Today’s Question: I have a question regarding exporting a photo for a contest that specifies file size limit of 10 MB. When I go into the Export dialog [in Lightroom] I specify 10,000 KB as my limit but the file that is produced is actually 5.85MB. Why is it not producing a file closer to the 10 MB limit? Will this affect the final viewing experience?
Tim’s Quick Answer: The “Limit File Size To” setting in Lightroom’s Export dialog only establishes an upper limit. The other settings you establish will determine the actual file size you achieve, which may result in a file that is considerably smaller than the limit you’ve defined.
More Detail: The “Limit File Size To” option is only available when exporting a photo as a JPEG file. The file size limit really just provides a different way to describe the Quality setting. In other words, if you’re trying to achieve a file that is smaller than a particular size, it is easier to specify the size rather than trying to guess which setting to use for the Quality option.
However, the “Limit File Size To” option won’t guarantee a specific file size. The other key factors related to file size with a JPEG image are the pixel dimensions and the content of the image (since that content impacts the effectiveness of the JPEG compression).
If you specify pixel dimensions that result in a file size of less than 10 MB even at the maximum Quality setting, then the file will simply be smaller than 10MB. In that case, since the maximum Quality setting would be used, the only way to achieve a larger file for a given photo would be to increase the pixel dimensions.
If you specify pixel dimensions that make it impossible to create a file size smaller than the limit you specify, Lightroom will present an error that the file could not be exported because a file of the size you requested couldn’t be created.
If the contest specified pixel dimensions to be used for the files you submit, my recommendation is to export the photos at those exact pixel dimensions, and use the maximum value of 100 for the Quality setting. Only if the file were too large would I consider reducing the Quality setting. Provided you have maximized the pixel dimensions based on the submission guidelines and used a high value (80 or above) for the Quality setting, the image quality will be maintained very well.
As an aside, I also wish that the people running photo contests (or otherwise defining image submission guidelines) would stop including a maximum file size as part of their criteria. Including a file size limitation can lead to confusion for the photographers trying to submit images. If photos must be submitted as JPEG images, and if the pixel dimensions are limited to a reasonable value (such as around 2,500 pixels on the long side) the file sizes will never be extreme in the first place.