Adobe Releasing New Photo Format 422
salmonz writes "Toronto Star just posted a story that Adobe is releasing a new digital picture format; the Digital Negative Specification,or DNG.
" Supposed to be use in raw photo formats; without the lossyness of JPEG.
Re:Why? (Score:5, Informative)
Re:Why? (Score:3, Informative)
Great idea. Some info. (Score:5, Informative)
Re:Why? (Score:5, Informative)
A raw format file, while still storing all the data that has come off the image sensor, can be one third the size of a PNG because it knows that the first pixel has only red channel information, the second only green and so on.
Re:Why? (Score:5, Informative)
DNG is about unifying the mess of "RAW" formats - camera-specific proprietary file formats containing raw dumps of unprocessed sensor information and shot metadata.
Furthermore, DNG is not immediately about getting camera manufacturers to use it themselves - though that would be the ideal. DNG is a bridge format - something you can convert all of your RAW files to for the purposes of long-term preservation/storage. It is open and documented, and based on TIFF so there are existing reader libraries that can handle the basic format (they will need extensions to do anything with it of course).
Adobe has provided DNG Converter which will enable anyone - even non-Adobe users - to benefit from the ongoing R&D Adobe does to support the variety of RAW formats out there. This will simplify the task of building quality RAW converters by allowing small developers to focus on excellent RAW processing and not have to exert to support the many camera RAW formats out there.
Sorry, I just woke up so I'm not going to touch on everything - but this is a major announcement whose importance will become more clear in time.
Full press release link from DPReview (Score:5, Informative)
Adobe Systems has today announced a new unified public format for raw digital camera files and a free software tool, Adobe DNG Converter, for translating raw photo formats into the new .DNG format, which is
compliant with the Digital Negative Specification. There is no standard
format for raw files, which vary between manufacturers and cameras.
Digital Negative Specification will introduce a single format that can
store information from a diverse range of cameras. An updated Adobe RAW
File Converter adds support for DNG as well as several other cameras.
Click here for more information on Adobe DNG [adobe.com]
Press Release:
Adobe Unifies Raw Photo Formats with Introduction of Digital Negative Specification Free Converter Tool Kick Starts New Digital Negative File Format by Translating Raw Formats into Easy-to-Use, Archive-Ready Files
SAN JOSE, Calif. -- Sept. 27, 2004 -- Adobe Systems Incorporated (Nasdaq:ADBE) today introduced the Digital Negative Specification, a new unified public format for raw digital camera files. The company also launched a free software tool, Adobe DNG Converter, which translates many of today's popular raw photo formats into the new .DNG file format, compliant with the Digital Negative
Specification.
Raw files, which contain the original information captured by a camera sensor prior to any in-camera processing, have become popular due to their promise of greater flexibility and image quality. Until today there has been no standard format for these files, which vary between manufacturers and individual cameras. The Digital Negative Specification solves this problem by introducing a single format that can store information from a diverse range of cameras. Technology leaders, major customers, and professional photographers today also endorsed the new specification (see separate quote sheet).
"Professional photographers and other creative professionals are moving to raw camera workflows because of the outstanding creative control they get over digital images," said Bryan Lamkin, senior vice president of Digital Imaging and Digital Video products at Adobe. "However, clients and publishers have difficulty working with disparate raw file formats and nobody can be sure that today's raw formats will be supported ten years from now. Adobe customers asked us to work on a unified, public format for raw files and that's what we've delivered with the new Digital Negative Specification."
Serious photographers want to store raw files in long-term image archives, because -- unlike standard JPEG's and TIFF's -- these files represent the pure, unaltered capture. Current raw formats are unsuitable for archiving because they are generally undocumented and tied to specific camera models, introducing the risk that the format will not be supported over time. The unified and publicly documented Digital Negative Specification ensures that digital photographs can be preserved in original form for future generations. The new .DNG file format also simplifies digital imaging workflows for
creative professionals who today have to juggle multiple file formats
as they bring raw images, from different cameras, into print and
cross-media publishing projects.
New Specification Built on Existing Standards
The Digital Negative Specification is based on the TIFF EP format, an accepted standard, and already the basis of many proprietary raw formats. The power of .DNG format lies in a set of metadata that must
be included in the file to describe key details about the camera and
settings. .DNG-compliant software and hardware can adapt on the fly to
handle new cameras as they are in
Re:Why? (Score:4, Informative)
And, again, PNG is totally the wrong format for this. You'd be taking a huge hit upfront in terms of indexing -- or your images would be outrageously huge.
