.
Art 122 Digital Photography
X
.
File Formats
X

-RAW-

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The RAW file format is digital photography's equivalent of a negative in film photography: it contains untouched, "raw" pixel information straight from the digital camera's sensor. The RAW file format has yet to undergo demosaicing, and so it contains just one red, green, or blue value at each pixel location. Digital cameras normally "develop" this RAW file by converting it into a full color JPEG or TIFF image file, and then store the converted file in your memory card. Digital cameras have to make several interpretive decisions when they develop a RAW file, and so the RAW file format offers you more control over how the final JPEG or TIFF image is generated. This section aims to illustrate the technical advantages of RAW files, and makes suggestions about when to use the RAW file format.

A RAW file is developed into a final TIFF or JPEG image in several steps, each of which may contain several irreversible image adjustments. One key advantage of RAW is that it allows the photographer to postpone applying these adjustments-- giving more flexibility to the photographer to later apply these themselves, in a way which best suits each image. The following diagram illustrates the sequence of adjustments:

Demosaicing and white balance involve interpreting and converting the bayer array into an image with all three colors at each pixel, and occur in the same step. The bayer array is what makes the first image appear more pixelated than the other two, and gives the image a greenish tint.
Our eyes perceive differences in lightness logarithmically, and so when light intensity quadruples we only perceive this as a doubling in the amount of light. A digital camera, on the other hand, records differences in lightness linearly-- twice the light intensity produces twice the response in the camera sensor. This is why the first and second images above look so much darker than the third. In order for the numbers recorded within a digital camera to be shown as we perceive them, tone curves need to be applied.
Color saturation and contrast may also be adjusted, depending on the setting within your camera. The image is then sharpened to offset the softening caused by demosaicing, which is visible in the second image.

The high bit depth RAW image is then converted into 8-bits per channel, and compressed into a JPEG based on the compression setting within your camera. Up until this step, RAW image information most likely resided within the digital camera's memory buffer.
There are several advantages to performing any of the above RAW conversion steps afterwards on a personal computer, as opposed to within a digital camera. The next sections describe how using RAW files can enhance these RAW conversion steps.

Demosaicing

Demosaicing is a very processor-intensive step, and so the best demosaicing algorithms require more processing power than is practical within today's digital cameras. Most digital cameras therefore take quality-compromising shortcuts to convert a RAW file into a TIFF or JPEG in a reasonable amount of time. Performing the demosaicing step on a personal computer allows for the best algorithms since a PC has many times more processing power than a typical digital camera. Better algorithms can squeeze a little more out of your camera sensor by producing more resolution, less noise, better small-scale color accuracy and reduced moiré. Note the resolution advantage shown below:

High Bit Depth

Digital cameras actually record each color channel with more precision than the 8-bits (256 levels) per channel used for JPEG images. Most current cameras capture each color with 12-bits of precision (212 = 4096 levels) per color channel, providing several times more levels than could be achieved by using an in-camera JPEG. Higher bit depth decreases the susceptibility to posterization, and increases your flexibility when choosing a color space and in post-processing.

Bits Per Pixel
Number of Colors Available
Common Name(s)
1
2
Monochrome
2
4
CGA
4
16
EGA
8
256
VGA
16
65536
XGA, High Color
24
16777216
SVGA, True Color
32
16777216 + Transparency
48
281 Trillion

Dynamic range

The RAW file format usually provides considerably more "dynamic range" than a JPEG file, depending on how the camera creates its JPEG. Dynamic range refers to the range of light to dark which can be captured by a camera before becoming completely white or black, respectively. Since the raw color data has not been converted into logarithmic values using curves (see overview), the exposure of a RAW file can be adjusted slightly-- after the photo has been taken. Exposure compensation can correct for metering errors, or can help bring out lost shadow or highlight detail. The following example was taken directly into the setting sun, and shows the same RAW file with -1 stop, 0 (no change), and +1 stop exposure compensation. Move your mouse over each to see how exposure compensation affects the image:

 

 

-JPEG-

 JPEG stands for "Joint Photographic Expert Group" and, as its name suggests, was specifically developed for storing photographic images. It has also become a standard format for storing images in digital cameras and displaying photographic images on internet web pages. JPEG files are significantly smaller than those saved as TIFF, however this comes at a cost since JPEG employs lossy compression. A great thing about JPEG files is their flexibility. The JPEG file fomat is really a toolkit of options whose settings can be altered to fit the needs of each image.
JPEG files achieve a smaller file size by compressing the image in a way that retains detail which matters most, while discarding details deemed to be less visually impactful. JPEG does this by taking advantage of the fact that the human eye notices slight differences in brightness more than slight differences in color. The amount of compression achieved is therefore highly dependent on the image content; images with high noise nevels or lots of detail will not be as easily compressed, whereas images with smooth skies and little texture will compress very well.
 

 

-TIFF-

 TIFF stands for "Tagged Image File Format" and is a standard in the printing and publishing industry. TIFF files are significantly larger than their JPEG counterparts, and can be either uncompressed or compressed using lossless compression. Unlike JPEG, TIFF files can have a bit depth of either 16-bits per channel or 8-bits per channel, and multiple layered images can be stored in a single TIFF file.
TIFF files are an excellent option for archiving intermediate files which you may edit later, since it introduces no compression artifacts. Many cameras have an option to create images as TIFF files, but these can consume excessive space compared to the same JPEG file. If your camera supports the RAW file format this is a superior alternative, since these are significantly smaller and can retain even more information about your image.
   
 
  Adapted from the Caimbridge in Colour Website: http://www.cambridgeincolour.com