Vuescan File Formats: "RAW DNG" vs. "TIFF DNG"

[mafoofan]

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Okay, so I think I understand the difference, but it seems like there is a lot of confusion on the internet about it and I thought it would be worth getting some confirmation here.

 

My understanding is that there are three ways to save a "raw" output file using Vuescan: (1) an unadjusted TIFF (non-DNG), (2) an unadjusted TIFF in a DNG wrapper ("TIFF DNG), and (3) as a "RAW" file in a DNG wrapper ("RAW DNG"). What Vuescan calls a "RAW" file is not the same as what comes out of a digital camera and should not be treated that way. It is merely an untagged record of RGB values as first recorded by Vuescan before permanent adjustments have been made.

 

I would have thought the "RAW DNG" is the purest, most unaltered output, and therefore the best for those who want to save all adjustments for later processing in Lightroom, Aperture, Photoshop, etc. However, apparently, this is not the case. Because the file is not tagged, ACR and other RAW converters may perform default adjustments "under the hood" the moment it is opened, unbeknownst to the user. From my understanding, these include changing the color space and increasing the gamma. I can confirm through my own comparison test that "RAW DNG" files out of Vuescan open up brighter in Lightroom than the same files that have first been converted to "TIFF DNG."

 

When Lightroom or ACR in Photoshop opens up a "TIFF DNG" from Vuescan, it recognizes that the image embedded within the DNG wrapper of the file is in fact an already-processed TIFF. Thus, no automatic changes are incurred.

 

So, it turns out, if you want the most "raw" output file from Vuescan for further editing in other software, you should--very unintuitively--avoid "RAW DNG" and choose the "TIFF" or "TIFF DNG" formats. Saving in either of those latter two formats locks-in all the processing done by Vuescan.

 

Then why the existence of "RAW" (again, unintuitively, saved as a TIFF unless "RAW DNG" is selected) and "RAW DNG" in Vuescan? If I am not mistaken, they are still the most "raw" files relative to Vuescan itself. Meaning, you can still re-"scan" such files in Vuescan to affect color balance, infrared-based corrections (if 64 bit RGBi has been selected), cropping, etc.

 

Does this all make sense, and am I right?

Edited March 28, 2012 by mafoofan

[tobey bilek]

It can not be a raw in digital camera sense because the negative film has already TIFFed or JPEGed it. The film recorded it based on color balance built into the film and discarded the unneeded information, ie red in the case of daylight film. Gone for good and you can not get it back. You can not rebalance daylight to tungsten. That is what camera filters are for.

 

You need to do two things successfully when you scan. Get the color balance as close as possible and get the histogram correct. Photoshop, acr, or LR can do everything else.

 

ACR or LR does nothing you do not ask it to do. It may assign default settings based on ?, but feel free to change the defaults or make a preset to your liking

 

I bring my scanner files out a plain tiff. ACR does not change it I never could get Vuescan to work when I installed the KM5400 on a new snow leopard machine. I found a KM program for OS 6 and went back to KM software which works perfectly.

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[mafoofan]

Yes, your TIFF files shouldn't look different in either application. But had you tried to view a "RAW DNG" file from Vuescan in Lightroom, you would immediately notice the gamma has been adjusted. I don't know if there is a way to prevent this change, but from my reading on the internet, it seems automatic and unavoidable. Also, I cannot find the ability in Lightroom to toggle it off. Hence, if you want to maintain absolute fidelity, saving your scanned data in TIFF (DNG wrapped or not) is the best way to go.

 

Don't believe me? Run your own test. Scan a frame without adjustments, check "RAW" and "RAW DNG." Re-scan the resulting "RAW DNG" and process into unadjusted "TIFF DNG" or "TIFF." Then compare in ACR or Lightroom.

[philipus]

I'm not sure which format retains the most information. I have been (occasionally struggling with) scanning for ten years and tried various output formats. Recently I came across ColorPerfect which for me this is the best solution to get accurate colour scans. It requires RAW File to be ticked and that RAW DNG is unticked in order to obtain a linear RAW scan (see here).

 

I am very pleased with the results.

 

Edit: ColorPerfect has been discussed in this forum as well, for instance here and here.

