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File Formats for Digital Imaging

One of many confusing decisions confronting the new user of a higher-end digital camera is the choice of what file format to use. In order to decide, you'll first need to know a bit about each.

Most every digital camera supports JPEG capture. JPEG stands for the organization that developed the format, the "Joint Photographic Experts Group." I've written previously about JPEG files and have probably impressed on at least some that they are not a good format for image editing. JPEG is a "lossy" compression algorithm that discards at least some information every time you resave it. It makes a very good capture format though. Since the files are small, you can fit a lot of them on even a modest sized compact flash card. For the same reason, you can also fit more in the camera's internal buffer, meaning you can shoot more images in a row without having to wait for them to be moved to the CF card. And guess what? They take less time to write as well.

With JPEG, the camera applies all the settings you have chosen including sharpening, exposure, white balance, color space and so on. All JPEG images are 8-bits per channel. If you do a good job taking the picture and get it right in camera, all is well, just as it is would be on film. There are better choices though if you want to maximize the potential of your images post-capture. One nice thing about JPEGs though is that they are all but universal in terms of support. You can take the images straight from the camera and use them in a wide variety of applications. Although smaller than other image formats, even jpegs generally have to be downsized some to send by email. A typical JPEG from the Nikon D100 will be between one and three megabytes in size.

If you want to do any editing of your images, it's best to resave them in a format other than JPEG after downloading them to your computer. This way, you get the benefit of small file sizes when it counts in camera, but don't have to live with the image degradation inherent in the jpeg format when you edit them later.

An option present on at least many digital cameras is TIFF, an acronym for "Tagged Image File Format." TIFF files can be either 8-bits per channel or more commonly 16. Most digital cameras only have 12-bit analog to digital converts so 16-bits per channel is somewhat overkill, but does mean you lose data just by saving the file. Just as with Jpegs, the data is already "fixed" as an image though. Any settings chosen in camera will have been applied to the data so you have to do a good job of taking the picture or it will be unusable.

TIFF is a very portable format as it is supported by a wide variety of photographic and publishing applications. It makes a poor choice for in camera use though. An image captured as a TIFF is much larger than the same image captured as JPEG, but will only marginal added benefit. If you want to maximize storage capacity or throughput, stick with JPEG. D100 TIFF files are over 17 megabytes each. If you want to go for maximum potential for quality, read on to find out about RAW.

RAW isn't really a file format so much as it is category of similar file formats. Each camera manufacturer has their own version, due in part to differences in sensor technologies but also in an attempt to achieve competitive advantage. A raw file stores the data exactly as it comes off the sensor. For most cameras, this is a single channel of pixels in what is known as a Bayer mosaic. Every pixel only senses a single color and these are then arranged in a checkerboard pattern. Every other row consists of alternating blue and green pixels, with the rows in between composed of alternating green and red pixels. Thus, half of the total pixels will be green with one quarter each of red and blue. This may sound odd at first, but since human vision is more sensitive to green than the other two colors, it actually works.

The raw data is called "raw" since it is totally unprocessed. At this point, white balance and other adjustments have not been made and the data is still an exact recording of what each photosite recorded. One photosite makes one pixel. Enough pixels and you have a raw image. It won't look like much until it gets processed into a visible image, a process that happens whenever you open a raw file in Photoshop Adobe Camera Raw, Nikon Capture or other raw file converter. Raw files contain the absolute maximum amount of data possible, giving you tremendous power to post-process them to maximize your creative efforts. Nikon RAW files (also known as NEF, or "Nikon Electronic Format" files) are 12-bits per channel, as are the raw files created by many other current digital cameras. This makes them ideal to archive as "digital negatives" but does mean that you have to process every file at least to some extent before it can be used as an actual image. NEF files shot on the D100 are right around 10 megabytes each.

To offset the size issue for RAW files, some cameras compress them to a degree. My trusty D100 cameras were one of the first to offer compressed NEF, but they were far to slow at it to be of any real use. Newer ones do much better at this. Some cameras now also offer a RAW+JPEG mode that gives you JPEGs you can use straight out of the camera for convenience, while still proving RAW files to be processed later.

More on raw files next week.

You may be left wondering why different formats even exist. If someone were to put well-made prints produced from JPEG, TIFF and RAW originals side by side, I doubt that either you or I could tell them apart. They look the same when opened in Photoshop too. So what is it that makes them different?

Lots of things have different formats. To pick just one example I've used in writing this article is how we represent numbers. Both "twelve" and "12" represent the same number, but they use different formats to do so. "TIFF" and "Tagged Image File Format" both refer to the same thing, but they are written differently, with the acronym "TIFF" being essentially a compressed format for the spelled out equivalent. Just as with different image file formats, each format has its advantages, and we naturally choose the one that best fits our needs at the time. File format means choice, and the choice is yours as to what you want to use to capture your images.

Update 12/04/2007 The new Nikon D3 and D300 cameras allow you to choose either the standard 12-bit NEF raw format or a new 14-bit NEF format. More bits means greater accuracy and improved shadow detail, so this is great news.

Date posted: February 27, 2005 (updated December 4, 2007)


Copyright © 2005, 2007 Bob Johnson, Earthbound Light - all rights reserved.
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Related articles:
The Ever-worsening JPEG Phenomenon
Compressed versus Uncompressed NEF

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