Difference between RAW converters

There is one big advantage of pictures taken in jpeg format compared to RAW: you have a fairly good idea what they will look like.

With RAW pictures, be it CR2 or NEF or any other version, it is just the raw data from the camera, and each RAW converter may interpret it in their own way. One converter may handle noise better than another. One converter may interpret the white balance or luminosity in one way, and another converter may have another idea of how things should work.

Here is an example where compensation for distortion is radically different. The pictures are taken with a Canon S90 camera.

The first picture is a CR2 file processed with Canon's own Digital Photo Professional. The lines and the tiles are pretty straight.



The next picture is the same CR2 file, processed by Picasa 3.6.1. Note the very bent lines. In the corners of the photos, there is actually more information than there is in the straight picture.



What likely is happening here is that DPP has information about how much the lens bends the picture. DPP corrects for it, and the tiles look like they should do. Picasa, however, probably takes pixel for pixel from the sensor of the camera, and the result is the bent aspect.

It is very possible that Picasa will update its converter for CR2 files from S90 in the future, so they will look closer to reality and further from the pixel data.

Note that converting to DNG is not a solution. If you convert to DNG, that does not remove the distortion, and Picasa displays the DNG as distorted as the CR2 file.

Incompatible "DNG" files

On the home page for Scott Kelby's book Lightroom 2 Book for Digital Photographers, one can download several hundred megabytes of sample files. Strangely enough, many of the sample "DNG" images cannot be opened in Adobe CS4 Bridge or Photoshop - at least not unless you have Lightroom installed. It turns out that many of them, like SaveYourPresets1.dng or SaveYourPresets2.dng are not dng files at all, but psd files with the wrong extension. Funnily enough, many of the correct dng files contain features that make them incompatible with Mac OS X 10.5.6, which in theory opens dng files.

I take this as yet another proof that dng is not a more compatible or safer format than many other raw formats, even though it probably is due to some bug in Lightroom.

It may seem strange that Scott managed to miss this, but Lightroom reads the files without problems, and if you double click on them within Lightroom a copy of them opens in Photoshop, so it looks like they are dng files, even though they are not.

How to fix it? Simply change the extension from dng to psd, and you will be able to open the files in programs that support the extension.

How I figured out that they were PhotoShop files? I opened the files in the Terminal text editor less, and noticed that they contain the code 8BIM, which is a Macintosh Photoshop extension.

Why does not Aperture read my DNG file?

If you have a DNG file that Aperture refuses to open, is it possible that the DNG file is from a camera which has a strange format. Aperture 2.1 has support for DNGs from many cameras, but not all.

In some cases Aperture opens some DNG from a particular, camera, but not all. It then may depend on the lens, like in the case of an Olympus E-P2 ORF file.


Another possible explanation is that the DNG was generated with the "Convert to Linear Image" option turned on in Adobe DNG Converter. It has to be turned off. This is not obvious. The Linear Image option is there to increase compatibility compared to "Preserve Raw Image", but it is increased in a way that Aperture cannot handle.


Wrong setting in Adobe DNG converter.


Then, there is of course also the possibility that you cannot import the file, because you do not have any project, or because you highlight the wrong part of the screen, when you do the import. But in this case no files can be imported, DNG or not.

The Lure of DNG

DNG, Digital NeGative, is Adobe's attempt at a unified RAW file format for all cameras world wide. Some people are all worked up about this and list numerous advantages.

I am not one of them.

There is nothing wrong with dng, it just so happens that it is not much better than other things either. Some people duly convert all their Nikon nef files to dng, but that may be a waste of time and a loss of some data.

However, if your camera produces dng files, by all means, keep them. They are not worse than other raw formats.

Let's look at some of the alleged benefits.

Archiving is one of the often quoted arguments for dng. As dng is supposed to be a universal standard, it seems reasonable to assume that support for it will be around for ever, right?

The answer is perhaps. A standard does not become a standard by someone calling it a standard. The internationally defined format for writing dates is year-month-day, like 080619 for today. However, apart from Finland and Sweden hardly any country uses that date format in daily life.

Adobe supports dng, and a few camera makers support it, but most camera makers do not see much advantage. The by far biggest SLR camera makers, Nikon and Canon, do not support it. That means that most raw pictures that are taken today use other formats than dng.

It is very possible that there will be dng readers in 50 years time, but it is equally likely that there will be readers for Nikon's nef files and Canon's cr2 files.


And it seems reasonable to assume that a program that can read one dng can read all, right?

But that is not the case. Just like for tiff or psd files, there are many flavours and extensions. If a program cannot read the native raw file from a particular camera, it is quite possible that the program cannot read dng files from that camera either. It is also possible that it can read some files but not all, depending on the lens used.

In some ways, a dng is simply a wrapper around some other file format. A dng can even embed the full original of another raw file. If the program cannot read the original, it cannot read the dng.


One claimed benefit for dng is the file size. The story is that raw files for early cameras were only lightly compressed. The nef file of a Nikon D1X from 2001, for example, weighed in 7.8 Megabyte, while an equivalent dng file only was 4.8 Megabyte. A Canon 300D from 2003 had a crw file of 6.0 Megabyte, while the equivalent dng was just 4.2 Megabyte.

However, there is nothing inherently large with Nikon or Canon files. The companies can modify the formats, and they compress them more and more. A lossless compressed 14 bit nef of 14.4 MB from a new Nikon D300, for example, is converted into a 13.3 MB lossless compressed dng file, which hardly is any significant gain. A non-compressed dng with embedded jpeg preview turns out to be 25.4 MB - much bigger than the original.

There are still cameras that have huge raw files, and in these cases a conversion to dng has a certain sense. However, often it may be as useful to convert the raw to tiff or even jpeg - formats that are easier to manipulate for other purposes.

If you convert a raw file to dng, you may not lose any information - at least not unless you use the "Linear Image" option. However, there may be camera specific information in the raw file, like the Picture Control data in a NEF file, that no program can interpret correctly when it hides in a dng. The information is definitely there, but it may in practice be impossible to actually use it.


But then, as said before, there is nothing wrong with a dng. If you see any use for dng, by all means use it. I for example convert my nef files from my D300 to dng in order to open them in Photoshop CS2. It is either that or upgrade to Photoshop CS3 or use Aperture. Most often I actually use Aperture, as I prefer to work on the original file.