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The Long Beach Digital Photography/Photoshop Meetup Group Message Board › 8 bit vs 16 bit image

8 bit vs 16 bit image

A former member
Post #: 50
For those members who are new to digital photography, here is a basic description of 8 bit vs 16 bit image capture and why. This is from Tim Grey at

http://campaign.r20.c...­

Today's Question: I was wondering what the difference was between an 8-bit image and a 16-bit image and how and when you would use one or the other.

Tim's Answer: The bit depth of an image determines the total number of possible color values for each pixel within the image. A single bit can have one of two possible values, generally described as being either zero or one. So with 8 bits, there are a total of 256 combinations. This represents the two possible values raised to the eighth power, or two multiplied by itself eight times.

For an RGB image there are three channels, so to determine the total number of possible colors you need to raise 256 to the third power, or multiple 256 by 256 by 256. That yields a total of nearly 16.8 million colors. And it just so happens that "typical" human vision is generally regarded as being able to distinguish around 16.8 million colors, so this is fortuitous.

For 16-bit per channel the numbers get much bigger. Two raised to the sixteenth power is 65,536. For an RGB image that means you need to raise 65,536 to the third power in order to determine the number of color values available. Let's just say it is a really huge number that is a bit more than 281 trillion.

The key question then, is if an 8-bit per channel contains about the range of color values the human visual system can discern, why would you ever need more? The answer is adjustments. Every adjustment you apply to an image causes some degree of loss of information at the pixel level, in terms of pixel variations. With an 8-bit per channel image you don't have as much headroom for strong adjustments. With a 16-bit per channel image you don't have to worry much at all about strong adjustments.

The key difference is the potential for posterization, or the loss of smooth gradations of tone and color, with 8-bit per channel images. However, this is really only a risk if you apply rather strong adjustments to the image. The risk is a bit higher for black and white (grayscale) images, since those images only have a single channel.

For most uses an 8-bit per channel image will serve you well, especially if strong adjustments aren't needed. But a 16-bit per channel image does provide you with a bit of insurance. But it is important to keep in mind that a 16-bit per channel image is only meaningful if the source data had a bit depth that is higher than 8-bit per channel. That means, for example, that you need to start with a RAW capture converted at 16-bits per channel, or a film scan performed at 16-bit per channel, if you're going to get any benefit of working in 16-bit per channel mode for a given image.
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