Today’s Question: Some years ago I scanned a 35mm slide. Lightroom [Classic] shows me the file size to be 108MB, and when I send the image to Photoshop I get the same size in the Image Size dialog. I have re-scanned the slide, and when I send it to Photoshop to prepare a JPEG image for submission to an agency, the file size shows as 54MB. What is causing the difference?
Tim’s Quick Answer: In this case the difference is the bit depth. The first image was set to a bit depth of 16-bits per channel, with the second image set to 8-bits per channel.
More Detail: The Image Size dialog in Photoshop provides what appears to be a file size value to the right of the Pixel Dimensions heading, but this value will not necessarily match the actual size of the file on your hard drive. However, the bit depth setting for the image will impact this size value, just as it will impact the actual size of the file when saved.
The file size shown in the Image Size dialog in Photoshop can be thought of as the estimated size of the image file if it is saved as a TIFF file with no layers and with no compression applied. Saving with a different file format or settings will result in a file size that differs from what is shown in the Image Size dialog.
However, the file size estimate does take into account bit depth. In this case both source images were likely scanned at a bit depth of 16-bits per channel, although it is possible the second image was scanned at 8-bits per channel. It is also possible, however, that the image had been converted to a JPEG image before the size was viewed in the Image Size dialog. JPEG images do not support 16-bits per channel, so that conversion would necessarily involve converting the image to 8-bits per channel. That, in turn, would cut the file size in half.
Again, the file size estimate in the Image Size dialog in Photoshop should only be viewed as an estimate based on pixel dimensions and bit depth, without reflecting the file format and related settings for the final saved image file.