How DNG differs from TIFF, I don't know. I would have thought TIFF would be the obvious answer. (TIFF, for those who don't know, aren't compressed but can be losslessly compressed)
Re:Newsflash (Score:5, Informative)
Actually no it's not useless, this would be very useful, especially if the format is open. Remember Adobe also cerated PDF - they know about making money from open standards.
You see proper digital cameras - especially the ones that cost $10,000 and are used by photojournalists and the like all let you save the image in raw format - that's a copy of the actual data that was captured before any processing. By doing so, you can take the image home and adjust it - white balance, satuaration and everything else - with photoshop et al. Rather than letting the camera make the adjustment and possibly messing things up, you know you still have the raw data so you can undo your changes. Trouble is, all the camera manufacturers ahev their own standard for raw data, so to get it into photoshop, the gimp or whatever you want to use, you must first run the raw image through software provided by your camera manufacturer - and you can bet that software won't run on Linux.
So this is good, 1 because it encourages interoperability and 2 because it further opens up proper image processing to Linux users.
Re:Yep. (Score:2, Informative)
Re:Why? (Score:5, Informative)
Plus, correct me if I'm wrong, but doesn't PNG assume all pixels have RGB information? Oftentimes this isn't the case in digital cameras (unless you're using a Foveon sensor). Google "Bayer Pattern" if you're interested.
The article is light on details, but I don't think Adobe is aiming this solely towards digital cameras (even expensive digital backs for medium-format cameras), but also towards medical imaging and what not. There is a reason why ImageMagick can be built with 24bits/channel and up.
Re:Why? (Score:2, Informative)
Re:Maybe (Score:5, Informative)
I'm not horribly concerned if Nikon doesn't release an update for my particular camera since Adobe will be providing an image conversion utility that supports many of the proprietary raw formats.
Re:Why? (Score:2, Informative)
Adobe's take on the issue (Score:5, Informative)
There are currently two image formats in wide use for high-end cameras. RAW is the format of choice for people who demand high-quality shots with no compression artifacts. Unfortunately, different camera manufacturers have implemented their RAW encoding differently, which means that two cameras that can save to RAW don't necessarily use the same format. As a result, professionals often have to convert between their vendor's RAW format, and that used by their software.
The other format is good old JPEG, but as you probably know, JPEG is a lossy compression [webopedia.com] algorithim, making it unsuitable for those who demand a certain level of quality in the shots as captured.
The new format is designed to provide the same advantages of RAW, without the cross-vendor incompatibilities. Adobe is calling it "a publicly documented and readily available specification," although I didn't see any kind of license data around the download of the spec (which is on the Adobe page listed above).
Re:Why? (Score:5, Informative)
First, PNG is not always indexed. However, it would have required massive extensions to PNG to turn it into something capable of being DNG, and TIFF is already well placed for extension (TIFF is a container format - most people think of it as a simple image format, but it is very flexible and capable of adaptation).
TIFF supports a huge variety of compression modes, including uncompressed, JPEG, LZW, and ZIP, and a variety of color modes.
DNG is an extension to TIFF, to allow the additional properties of a RAW to be expressed without losing the efficiencies of RAW (linear data, typically one color channel per pixel until processing). Just as a for instance - you can take your DNG into most any TIFF reader today and it will at the very least be able to read the preview embedded in the DNG without any mods to your TIFF library.
Re:Why the need to diss Adobe? (Score:5, Informative)
For more details about this announcement, go to dpreview.com
Adobe announces new format for raw files [dpreview.com]
The Digital Negative Specification is being posted to the Adobe Web site free of any legal restrictions or royalties, enabling integration of the
Re:Why? (Score:5, Informative)
The JPEG standard includes a lossless option too; professionals don't want to use JPEG because lossless JPEG is inefficient, not because it doesn't exist.
Re:Why? (Score:5, Informative)
This is not processing RAW into a TIFF - you can do that now with many tools. This is repackaging a RAW file into a new, universal RAW - this should open the RAW processor world to a new level of competition (as the greatest amount of R&D time was always wasted on reverse engineering RAW formats - something Adobe is now doing for you with DNG Converter).
