- there is no visible differences between the untouched camera raw file and the LR generated DNG
To be clear, the raw file, and a not-yet-demosaic'd DNG (which is what LR produces) cannot be viewed without conversion. The conversion is internal and automatic using Adobe Camera Raw (built into Lightroom), but it does occur. So that there are no visible difference means both ran through the same raw conversion inside LR before you saw them.
If you take both into Photolab without LR involved (other than to produce the DNG), Photolab should show both identically also to each other, but slightly differently to LR.
- the returned DNG is visibly different from the original
- a returned TIFF file is different again, perhaps "in the middle"
I'm not sure what "returned DNG" vs "returned TIFF" means here. If you mean you are calling Photolab from within Lightroom, and have a choice how Photolab returns the result, and that return makes a difference, it is odd if that is true. Returning an edited photo should be the same regardless of whether it is returned as a DNG or TIFF (and mostly the same as a JPG). At that point the DNG and the TIFF are just containers to hold the data, neither is raw. If they look different afterwards, then the two are going through some different type of either editing or conversion either before leaving Photolab, or in returning to LR. It is possible that LR is importing them differently on return and applying some preset or edit to the result.
It is important as you use two different editors to keep track of which one is applying change (and what type of change), so you do not accidentally have both altering your image.
Note, just to keep this even more confusing, going from one editor to another and back can also have issues with color space. Color space is not directly related to raw conversion, and color space if properly handled has no significant effect on what you see -- but if color space gets mixed up between products you can get anywhere from subtle to dramatic changes in appearance; usually bad ones. Both products likely have controls over color space (on LR they are in the transfer process, either in the plugin setup or under Preferences, External Editing.
While the ability to edit in a "plugin" editor in Lightroom is really cool, it does require both that the product work smoothly with lightroom, and that you really understand the transfer process back and forth to ensure you are not compounding edits in some fashion.
Not that it makes much difference to me but which of these files is entitled to be called a raw file?
Generally speaking a raw file is one that has original sensor data, before the de-mosaic process. Properly speaking this is not a "color" file; it has red/green/blue/green sensors (yes, green twice usually), but these are real color sensor readings, and are linear (200 photos is twice the value of 100 photos).
A color image (commonly speaking) is one that has been changed to pixels, and each pixel has a separate red, green and blue value for that same dot, that yields a blended color for that dot. Each of these pixels is calculated (and not in a linear fashion, but more of a log process) from the sensor readings that surrounded it; each sensor contributes to many pixels. Exactly how this calculation occurs is the "raw conversion algorithm" and is quite different between different manufacturers, and what yields subtly different colors when you do the conversion.
Once converted there is no going back; everything afterwards is editing. People often then say that certain aspects are "baked in".
Lightroom by default does not raw-convert-and-save (even in the import conversion to DNG, that's just format not raw conversion). So all the time, every time you look at an image in LR, it is re-doing the raw conversion, redoing the de-mosaic process. This is partly why it is called a "non-destructive" editor.
When lightroom exports to a TIFF, it is baking in the de-mosaic process; the resulting color image is no longer raw.
Some programs like photoshop, cannot edit raw at all. It must be converted (de-mosaiced) before it can edit. Often this is invisible; photoshop uses the Adobe Raw Converter from Lightroom if you ask it to edit a raw image (and you may or may not actually see that it does this depending on how it is used).
So you could (for example) convert in LR, and edit in Photolab. You could convert in DxO and edit in Lightroom. There are dozens of converters, and dozens of editors. Most people convert and edit in the same tool, but not all.
Just to make things more complicated, the conversion process and the editing process are blended together in many tools, so when you convert in Lightroom some of the "edits" are actually applied during the conversion, not separately -- white balance is a good example. Doing this provides more "leverage" (for want of a better term).
Why does this matter? Consider using Photoshop. If you adjust white balance in lightroom, then send an image to Photoshop to edit, you had a lot more ability to edit the white balance. If on the other hand you leave white balance alone, and then send the image to photoshop (which does a raw conversion on the way), you can change white balance in photoshop but your ability to do so is considerably limited. Small changes work fine, but large ones work much better if done prior to raw conversion.
Sorry... I'm getting long winded again. One last: DNG and TIFF are both really containers, meaning they hold data. They can both hold raw data and both hold converted (demosaic'd) data. NORMALLY though, by the nature of how workflow is designed in these tools, DNG's hold raw data, and TIFF's converted. But that's more like finding milk in a milk carton and water in a water bottle -- it's convention, but nothing keeps you from swapping contents. There are a lot of processes (LR merges, enhance details for example) that return edited non-raw data in a DNG. And there are a few programs out there that convert raw data to TIFF containers for further processing, but they are much more rare and specialized.
And just to confuse things, there are a lot of people who will say that once repackaged in a DNG, raw data is no longer really raw. I get where they come from, but technically that's not correct, any more than putting milk in a water bottle makes it no longer milk. Indeed, there's another similarity -- if you push that analogy a bit further, the labels on the milk carton describe the milk; moving to a different bottle you may lose some description of it, but the real milk liquid is the same. Same with raw data -- converting to a raw DNG you might lose some descriptive data, but it's still raw.
Wow... Victoria is going to start charging me for writing so many words.
Sorry... hope that helps...