On Oct 31, 2005, at 9:35 AM, Shel Belinkoff wrote:
When converting PEF files to DNG, the converted file is roughly 50%
of the
original. That suggests that some, perhaps a lot, of information
in the
original file is discarded. What's lost?
Nothing, with DNG Converter v3.1 and up.
When working with DNG and PEF files, and converting them to PSD or
TIFF
files after going through the RAW converter, the file sizes (for
DNG and
PEF "originals") are about the same size, and I see no difference
in the
results obtained from either format. How is it that the DNG, which is
about 1/2 the size of a PEF, and the PEF, yield similar sized files
when
converted?
All this suggests that the PEF, and I suppose other RAW format files,
contain a lot of unnecessary information. Given that the results
from a
PEF and a DNG file are identical, is there really any reason to
save the
PEF file?
A Pentax RAW format file is essentially a Tagged Image File Format
(TIFF) file with a Pentax specific data layout. TIFF is simply a
container format standard which is generic and broad enough in
definition to store all kinds of image data. Rendered RGB files
stored in TIFF format follow a widely accepted protocol standard for
what the tags and data elements mean in terms of data organization;
PEF files use the format but use their own sets of tags to determine
the component contents.
What's in a PEF file?
- All the image and camera metadata (exposure settings, EXIF format
data, some image and camera identification data, and a few bits and
pieces of other data in proprietary format that Pentax feels they
want to stuff in there for present and future use).
- The sensor data itself.
- At least two JPEG renderings ... the thumbnail used for whole image
display on the LCD, a JPEG preview rendering for sure, and (I think)
a full resolution, high compression JPEG rendering used on the LCD
for when you zoom and scroll around.
DNG files also adhere to TIFF format protocol, but DNG is an openly
distributed format so all the components have easily discoverable
tags and organization. It was conceived of as a modifiable container
format so developers can use and modify the contents easily. What
does it contain? Well, pretty much the same thing that PEF files
contain (metadata, JPEG preview and thumbnail, sensor data). It can
also contain an encapsulated PEF file if you turn on that option.
Why are DNG files smaller?
- Pentax applies no compression to the sensor data. Because the
compression is bit-for-bit lossless, no information is lost.
- The DNG Converter applies lossless compression and can save up to
30-40% on size.
- Pentax JPEG preview files are somewhat large, the DNG Converter
produces a better quality JPEG at smaller
size.
All RAW format files contain some metadata that could be considered
"unnecessary", but the proportion of such unnecessary data to sensor
data content makes that insignificant. They might have 100-500K worth
of metadata, the sensor data is much much larger than that.
Losslessly compressing the sensor data is the key to their smaller size.
Why are PSD and TIFF RGB files the same size when made from PEF and
DNG files?
Should be obvious now, but basically since the DNG and PEF files
contain the same information, the RGB rendering from either will also
contain the same information and be the same size when constructed by
Photoshop.
Is there really a reason to save the PEF file?
There are arguments for archiving one, the other or both. Some
argument points to DNG being an open standard and therefore having a
better chance at 'archival' usability than PEF files (which is true).
Some arguments suggest that you can always derive a DNG from a PEF,
but cannot do the opposite, so you should save the PEF (logical). DNG
Converter includes the ability I mentioned to embed the PEF file INTO
the DNG (and similar capability to extract it), so that's another way
to archive both, but it makes the DNG files larger.
Personally, since the DNG Converter is now in its third revision and
I never use the Pentax tools for RAW conversion (which are the only
tools that can use the Pentax private, proprietary data in the
makernotes metadata), I'm beginning to move over to the notion that I
should archive only the DNGs and to heck with the PEFs after I get
them out of the camera. But I still linger on the notion that the PEF
output is the actual original file that I can always go back to, so I
am archiving PEFs as yet and working with DNGs.
Godfrey