More info available ... specs too (Score:5, Informative)
Adobe already has a page [adobe.com] on DNG. Its is a free format and the specs are right there on the page, so GIMP won't lose out.
I believe the format is a) to save Adobe money long term (they don't have to support yet another specific sensor) and b) reduce headaches and complaints from the user. We'll just see how the camera companies and digital photography professionals react.
Re:JPEG-2000? (Score:2, Informative)
Re:Why? (Score:5, Informative)
Re:JPEG-2000? (Score:4, Informative)
There are no royalties, no licencing, it has 2x to 5x the compression efficiency, and it's inherently multiresolutional.
Actually, Adobe did a very similar thing: they took the TIF format (the industry standard storing images with lossless compression) and added a few special fields, using the extension mechanism already provided by TIFF. As far a I can see, Adobe doesn't intend to charge royalties for DNG. It looks quite open -- even the DNG guide for manufacturers doesn't mention any licensing requirements.
(Adobe's DNG web site [adobe.com] is already online.)
Re:Why? (Score:3, Informative)
Re:Why? (Score:2, Informative)
The spec. appears to be unencumbered. Although there aren't bright yellow "FREE FOR ALL TO USE!" disclaimers, it does not bear any mention of a patent and it does state that the spec. is "free". Since it would be in Adobe's interest (as a market leader in photo editting software) to minimize writing kludgy compatibility code in the future, I don't see why anyone should assume that this offer is being made "in bad faith".
Oh yes, there might not be all of this "OMG PATENTZ!" hysteria, if the posting had included this link [adobe.com] to the Adobe site, which features a pdf of the specification itself.
Even more happily, the pdf was a simple one & rendered promptly under xpdf/gs.
Re:Why? (Score:4, Informative)
Re:Why? (Score:5, Informative)
Couple of notes...
That chessboard layout is called a Color Filter Array, usually arranged in a Bayer Pattern. [dpreview.com]
Digital cameras these days are 10 bit in RAW mode.
And some even have 4 color sensors. [dpreview.com]
dpreview is THE site for camera buffs, much the same way avsforum is for us audio & vidio philes. Now if only I could find sites for other categories....
--
"Geometry is frozen music"
- Pythagoras
Re:Maybe (Score:3, Informative)
Perhaps because it's an effective method to get everyone playing nice. There *are* other lossless formats that camera manufacturers could support (and I'm sure some do), but with no real direction, everyone would just do what suits them and there would likely be no common format between them. Adobe is just trying to help give the camera manufacturers something to agree on (barring other possibly nefarious intentions).
Re:JPEG-2000? (Score:3, Informative)
Re:Why? (Score:4, Informative)
Hell yes. Unlike with a raster image such as JPEG or PNG, the data from a camera sensor is most likely a Bayer array - alternating lines of Red/Green and Blue/Green sensors, rather than RGB triplets, so it's not so much RGB, as RGBG. (There are some varients/exceptions in the in sensors from Foveon, Fuji and Sony). There is also a lot of data specific to the exposure; duration, ISO, lens details, etc. which would need to be applied in camera before a raster image could be produced. With RAW, you can apply these settings after the event in Photoshop or whatever. Exposed the sky correctly, but got the ground off by a stop? No problem; "develop" the RAW twice and use the sky from one shot and the ground from the other for a much better result than "enhancing" the ground in an image editor.
Yes, you could have most of this with a tweaked version of PNG and a bunch of ID3 type tags (and maybe that's exactly what Adobe has done, I haven't looked at the file format yet). The main benefit though is to make it very easy for data exchange and solve the nightmare situation whereby each new sensor has it's own RAW format. The state of play at the moment is a nightmare for vendors like Adobe who need to update their software for almost every new high-end camera release. Likewise for the makers of those "digital photo stations" that are cropping up like Starbucks, or their little brethren; the printers you can plug a camera into directly. With a standard like DNG to support you gain the much larger colour gamut of the RAW format and more flexibility in tweaking the image for a better print.
Anyway, you can read the actual Adobe press release, or download a free (beer) DNG converter here [adobe.com] to find out a little more.
Re:Anyone who uses JPEG... (Score:4, Informative)
photographs are usually TIFF documents, but we've ran accross many customers who supply all of their graphics and photos as high-res JPEGs
Adobe lockin v.s. Quark? you obviously don't work in a printing house. Quark is basicly the only tool used in creating anything for print. Quark can create Adobe PDF Files without issues from any other software we use.
