Unexpected Shift in RGB values when exporting Uncalibrated tiff to sRGB tiff

[User30107510r301]

I have a large number of scanned images in 8 bit RGB tiff format with an 'uncalibrated' color profile. My Mac assumes it is sRGB if viewed in Preview although the EXIF data shows 'uncalibrated' for the [EXIF:ExifIFD] ColorSpace tag. I asked the company that did the scans what color profile they used in the scans and they said sRGB.

Since the tiff images were uncompressed they were extremely large so I decided to compress them with Adobe Deflate (zip) in Lightroom Classic 14.0.0 (Mac running Sequoia 15.1). This required to export them as 8-bit tiff and to assign a color profile, so I chose sRGB (sRGB IEC61966-2.1 to be exact). There were no edits to any of the images so I expected the exported tiffs to be identical, except for being compressed.

Visually, the exported tiffs with Adobe deflate (zip) were identical. However I happened to do some image comparisons of the original uncompressed tiff and the exported zip tiffs with BeyondCompare  and was surprised to see small changes in RGB values.

Specifically, RGB values would shift by no more than one value, for example 40, 53, 38 to 39, 53, 38. Occasionally, each channel might shift but never more than 1 value, for example, 12, 18, 20 to 11, 19, 19.

It was never all the pixels, and usually not all adjacent pixels.

I did trouble shooting with Photoshop 26.0.0 (Mac) and one of my tiff files with with an uncalibrated color profile.

1. Open file, don't apply any profile. Save without applying profile as copy tiff (zip) - No rgb shift compared to original

2. Now on same open file, apply sRGB file and save a copy as tiff (zip) -> No rgb shift compared to original

3. Now on same open file, convert to sRGB profile and save copy as tiff (zip) > No rgb shift compared to original

Then I took the three saved copies from Photoshop above, imported them into Lightroom Classic 14.0.0 (Mac), and exported to tiff (zip). In all three cases, I saw the RGB shift I had noted in the original post. Interesting, the RGB shift in all three files was the same.

So it seems like Lightroom is somehow processing this file erroneously as Photoshop is able to do it without any RGB shift. In facct no matter what I tried I couln't recreate the error in Photoshop so I'm gueessing it's a bug in Lightroom Classic.

[MadManChan2000]

Images that are imported into Lightroom and then exported (even in the same format and same color space with lossless or no compression) always go through a 8-bit to 16-bit (or 32-bit) back to 8-bit round trip process.  The last step, from 16-bit back to 8-bit, involves adding dither (very small random perturbations) to avoid visible quantization issues such as banding.  For most cases, this is the desired process because it improves quality.  However, for the use case that you describe, it will result in some changes in the numerical values, because of this step.

[User30107510r301]

Thank you, that makes a lot of sense. Does Photoshop do something different then since it doesn't replicarte the same error as Lightroom Classic? Even though it's a small error, it makes me think I will use Photoshop instead to batch convert images in the future.