ACR to PS, or why you should use 16 bit if using ProPhoto

Jan 29, 2005
This is an e-mail from Andrew Rodney I think may help:

"...its quite clear to me that the 16-bit file is showing vastly superior quality with respect to noise and artifacts compared to the 8-bit file.

The image was shot with a Canon 350D (ISO 100). I used Adobe Camera RAW 3.X with all defaults and auto settings OFF. No sharpening in ACR either. The file was brought into Photoshop in 16-bit in ProPhoto RGB from ACR. Then I duplicated the file and converted the dupe to 8-bit. I applied levels corrections (nothing super radical), USM and a boost in saturation (+20 in Hue/Sat). The IDENTICAL corrections were made on the high bit file (hold down option key and the other key command to call levels, USM etc to get exact values) or drag and drop history from one to the other.

Cache is off In histogram. The 8-bit Histo isn't awful like I see in Bruce's book. But there is a very noticeable amount of noise in the 8-bit file not seen in the high bit file. I also converted from ProPhoto into LAB and did the same corrections (well not exactly since levels in LAB can't be duplicated exactly as you would from an RGB file). Again, the 8-bit file shows severe noise introduced by the corrections that simply don't show up in the high bit file. At 200% zoom, it shows up like a sore thumb.

I've taken a section of the image since in high bit, it's quite large and cropped it down as the 16-bit ProPhoto RGB file. In the zip archive are screen dumps of the corrections made. I also generated a Photoshop action; one duplicates the 16-bit file, converts to 8-bit and applies the three corrections. The 2nd would be used on the original (doesn't duplicate) making a bit easier to apply both sets of corrections. After that, zoom into the green (slightly out of focus) bird feeder at 200% and look at the differences. The biggest issues in the 8-bt file appear to show up in shadows which makes sense. This is another reason why even superior quality would be produced on linear encoded data within ACR. It also illustrates the need to 'expose to the right' for RAW data since the first 2048 steps of data are all within the first stop of highlights.

This is a real world image and the corrections are not severe and identical on each. The Zip archive is about 1.8mb.

I'm seeing this effect on other files shot and processed in this manner. I can of course supply the RAW data but it?s pretty large. If anyone wants it, let me know and I'll put it on my public idisk.

The file is here:

Andrew Rodney
Author "Color Management for Photographers" "
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