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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.

https://drive.google.com/file/d/1pLd9W4whVX9hUBosig-d-ZzJqXWWBiKu/view?usp=sharing

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I have downloaded your TIFF images and examined them. The reason why the images scanned in RGB and converted to Grayscale in Photoshop are larger than the images scanned in Grayscale is simply that it's not the same images!

It seems that when scanning in RGB you get much more detail in the light areas than when scanning in Grayscale.

This is how the two scannings of the blank paper looks like after having applied Image > Adjustments > Equalize (just to make the light pixels visible):

And this is the two scannings of a printed document, also equalized:

When compressing an image the level of noise in the image really makes a big difference. It's not surprising that the images originally scanned as grayscale can be compressed a lot more than the RGB version since they have large uniform areas.

Apart from the scans actually being different from the beginning, a great deal of the noise in the images converted from RGB to Grayscale in Photoshop is introduced during conversion, and it comes from having ticked Use Dither.

If you untick this, a lot of the noise is gone and the file size goes down.

When I copy/paste the pixels of the two versions into new documents and save them as LZW compressed TIFF files, I get almost the same file sizes, so I think my suspicion is right.

(One thing that is making the images converted to Grayscale in Photoshop a tiny bit larger (a few kB), is that they have an embedded color profile. You can unembed them by using Edit > Assign Profile > Don't Color Manage This Document.)

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TIFF or TIF format is not used to get small file sizes. It is a raw format for big files. In order to get small file sizes you can save as a 4 color GIF with size of 255kb or low quality JPG ~ 500kb. In other words, you need a format with compression.

Why PS increases the file size ... I have no idea. It probably saves color data even when the mode is grayscale. It is like a bug. I encountered something even worse. I left one channel from your normal scan and converted to Grayscale mode. Saved file is now 6.6MB but then I used Indexed color (32 colors) and the file size is 412KB.

My first attempt was not using Grayscale (I never use this mode). I remove color by pressing Ctrl+U and removing the saturation. This keeps the RGB channels and if you save your scan that way, it is 2.43MB

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  • TIFF actually does support, as a baseline, several forms of compression: originally 2 kinds of RLE encoding; and through (now commonplace, well-supported) extensions (1992), LZW and JPEG along with 2 others you never heard of.
    – Yorik
    Jul 6 at 16:40

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