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Converted a RAW image to DNG but photoshop resizes them from 30mb to 8.5mb ?

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As long as the appropriate information is there in the file, why would it be an issue to reduce the file size? Most people see that as a positive thing... –  John Cavan Jun 24 '13 at 21:42
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2 Answers 2

On the import tab choose "Copy" instead of "Copy as DNG", then you will retain the large data amount. One of the benefits of using DNG is that it uses less space. But normally it is only a 20% improvement.

It would be helpful to know your camera model, so we can see waht raw format it is (compression or not, number of previews, etc..

The raws contains previews. Canon uses half resolution previews (resulting in 1/4 the size of it) while Nikon uses full resolution (but I dont know if they compress it harder). DNG may use something else (or not have a preview at all in your case). A preview can easily be 2-4Mb and sometime is it bigger than the raw data if it is high quality because it stores the image as RGB data, while the raw is basically a greyscale image (bayer pattern).

You can tweak DNG settings in LR: http://lightroomsecrets.com/2012/03/lightroom-4-tip-new-dng-options/

Note that most of the options will affect your file size:

  • Embed raw file: your end result will be larger than the original
  • Preview Size: You have some control of the output size.
  • Lossy Compression: This is just wrong! You might actually have this one active.
  • Embed fast load data (V7.1) - Larger output file
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Are you sure there is actually a quality loss? It does seem likely, however it is possible that extra information is being discarded. Depending on the RAW format, it is possible that there are embedded JPEGs and possibly multiple embedded images or settings that may not be part of the raw image data itself.

Beyond that basic theory, you really need to provide more information about the case, perhaps post the image somewhere so that it can be looked at directly. A screen shot of your settings for the conversion might also help.

It is also possible that you are saving the 8 bit version to a DNG file instead of the higher bit raw data depending on what steps you are taking to do the conversion. Have you tried Adobe's stand alone DNG converter?

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I think it's most likely that we're going from an uncompressed RAW format to compressed DNG. –  mattdm Jun 24 '13 at 21:01
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