Now that i think about it, most of the software we use is based on Open standards from Adobe (Postscript, DSC(Document Structuring Convention), PDF, PJTF, JDF)
YIDIWIP (Yes I Do Work In Prepress)
Re:Why? (Score:3, Informative)
Re:Why not lossless JPEG/JPEG2000 then? (Score:3, Informative)
the RAW images basically give you direct data off the sensor and all the data about the cameras current settings, ie shutter speed, apeture, meter settings, focus settings etcetc.
having a single format would allow all the image processing apps to only have to load up the DNG file, rather thn having plugin's for nikon's raw file, sony's raw file, canon's raw file etc. right now if you don't have a plugin then you have to process the image through the manufacturers image application which can be a pain and seems unnecessary. also what if you have another platform, are all the raw input filters available for a mac for example? for linux? with a standardised free format then all the apps will have access and no companies will have to pay licence fees to load in the data (which soemthing like the gimp isn't going to do).
dave
Re:Why? (Score:3, Informative)
Photoshop and PSP allow you to save files as lossless JPEG, but the only camera I know is the newest Canon digital SLRs. Most other professional digital SLRs and prosumer cameras only store proprietary RAW, then some level of lossy JPEG.
Less why, more how (Score:4, Informative)
Ignoring the differences in the various RAW formats between manufacturers, what about differences between two cameras from the same manufacturer ? What causes that and would DNG cater for it ?
As an example, look at the Nikon D70 and the D100. Adobe had full support for the D100 with their ACR (Adobe Camera Raw) plugin. But when the D70 came out, we waited months for true compatibility with Photoshop (yeah, yeah, you could use the nikon supplied plugin, but that was worthless really).
I'm guessing Adobe want this because in order to keep selling Photoshop to photographers, they have to keep amending ACR everytime a new camera comes out. But can a fixed standard cater for everything that Canon and Nikon will be putting in their cameras, and want to store in the RAW files 2 years from now ?
Key benefits from DNG primer (Score:5, Informative)
Image format: DNG is based on the TIFF-EP format, but DNG specifies the inclusion of a number of additional tags that let the converter properly interpret the raw file.
Metadata: DNG enables inclusion of metadata in EXIF, IPTC, and XMP formats.
Compression: Files can be stored as uncompressed (either bit-packed or padded to 16-bits per pixel) or with lossless JPEG compression.
Color space: DNG fles are stored in a linear, nonwhite-balanced color space (usually the native color space of the camera).
Interpolation: DNG enables file storage either in mosaic (CFA) form or in demosaiced form. Generally, a mosaiced file is preferred because it represents the original data the sensor captured and enables maximum conversion fexibility. It is also smaller than a demosaiced file. In some instances, however, saving a demosaiced file can improve compatibility, particularly if the camera sensor contains an unusual mosaic pattern that all converters do not support.
Re:Why? (Score:3, Informative)
When you shoot in JPEG and even in TIFF with any current camera the internal processor applies white balance, color, contrast, sharpening and other algorithms to the data and saves it in the chosen format, typically JPEG at anywhere from a 1:2 to 1:8 loss compression ratio.
This is great for point, click, print. But for hobbyist and professional photographers this eliminates a lot of the post-shot production that can be done to the image later, even if it's a TIFF file. With a RAW image format you can apply those effects afterward, unapply them and generally tweak the output. My Nikon came with some software that allows me to set the white balance using the same algorithm the camera would do internally if I shot JPEG. But with the in-camera option if I don't like the results, tough. Sure, I can tweak it later in Photoshop but I can't fundamentally alter the effect. This is particularly frustrating for effects like white balance and sharpening. Having the RAW format, also known as the "digital negative", gives the photographer much more flexibility.
Why do we need a universal file format? Because every camera manufacturer out there has a different one that programmers have to figure out and photographers have to put up with (not that professionals change kit all that often). With Photoshop CS Adobe helped photographers out by putting in an excellent RAW reader but someone at a Adobe has to keep up with all these specifications. If all the camera manufacturers would adopt ONE format then Adobe AND open source developers could focus their time on the important business of digital photography -- producing excellent final photos.
