Alpha 900 DNG files

[Wes Gibbon]

When I have needed to remove CA (mainly from wide angle lenses) I have been converting my ARW files into DNG format and removing CA in Photoshop Raw Converter. However, I have just noticed that the DNG files are scarcely half the size of the ARW files. Is that because DNG files make more efficient use of space, or is Adobe's DNG converter losing some bits of data? Does it matter?

Is there a better way of removing CA, either economically, or if necessary with no expense spared? All suggestions gratefully received...

n.b. I have finally been able to give my A900 a good workout - a week or so in Assynt. I am particularly pleased with the Minolta 28mm and 35mm f2.0 lenses, though I cannot complain about my other lenses (all Minolta: 20mm, 50mm f1.4 RS & 'Big Beercan' - the original Minolta 75-300mm), though the latter is as expected subject to flare. Colours are lovely, though maybe that is also due to my increasing skill in using photoshop!

[David Kilpatrick]

The DNG raw conversion certainly can be smaller in size. Nothing is lost that you need worry about, though you may want to keep copies of the original ARW files as well. I have compared DNG to original in detail and I can detect no difference, except that the DNG file may with some camera models actually result in a better high ISO conversion (depending on your version of ACR).

David

[Wes Gibbon]

The DNG raw conversion certainly can be smaller in size. Nothing is lost that you need worry about, though you may want to keep copies of the original ARW files as well. I have compared DNG to original in detail and I can detect no difference, except that the DNG file may with some camera models actually result in a better high ISO conversion (depending on your version of ACR).

David

David,

Thank you for your amazingly quick and helpful reply.

The exact sequence might explain my puzzlement - I converted the ARW files (about 36Mb) to DNG (about17 Mb) and read them into ACR. Eventually, I noticed that the message along the bottom included '8bit'. I clicked on this and changed this to '16bit'. The next time I read a DNG file in it showed '16bit'. After this, I thought I'd experiment and saved this DNG file as another DNG file and it created a 48Mb file - over twice the size. Of course, i have no idea what I (or ACR) was doing, or even whether this was a sensible thing to do, but if you have found no loss of detail, that sounds OK to me. It occurs to me that I also changed ACR to use Adobe RGB instead of sRGB. Could this have made a difference?

All this experimentation came about because I had been working on some images and suddenly discovered that what I had assumed to be 16 bit images were only 8 bit. ACR doesn't exactly go out of its way to tell you how many bits it is generating for Photoshop to work on!

You can see a few prelimiary results on http://www.wesgibbon.com/page18.htm, but I hasten to add that most of the images were a quick conversion from jpegs, and in any case for the web they were reduced to 70Kb-100Kb, so no indication of quality (printed on A3 from converted raw files, I'm really pleased with them - seem to be as good as I used to get from my Mamiya 645, or better).

[David Kilpatrick]

You are saving back to DNG from ACR. That is not the way to do it, it creates a DNG from your converted image. To create DNGs, use DNG converter. Then, never ever save back over that DNG. ACR can convert the original original 12-bit DNG file to 8 bit (some loss) or 16 bit (redundant file size - there is not enough data in your original raw file to fill the 16 bit size). In the ACR saved DNG, the 8 or 12 or aRGB or sRGB is just a tag anyway (it is reversible). It can not resave or open as 12-bit. Your Sony is 12-bit raw data. The DNG converter version of the DNG (not processed via ACR) is also 12-bit.

If you do want to save new DNGs as 'adjusted raw files' from ACR, give them a new filename, don't overwrite your pristine original data. The DNG created by ACR actually does include the original file (it can also include your original ARW), but it also includes the crop and adjustments, and a new file. ACR does not need this, it can save all your crop and adjustments in an XMP sidecar file which lives in the folder with your original.

All you are doing is creating lots of extra data.

David

[Mike-Photos]

I've always been nervous about DNG. At one stage, when I had my 7D and A100, I went through a phase of converting to DNG and deleting the raw files, and now I regret it. The reason is that I was using Bibble, and I am now using DxO, and neither will read the DNG files. The only products I have that I can re-process these files is ACR or LightRoom, and some of the files I want to process are high ISO, where DxO is king.

If I have to keep my Raw files as well as the DNG files, why convert to DNG?

Maybe I misinterpreted the initial hype with DNG, but I thought I was converting to a fully open format that would always work no matter what. I came down to earth with a bump when RawShooter could not read DNG files that were created with the DNG converter after RawShooter faded away. Also, on the Bibble forum, I came to understand that despite the hype, even if you support DNG as a format, you still need to support each camera specifically to get the best out of the DNG version.

Until Sony produce native DNGs out of the camera, I won't touch DNG.