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Understanding RAW File Format
[Editor's note: In this tutorial we sometimes use the term "RAW
image" when technically we mean "RAW data".]
When you take a picture in JPEG file format with your digital camera, a couple
of things happen before the image is even saved to memory card:
The image sensor gathers the information from its photosites, converts it
from analog to digital, and holds it for further processing. At this stage,
the image data captured can be thought of as being "unprocessed"
-- or, RAW data.
If you have specified white balance, sharpening, contrast, saturation, image
effect, digital zoom, etc., these are applied to the RAW data.
If you have specified image quality and size, these are also applied to
the RAW data.
The resulting image is a JPEG image, processed ("in-camera process"),
and compressed, which is then written to your memory card.
After you have transferred the image from your memory card to your PC, you
may decide to further process it ("post-processing") in an image editing
software, such as Photoshop. Most photographers will usually adjust levels and
sharpen the image a bit.
RAW Defined
RAW file format is the uncompressed, unprocessed data file captured by the
camera's image sensor, before any in-camera processing has been applied (though,
in practice, depending on the camera manufacturer, some minimal in-camera processing
may have been applied to the RAW data). In this sense, an image saved in
the RAW file format is the digital equivalent to the (exposed but as yet unprocessed)
film negative.
In fact, the camera will ignore your white balance, sharpening, contrast and
saturation settings. Instead of applying them to the RAW data, it will save
those settings in a separate header associated with the RAW data.
[Editor's note 2004-08-18: Thanks to Stevens Noyes for his constructive
feedback. See "Feedback From Readers" below.]
Correcting Mistakes
The fascination with RAW is that it seems to magically give you the ability
to "correct your mistakes." How is that achieved?
Remember that when you save an image in RAW, your image settings are ignored
as far as applying them to the image, and are instead saved in a header.
When you open the RAW data in your image editing software (with the appropriate
RAW plug-in installed, since every manufacturer encodes RAW a little differently),
that header is read and used to display an image of the RAW data.
Notice that we say, display -- the actual RAW data is never affected.
You may now manually adjust the settings and see the effect on the RAW data
-- giving the effect of being able to magically "correct your mistakes."
When you are happy with the adjustments, you would then typically save a copy
as JPEG.
RAW is therefore a powerful option that most advanced digital cameras make
available to photographers who do not want the camera to apply any in-camera
processing to the captured RAW data -- preferring to do that themselves in post-processing.
Exposure Compensation
Some photographers have mistakenly thought that RAW allowed them to correct
exposure errors. Well, to a limited extent, that is true.
However, a picture has to be correctly exposed, whether you are using RAW or
not. You cannot take a grossly underexposed or overexposed picture in RAW, and
expect to be able to "correct your mistakes."
To explain how this is possible, we need to get a bit technical: While JPEG
captures 8 bits of colour per pixel or 256 shades of colour per pixel (16.7
million colours), RAW captures 12 bits of colour per pixel, translating to 4096
shades of colour per pixel (68.7 billion colours). This great amount of information
(extra pixel depth going from RAW to JPEG) is what allows the exposure to be
corrected up or down by one stop, and sometimes by as much as 2 stops (but you
might be pushing your luck). Applying this exposure compensation and adjusting
the other settings to your RAW data in post-processing may be enough to salvage
an otherwise ruined image.
[Editor's note: changed 2004-06-28 to correct for the explanation behind
why exposure compensation can be applied to RAW. Thanks to Mark Wickens and
Rick Sidwell for their constructive feedback.]
When To Use RAW
RAW data takes longer to write to memory card and there is a need to post-process
every single picture.
These two requirements can be a major nuisance or not depending on the type
of photography you do. If you need to take pictures in rapid succession, and
your digital camera does not provide a large enough RAW buffer, you will be
hampered by the extra amount of time it takes to write a large RAW image data
to memory card. Post-processing every single image is also a chore that not
many amateur photographers (and some professional photographers) enjoy doing.
This means that, for most practical purposes, amateur photographers will find
that saving in RAW is not an interesting option, and that shooting in JPEG is
more than sufficient. [There, I've said it, and you can now stop being intimidated
by all that "RAW talk."]
Unless...
You are unsure of the white balance to use or need accurate colour reproduction
of a subject (e.g. a wedding dress must be reproduced faithfully as
pure white, off-white, silk white, etc.);
You want optimum control over sharpness, contrast, saturation;
When you want to print extra extra large, RAW file format gives you more
room to maneuver since there are no JPEG artifacts due to in-camera compression.
Many photographers are finding that they do want the control that RAW gives
them and but still find the post-processing tedious. Especially when only one
picture out of a dozen or so might be a keeper.
RAW + JPEG
To address this issue, some advanced digital cameras now offer a RAW+JPEG option.
It takes even longer (slightly) to write to memory card, but now post-processing
of the RAW image can be reserved for only the keepers, or the images that need
adjustments. If the JPEG versions are fine, they would be used instead.
Summary
Should you use
JPEG or RAW?
For most of us,
saving in JPEG is more than adequate. JPEG files are compressed and, compared
to RAW, they are smaller, save faster, and more images can fit on a memory card.
Many professional photographers shoot at the highest JPEG image quality.
For ultimate control
over white balance, saturation, contrast and sharpening, and/or if you know
ahead of time that you intend to print a certain picture super extra large,
RAW file format (or, RAW + JPEG) is the ultimate answer. For many professional
photographers, there is no substitute to shooting RAW.
[Or, just for
the fun of it, to learn something new, to stop getting intimidated by all the
"pro" talk about the advantages of RAW, and to earn forum bragging
rights, why don't you just try it out with a few pictures?]
For a more technical
(and practical) explanation of RAW, including a typical workflow, consult The
Art of RAW Conversion.
"RAW file format is the uncompressed, unprocessed data
file captured by the camera's image sensor, before any in-camera processing
has been applied. In this sense, it is the digital equivalent to the film negative
that has been exposed but has yet to be processed."
---- Steve Smith
The statement:
"RAW file format is the uncompressed, unprocessed
data file captured by the camera's image sensor, before any in-camera processing
has been applied. In this sense, it is the digital equivalent to the film negative."
might be a bit misleading as it is not really true and
continues the propagation of this RAW myth.
There is processing done, in most cases, to RAW data.
This includes various NR (DFS and dark current), lossy compression (in the case
of the compressed NEF format), loss-less compression (Canon TIF/CRW/CR2 formats),
WB adjustments (in the original D1
and early D1x models) and (in the case of the 10D/300D) even sharpening applied
in the RAW format.
And these are just the adjustments to the RAW file that
are known about. The truth is, few people outside the camera companies know
the full extend of data tweaking that goes into the RAW files.
A better wording might be:
"RAW file format is the data file captured by the
camera's image sensor, with minimal in-camera processing applied. In this sense,
it is the digital equivalent to the film negative."
--Steven Noyes
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