Perhaps I am wrong, but I was under the impression that DNG was just a standard wrapper format that could contain one of many different types of image data.
So is NEF! (In fact, DNG is rather like NEF but with a lot more metadata. I could have used other examples, but in fact DNG is closer to NEF than some of the others).
http://www.barrypearson.co.uk/articles/dng/raw.htm#contents
In other words, there is no single "DNG" format data. Rather, DNG is standard way of packing the camera's raw data. There are different DNG formats for various classes of cameras, and they add extensions to DNG to support new technologies.
DNG has 2 main sorts of data. Image data, and metadata. The latter describes the camera/sensor configuration. So the metadata identifies the width and height of the sensor in pixels. It identifies the order of the R G & B filters of the CFA. It identifies whether the pixels are square, and if not what the ratio is. etc. A difference with DNG is other formats also allow variations, but they don't always say so in the file, so software has to be updated when the camera is launched.
Here are examples. Note they include the non-square pixels of the D1X:
http://www.barrypearson.co.uk/articles/dng/innovation.htm#examples
The raw data off a Bayer format sensor is certainly different than the raw data from a Foveon sensor. Sensors which have different pixel arrangements (i.e. 45° angle) will have yet another variation of raw data.
Foveon is a special case. DNG can't handle it properly because it would mean publishing Foveon's private design details:
http://www.barrypearson.co.uk/articles/dng/specification.htm#x3f
Fujifilm SuperCCD sensors have extra metadata in DNG:
http://www.barrypearson.co.uk/articles/dng/specification.htm#fujifilm
Further, each model camera has specific characteristics which must be taken into account when interpreting the raw data (what are the exact colors that are sensed at each pixel, what is the sensor's response curve).
http://www.barrypearson.co.uk/articles/dng/innovation.htm#examples
My understanding is that a conversion to DNG involves finding the closest internal DNG data format that matches the camera, converting the data, and adding camera specific data to describe the camera.
I don't understand what you mean by "closest internal DNG data format that matches the camera". The words suggest that you think DNG is something different from what it is.
I see a number of possible problems with using DNG for long term archiving.
- 1) There is a possibility that third party DNG software may not understand all DNG files. Perhaps the software developer felt it wasn't worth supporting Foveon, or other non-bayer format cameras.
True. This improves with time. But, of course, it is often the case that those packages didn't accept the
native raw image files for those cameras either.
- 2) DNG converters tend not to have the same level of camera specific knowledge. Canon's raw converter has a table of how various Canon lenses vignette on various Canon bodies. When converting from Raw, Canon's software can automatically compensate for lens vignetting. I don't think the economics are there for anyone else to implement this in a DNG converter. By using DNG you lose this ability.
If Lightroom and ACR can handle it, typically DNG can. (v1.3 of DNG added a lot for lens corrections). But - you are right; manufacturers can store information that only their own software can handle, so if you want those features, you can't use DNG. (Adobe calls this "secret sauce").
- 3) As part of their marketing, Canon has to maintain support for all of their Raw files .... I would expect keeping your files in the original Raw format makes it MORE likely that you will be able to open them in the future.
But will this be with your
future choice of software? The problem arises if future products have to build in profiles for today's cameras. How will they do that? DNG holds lots more metadata specifically to help with that future problem.
- 4) DNG may never catch on to be as popular as Adobe hopes, and they may abandon the format for something newer.
Adobe have submitted DNG to ISO, who are using it in their revision of ISO 12234-2. Sooner or later it will probably become an ISO standard. (A bit like PDF used to be an Adobe format, but is now an ISO standard: ISO 32000-1).
Apologies!
