Unless you use a large file size limit some of the exported JPEG images will probably have visible JPEG artifacting. You are better off determining your threshold of acceptability for JPEG artifacting (i.e. Quality 50-70?) Then use that Quality setting for your exports without restricting files size. The resulting "average file size" will be less than using a larger file size limit.
Tips for further reducing files size of Exported JPEG images:
1) Use conservative sharpening settings in the Develop and Export modules (i.e. don't over-sharpen)
2) Use sufficient Color and Luminance Noise Reduction settings to reduce visible noise in the image.
3) Make sure "Remove Chromatic Aberration' is checked in the Lens Corrections Color panel.
Following the above suggestions can reduce exported JPEG file size by 50% or more, especially for high ISO raw image files.