I am scanning a grayscale document using my EPSON ES-500W. When scanned in as Color I received a 4.7MB file. If I convert it to Grayscale in Photoshop and save the file it shrinks minimally to 4.4MB. The same file scanned as Grayscale directly from the scanner is smaller at 1.7MB.
I opened the original
1YIFEI_EM1700_0001_COLOR.tif in Photoshop and converted to Grayscale using
Image > Grayscale. The file is then saved without changing the default compression method of LZW using
File > Save. I am using Photoshop CS4.
I expected the file to reduce in size to something similar to scanning the paper as Grayscale but is about 2.75x larger.
For comparison, I used ImageMagick to convert the same file to Grayscale and then compress it as LZW. The resulting file is about 1.7MB, similar to the original Grayscale scan:
> convert color.tif -grayscale average gray.tif
> convert gray.tif -compress lzw gray_lzw.tif
In another test, I scanned a blank sheet of paper which resulted in 1.4MB as Color and 500KB as Grayscale. Converting this file to Grayscale in Photoshop using the same method actually increased the file size to 3.6MB. Converting the file to Grayscale using ImageMagick resulted in a file size of only 250KB.
Why is this Color file converted to Grayscale in Photoshop so much larger than expected? Is there an additional step necessary to discard unused color channels or meta data?
Although it would be interesting to know whether this is still an issue in later versions of Photoshop, my question is specifically asking why the resulting file is so much larger when there should be less information, and as a result a smaller file, as a direct result of converting to Grayscale.
I've provided a link here to all of the originals and conversions mentioned in this question as a file on my Google Drive.