Edited March 28, 2012 by philipus

[250swb]

I just save the scan as a TIFF.

 

Steve

[CalArts 99]

 

Then why the existence of "RAW" (again, unintuitively, saved as a TIFF unless "RAW DNG" is selected) and "RAW DNG" in Vuescan? If I am not mistaken, they are still the most "raw" files relative to Vuescan itself. Meaning, you can still re-"scan" such files in Vuescan to affect color balance, infrared-based corrections (if 64 bit RGBi has been selected), cropping, etc.

 

Does this all make sense, and am I right?

 

That's correct. And yes, the point of RAW in scanning is to easily re-scan directly off of the hard disk and with the infrared channel stored (that stored channel is the difference between camera RAW and scan RAW, btw.)

 

As you noticed, the RAW file option lets you choose between tiff and DNG. DNG files are essentially the same, but they have some extra DNG marking tags. But when Vuescan introduced a RAW scanning feature, the RAW tiff file was the first file format to be supported. It's still fully functional, but the DNG option is now offered, too. RAW tiff is just the previous file type.

 

It appears that LR's ACR engine is reading those RAW DNG scan file tags and making some sort of adjustment. I can't say what that is, as I don't use LR very much. I also save scans as tiff files, and use RAW tiffs only if I might need rescans for some reason (but that would only happen if you 'screwed up' your initial scan and also have plenty of storage space, etc..) I never save scans as DNG tagged, because of those embedded DNG tags (there is a color per pixel variant of DNG and maybe ACR is screwing around with that.)

 

And as Philipus points out, ColorPerfect also doesn't like those tags in RAW DNG.

[mafoofan]

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Thanks for all the discussion everyone. I've been experimenting more with the "RAW" files out of Vuescan, and don't know what to make of things.

 

If you open up RAW TIFFs in Lightroom, you get what you'd expect--a very dark, unboosted gamma image. If you open up RAW DNGs in Lightroom, you also get what you'd expect given the information discussed above: an image that has been gamma-boosted, but which is otherwise the same as the RAW TIFF.

 

If you "re-scan" the RAW DNG in Vuescan and output a RAW TIFF, the re-scanned RAW TIFF looks just like the other, original RAW TIFF in Lightroom: dark with an unadjusted, scrunched up histogram.

 

Okay, so far so good.

 

However, here is where things get tricky--and I'm not sure whether the problem is with Lightroom or Vuescan. If I convert the RAW TIFF to a DNG file in Lightroom (checking the option to embed the original raw file), two issues arise: (1) the file size drops from ~145mb to ~95mb, and (2) Vuescan is no longer able to extract the original RAW TIFF from the new DNG.

 

Similarly, if I save Lightroom adjustments to a RAW DNG file from Vuescan, the file size decreases and Vuescan can no longer extract the original RAW TIFF.

 

In short, DNG is not working the way I understand it should, as the original, unadjusted image file is not being preserved after Lightroom has made adjustments.

 

Thoughts? Am I being an idiot?

[CalArts 99]

You're not being an idiot but I believe you're probably torturing yourself unnecessarily. As I mentioned before, a DNG container has embedded tags in it (including the xmp which is inside rather than as a sidecar.) Whether it's Hamrick's issue of rendering the DNGs or an Adobe issue is unclear, but the point is why not just save your scans as RAW tiff files instead (if you feel you need the re-scan abilities, etc..)

 

Here is a discussion about this very issue of Hamrick's DNG implementation and how Adobe might interpret those DNG wrapped files. Although the person has a different issue when opening them in LR, it's still about the core issue. In the same thread, Jeff Schewe and Andrew Rodney also argue for using tiff because of this. Also I think I remember that the author of a Vuescan manual (Sascha Steinhoff?) once said the same thing. Maybe things have changed in LR by now, I don't know. But tiff isn't going anywhere and using a DNG implementation that's potentially creating issues (because of Vuescan or Adobe) isn't worth it, imho.

 

http://www.luminous-landscape.com/forum/index.php?topic=52578.0

 

Anyway, I think it's worth reading the thread all the way through. DNG also does some compression of its own, which is probably why you are getting different file sizes.

 

fwiw, attached is Adobe's DNG specifications.

dng_spec.pdf