Another benefit of the RAW format hinted above is that for programs like the GIMP developers waste a lot of time trying to reverse engineer these formats. As you can imagine a lot of the camera manufacturers aren't out there sharing the love with their specifications. The one format that's been best reverse engineered is the Canon RAW format with varying levels of success going down from there for Nikon, Minolta et al.
Lastly, let's not forget the benefit of having a common format for long-term archival and retrieval. Anybody out there have any binary files they can't get into any more? A common RAW format will reduce the chances of being orphaned with files you can't read.
In summary, then, this is a huge benefit for everyone as we can all focus on digital image production using a shared format and libraries and less on figuring all these formats out. The only question the article doesn't address is whether Adobe are releasing the format into the wild, as it were, or plan to setup a "toll" on it via patents.
Re:JPEG-2000? (Score:3, Informative)
They're filling the format with lots of structured metadata that can give you all the information you need (and probably also some you don't need
You could look at a photo and say "Hey, I like the way the details are emphasized in this photo. Let's see which settings he used".
Compression: they're proposing a "lossless JPEG" compression. Since they cited the DCT-based compression (for lossy compression), I guess that the "lossless JPEG" will be the "old" one and not the JPEG2000 wavelet based format (probably to avoid heavy modifications to today firmware).
My opinion is that the main feature of this format is the presence of photo-specific metadata (while JPEG2000 is "general-purpose"), but it probably could have been substituted by a "sub-specification" of the JPEG2000 metadata.
Re:Why? (Score:3, Informative)
No, it is not [libpng.org]. It *can* do that, but PNG supports full 24-bit color, with 8 bit alpha. And possibly higher color depths if necessay (there's a byte or so in the header for bits-per-color).
Finally Raw interoperability. (Score:5, Informative)
This provides a common RAW format for cameras. This is not a png or jpg replacement, but a RAW replacement.
There are a number of third party RAW converters on the market right now. Many have limited camera support. You can bet they will quickly moving to support DNG. Which will instantly open up their usage to almost all current cameras.
With DNG support and Adobes converter you will soon be able to open just about any RAW image with any converter.
Even without camera output this is a benefit. As you can get one converter to support all your cameras.
You can archive all your RAWs as DNG and not have to worry that you kept all the software that came with the camera that generated the original RAW.
Camera support would be even better, but that may be slow as the manufactures may suffer "Not Invented Here" syndrome, or see value adds to their own format quirks.
Parent is uninformed (Score:5, Informative)
This format is about putting all RAW files under one (DNG) format.
Eg. Nikon has NEF, Canon has CRW, Olympus has xxx, adsf has yyy....
Isn't it better to have one open/standard format which all manufacturers support/endorse?
If you are skeptical read this. [adobe.com]
Another Standard? (Score:1, Informative)
Why don't they call this YAIF - for Yet Another Image Format. Really guys would the others not suffice?
tbc++
Re:Nope, just have to get photographers using it (Score:3, Informative)
TIFF is also a format that most camera makers are already comfortable with. The files that come out of most cameras conform to the Exif spec. Exif is the JFIF (JPEG) file format with metadata embedded using TIFF tags. It will be interesting to see (I haven't read the specs yet) whether Adobe wants to keep any or all of the Exif tags, or wants to ignore them and invent their own. It would be smart of Adobe to try to design something that can live happily with what users and manufacturers are already familiar with
Re:Why the need to diss Adobe? (Score:5, Informative)
Those two together let you play with the image composition before its set into any format. Don't like the exposure? Check what the exposure setting was, then recalculate the pixels based on the original source data. Bad aliasing effect? Try again from the raw data at a different resolution or different interpolation between the CCD sensors and pixels and see if you can save the picture.
Re:Customer oriented naming (Score:4, Informative)
my digital rebel I take a "black mask" shot every night I start a sky photographing session. I'm looking at 25-60 second exposures (multiples to get even longer exposures) and when I subtract a black mask from the images I remove most of the unwanted ccd noise. (simple script under linux in C to work with the RAW files.) I can not accomplish the same quality by doing a black mask on Jpegs or in photoshop with the RAW plugin I bought for it.
The same goes for stacking images to get a better exposure. performing this on the native RAW images produces crisper results.
Supports EXIF and more (Score:3, Informative)
DNG is TIFF -- Dont Panic ! (Score:5, Informative)
A DNG-format file is fully compliant with the TIFF 6.0 Specification Standard and the ISO TIFF-EP codification of that spec, which was designed from day one as a fully extensible raw, processed, or whatever image / metadata annotation spec.
