RAW or JPG?

Digital SLRs and some compact digital cameras have the ability to store images in RAW format ("not processed") instead of the common JPG.

Is this format useful for the digital underwater photographer? Read on to find out.

Basics of RAW

The RAW format stores the unprocessed data of the camera's CCD/CMOS sensor separate from the intended processing for the image. For example, modifications to the white balance setting on a RAW image only change stores this specific setting without any alteration to the raw data. It is only upon displaying or converting the image that the raw data is combined with these settings to show the final result. With shooting in JPG, this has already taken place before the image is written to storage.

RAW can thus be regarded as a digital negative which will retain all image information for future use.

Continuing the comparison with JPG, RAW has another big difference: the number of bits for storing color data. JPGs have 8 bits, allowing 256 (=2^8) variation in each color channel (red, green or blue) to be represented per pixel.

RAW usually have 12 bits, making 4096 (=2^12) variations for each color possible. Although the human eye is not capable of discerning these minute variations, the extra bits do carry useful information when editing the image. When increasing brightness on areas of shade for example, details make become visible again that would have been lost in a similar photo shot in JPG.

But as most image appliances (e.g. monitors, DVD-players and printers) and software (web browsers, photo viewers etc.) are not capable of handling 8 bits per channel, RAW images must be converted to a more common 8-bit format such as JPG before they can actually be used.

Camera manufacturers do include tools for this conversion, and software like Photoshop CS and PaintShop Pro is also able to handle many RAW formats.

Benefits of RAW under water

As commonly known, the light and color conditions under water make photography much harder than on the surface. Incorrect interpretation of color and white balance by the camera is one of the commonest issues, as explained in the underwater color correction article.

The RAW format proves to be an ideal aide in correcting the incorrect results afterwards. Adjustment of the white balance can then be done without loss of quality, making the colors look vivid and naturally again. The 12 bits also proof handy here: even tiny traces of color are still stored in the file and can be "cranked up" by color correction options in the software.

Over- and underexposed photos due to incorrect strobe settings will also show more details after correction, and posterization effects in large blue areas (shown on the right) will be greatly reduced.

Drawbacks

The storage techniques used in the RAW format to bring along some drawbacks. The first comes from the extra bits make the RAW file size increase to multiple megabytes. The Canon EOS 300D, for example, produces files of approximately 7 MB, the Nikon D70 use around 5,5 MB of storage.

These sizes have a negative impact on the time required to write a shot to the storage media. A Nikon D100 may take as long as 40 seconds for storing only one RAW, the Canon 350D and Nikon D50 on the other hand only take half a second. The storage medium should of course also have of sufficient space, a 512 MB card is not big enough for a decent photo shoot in RAW.

Another drawback is that (as of yet) no common standard exists for storing RAW images. Most manufacturers use a proprietary format and file extension, like Nikon NEF, Canon CRW, Minolta MRW and Sony SRF. This inhibits exchanging these files with other people. Together with the limited RAW support of software and appliances makes conversion to a common format (usually JPG) an absolute must and requires more time for the whole process ("workflow").

Summary

The extra bits and separate storage of data in the RAW format provide a large margin to the photographer to correct mistakes and common issues of underwater photography.

Because of the limited support of RAW in many areas, additional time has to be put in processing and converting this type of images, but this investment is definitely worth it.

Also read: Image processing for Digital Underwater Photos

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