Photography - Understanding Digital Image Formats

Images produced by digital cameras now rival the quality of our finest photographic film stocks. But the nature of a digital image shares almost nothing in common with the analog image captured in a film emulsion.

An image captured in film is an incredibly complex physical object that has a life of its own, and can be interpreted directly by inspection with the human eye. A digital image, on the other hand, is an electronic representation of a scene - a sequence of numbers specifying red, green, and blue light intensities that requires some form of software to render it into a visual form that can be displayed on a suitable imaging device, like a photo-printer.

When an image is captured digitally, it is done with a mosaic of light-sensitive electronic pixels. These pixels are actually independent square-shaped photodiodes which are arranged in the form of a large tiled surface. Well, large from the point of view of a single pixel, since if we were to enlarge the pixel to the size of a kitchen floor tile, then the area covered by the entire image sensor would be about the same as that of a football stadium.

A typical medium-resolution digital camera might have about 4000 electronic pixels arrayed along one edge of its image sensor, and about 2500 along the other, making for around 10 million pixels overall. The image sensor in this case would be said to have a 10 megapixel resolution.

Now, when an image is recorded electronically, what each pixel on the sensor measures is the amount of energy the light imparts to it during the photographic exposure. Or in simpler terms, the brightness of the light. This large array of numbers is known as the RAW format of the image. It is, in effect, the digital equivalent of the film negative (or positive in the case of slide film), since it carries ALL the information associated with the exposure.

As it happens, you cannot simply interpret these RAW image records in a color-by-the-numbers type fashion. If you were to assign the color and brightness of each pixel to a corresponding printed pixel on a piece of photographic paper, or on a computer screen, you would not see a pleasing representation of the scene that was photographed.

The reason for this is that the way our eyes respond to color brightness is different than the way electronic pixels respond to it. Our eyes are less responsive to large changes in brightness than are electronic pixels. The RAW numbers need to be processed in a way that compensates for this difference.

What this means is that a lot of number crunching needs to be performed to get the best result from our RAW image before it is printed in any form. This might be done inside the camera if you want to immediately see a preview of the result on your camera's LCD screen. Or it might be done using complex image processing software on your PC, once you have downloaded the image. Until then, the RAW image needs to be stored for later use.

Unfortunately, in the race to conquer the digital photography landscape, digital camera manufacturers adopted a first-to-build is first-to-dominate philosophy and created their own proprietary versions of the RAW image format. A Canon RAW image, therefore, is formatted differently than a Nikon RAW image for the exact same image. Due to the proliferation of RAW formats, image processing software now has to cope with hundreds of competing RAW image formats. In practice this is just not possible, so your imaging processing software (if it comes from a vendor other than your camera manufacturer) is likely to support only the major RAW formats, like for example Nikon's NEF format, Canon's CR2 format, and Fuji's RAF format.

This situation is likely to improve in time, however. Adobe has entered the digital imaging fray by publishing an open standard for a RAW image format that it calls Digital Negative, or DNG. Slowly, camera manufacturers, like Hasselblad, Leica, Ricoh, and Samsung are building DNG support into their cameras, and with luck the larger players in the field will follow suit.

What this means, assuming that a standard such as DNG is adopted, is that when a photographer captures an image, stores it in RAW format, and then forgets about it for 10 years, they won't discover, when they get around to retrieving it again, that their image format has been obsoleted and there is no longer any software that can render the file into a viewable and printable image. For large corporations with millions of archived images to preserve, this kind of problem represents a logistic nightmare, and it is very costly to stay on top it.

In the long run, a standardized RAW format will ensure archival integrity of images, reduce headaches for unwary photographers the world over, and save them both time and money. DNG support is currently available in Adobe software packages such as Photoshop, and Photoshop Elements, and will likely migrate to third party image software packages as the standard is embraced. Adobe also offers a free Digital Negative Converter from its site which allows forward-thinking photographers to convert their existing RAW image format files into a DNG version as well.

As has been mentioned, software is needed to convert a RAW format image into one that can be displayed and printed. This is analogous to the "development" process for negative film. The most common image display format is JPEG (which stands for Joint Photographic Experts Group). The JPEG format is one that can support a great deal of compression, so that the final viewable image is substantially smaller in size (number of bytes) than the RAW image file. This means it can be sent on to others easily, via email for example. The JPEG format is also an industry standard image format, so the file can be opened and read by all commercial image processing software and a large number of open source image software packages.

Another standard image format is TIFF. However, TIFF file sizes are generally much larger than those for the equivalent JPEG image, so they are used mostly by professionals who need to produce large print reproductions with high resolution. In fact, the DNG standard is based on a version of TIFF.

Various image processing algorithms are applied to RAW images to convert them into printable form. This includes performing white balancing, which is the means by which an unwanted overall color cast is removed from the image. When a color cast is present, a photographed all-white object will render with an off-white component that subtracts from image fidelity. The RAW image stored by your digital camera will likely have a record of the white balancing correction used when the image was created, but you are free to adjust this when editing the image derived from the RAW format.

It is important to appreciate that when you are trying to the create the best possible printable image, you need to start with the original RAW image file. Once a printable version has been created, such as a JPEG version, the applied image processing algorithms have "tossed out" a great deal of image information that was deemed unnecessary. These lossy operations are irreversible, and they limit your remaining options for tinkering with the image should you decide that the result is not quite what you are after. The solution is to return to the RAW format file and start over.

Because the differences in file sizes are so great, if you are not concerned with collecting RAW image files and processing them for the perfect image at a later date, you should consider allowing your camera to create JPEG images as the default, and ignore the RAW format altogether. This will improve the responsiveness of your camera, because you do not have to store the large RAW images to your memory card. If, for example, you are photographing a sports event, your frame-rate when shooting in the continuous mode will be greatly improved. Also, you will be able to record a much higher number of images to your memory card before it fills up.

On the other hand, if you will be photographing something of importance, do consider the implications of not using the RAW format to record your images. You might regret it later.

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