BTW, TIFF was originally designed for offset printing folks, and in the 6.0 standard already supports a huge number of colorspace models besides RGB, and has an extensible mechanism for specifying color-data encoding and compression schemes (you can even store JPEG encoding in TIFF).
When I worked at the ground-data processing section of the Jet Propulsion labs, TIFF was occasionally used to store and transmit raw multispectral satellite data, which consisted of over 256 separate color-spectra bands from far infrared to ultraviolet, stored spatially in separate tiles.
Working together with Spot Image and other satellite providers, NASA also helped develop the GeoTIFF extension to TIFF, which annotates an image with exact georeferencing information.
It looks like Adobe went the route of using SubIFD's to define the extended data. A little bit unfortunate, since that data will not show up in a "tiffdump" listing of the file, but in any case I have no doubt that folks are already taking the spec and writing "libtiff" extensions to parse the stuff.
For more information on TIFF, see my old, clunky website that is chock full of invalid links,but still has a few useful things to say:
http://home.earthlink.net/~ritter/tiff [earthlink.net]
--Niles (original GeoTIFF and TIFF webpage author)
Re:What has not been said (Score:5, Informative)
Maybe that's because it's brand new and it's just been released.
Adobe in it's traditional greedyness did not release this as Open Source, they released a spec but not code. They just don;t get how to work with the open source community.
The way to work with the Open Source community to to release non-proprietary specifications for which we can write code ourselves. SO FAR they've done a pretty good job with PDF and DNG. What? Do you think no one is going to write an Open Source library for it?
Sheesh, talk about greedy; they could've colluded with camera manufacturers to monopolize on a closed, proprietary format, and you complain because they didn't give you the frickin' code? Get off your rear-end and write it yourself. That's the Open Source way.
PS: Adobe sucks for entirely different reasons (e.g. Dmitry Skylarov). This, however, is not one of those reasons. Adobe did right this time.
Re:Irony of life, DNG can be lossy too (Score:3, Informative)
So in other words, the Slashdot writeup that stated this was a new format that was better than JPEG was completely incorrect, and in actual fact this is simply a container format that uses existing JPEG algorithms? Sounds about usual for Slashdot these days.
According to the JPEG FAQ, PNG is more efficient than lossless JPEG for most images. Unfortunately, this specification doesn't allow for that; as far as I can tell this has little to do with picture quality and more to do with metadata and interoperability.
RTFA (Score:3, Informative)
Camera Raw Updates (Score:1, Informative)
Mac:
http://download.adobe.com/pub/adobe/magic/photosh
PC:
http://download.adobe.com/pub/adobe/magic/photosh
Re:Why? (Score:3, Informative)
And my whole point was... (Score:4, Informative)
This is just a graphic file format like any other, using bog-standard compression and metadata standards. It's just a more standard bag for raw camera data than what we've seen before.
Re:JPEG-2000? (Score:3, Informative)
Unisys never did the same with their submarine patent. They simply said nothing and then enforced it. Adobe has already precluded themselves from that.
Calibration images for astronomical photos (Score:4, Informative)
To get a corrected image, use this formula for each pixel:
newimage = (image - dark) / (flat - dark)
Better yet, take a bunch of darks and flats and median-filter them to get rid of cosmic rays which can introduce spurious glitches in the images.
"Raw" scanner files (Score:2, Informative)
Re:Great idea. Some info. (Score:2, Informative)
Re:Why the need to diss Adobe? (Score:4, Informative)
Well, a raw file is a lot more compact, and it does not do anything to change the artifacts and or "lossyness" of the sensor itself.
For instance the Canon cameras use a Bayer sensor. Imagine a checkerboard where half the sensors, are green , arranged in a checkerboard pattern, and the other half the sensors alternate between red and blue. The 12 bit value of each sensor site is encoded as the eight bit difference between it and the nearest same color site to its left. ( Amiga developers will find this vaguely familiar) The theoretical case of there being a full 12 bit difference between 2 sites never happens due to the optical properties of the camera and lens.
Re:Nope, just have to get photographers using it (Score:3, Informative)
The most useful tag is DateTimeOriginal, which tells you when the photo was taken. Some cameras also record info like shutter speed, flash setting, aperture, focus distance, lighting conditions, and other interesting info.