File Size Mystery


Today’s Question: When I take a 40MB DNG file from Lightroom to Photoshop, the resulting TIFF file is 207MB. Why does the file size increase by a factor of five and is there some way to reduce the TIFF file size?

Tim’s Quick Answer: There are three basic factors at play here. First, if the Adobe DNG file was created from a proprietary RAW capture format (or was a DNG capture out of the camera), the image data within the DNG file only contains one-third of the total information in terms of pixel values (more on this in a moment). Second, the DNG format uses compression that typically results in a file size that is around 20% smaller than the original RAW capture. Third, the image sent to Photoshop was likely created as a 16-bit per channel image, which doubles the base file size.

More Detail: As you may already be aware, a RAW capture only includes one-third of the total pixel information for the image. There are exceptions and variations here, but with most cameras each individual “pixel” on the image sensor is only recording a single color value (generally red, green, or blue). Part of the process of converting the original RAW capture to a “real” image involves calculating the full color value for each pixel. The result is that the RAW capture is generally about one-third of the file size of the final image, all other things being equal.

The DNG format makes use of lossless compression that results in files that are generally smaller than the original RAW capture, assuming the DNG file was created from a proprietary RAW capture. On average I find that this compression results in a DNG file that is about 20% smaller than the original RAW capture, but your results may vary.

The original RAW capture would contain information based on either 12-bit, 14-bit, or 16-bit per channel information, depending on the specific camera. Once you convert the original capture to a pixel-based format, you have the option of rendering the image in either the 8-bit or 16-bit per channel mode. The 16-bit per channel mode image will be twice as large as the 8-bit version, all other things being equal.

Finally, there is the potential for applying compression to the TIFF image, which results in a smaller file size. For example, you can make use of LZW or ZIP compression for TIFF images created by Lightroom as part of the process of sending a photo to Photoshop. Both of these options provide lossless compression that reduces the size of the image file without any reduction in image quality.

In this case, based on the information you’ve provided, I would guess that the TIFF image is being created in the 16-bit per channel mode, with LZW (or ZIP) compression enabled. In that case, the only real option for reducing the file size (other than reducing the pixel dimensions) would be to create the TIFF file as an 8-bit per channel image rather than a 16-bit per channel image. However, this would also result in less overall information in the photo, and a higher risk of some degree of posterization in the image.

On the assumption that you want to retain the full pixel dimensions of the image, and that you want to keep the image in the 16-bit per channel mode, the only other option for reducing the file size is to be sure to make use of LZW (or ZIP) compression for the TIFF file. These options can be found in the External Editing section of the Preferences dialog in Lightroom.