Wednesday, May 30, 2007
The Bit-Depth Decision
8-bit versus 16-bit workflow is among the least understood aspects of photography for most professionals. This primer will get you up to speed quickly.
Suppose you build a smooth linear gradient in Photoshop from black to white in 8-bit and use one pixel for each value. You'd have 256 tones. Level 0 is used to represent pure black, Level 1 represents the value nearest to that black but just a bit lighter, and so on, going all the way to a pure white at Level 255. The Photoshop Histogram plots these values using a single black line to represent each of the 256 values running left (Level 0) to right (Level 255).
Now you apply a simple edit in Photoshop, say you pull a curve to lighten the image. You change many of the numeric values and their relationship to each other. Multiple and differing levels are now represented by a single value while individual values may be discarded. For example, Levels 1, 2 and 3 are all now defined as Level 1. Levels 128, 129, 130 and 131 may now be represented by a single value, perhaps Level 131.
Depending on the edit, levels may be stretched, again resulting in fewer tones. If you pull a very steep curve in Photoshop, you can see how the image appears to posterize, also known as banding or more accurately, aliasing. In fact, the Posterize command in Photoshop is an excellent tool to see how reducing tones affects smooth gradations and detail. Set the Posterize dialog to 21 on your black to white gradient, and you'll see how the 256 steps are now reduced to 21 steps. The smooth, continuous-tone gradient you built is no longer smooth. If you do make such an 8-bit test file, examine the Histogram before and after editing the image. Notice the white spikes in the histogram after the edit. This represents the tones that were discarded!
In order to improve an image, you often need to change the values of your pixels. The result of editing pixels is data loss, but it isn't something to lose sleep over because on the whole, you're improving the overall look of the image. Ideally, you want to send the best and most pristine 8-bits of data to your printer. Since very few output devices can use more data, the goal is to edit your images without undo degradation. Here's where working with more than 8-bits of color comes into play.
Most modern capture devices, such as digital cameras and scanners, collect more than 8-bits per color channel. The manufacturers of these devices do this for a reason. The vast majority of digital cameras can produce 12-bits per channel, while some can capture 14-bits, and a rare few can capture 16-bits per channel.
What's important here is that these devices provide a finer degree of numeric values between pure white and pure black per color channel. A 12-bit file can define 4,096 steps from black to white, a far cry from the 256 steps in an 8-bit file. When you edit these documents, you have plenty of head room due to the extra data provided, so ultimately you can send the best 8-bits per color of data to your printers.
The downside you ask? Well, any file that's more than 8-bits per color channel will be larger in size. That means each file will take longer to process and take up more space on your hard drive. A 50 MB 8-bit file would be 100 MB, or twice the size in 16-bit. Also, prior to Photoshop CS, you had far fewer useful image-editing functions that would operate on documents with a bit depth greater than 8-bit. Today, nearly all the necessary image-editing controls in Photoshop work equally on 8-bit or 16-bit documents. Not all third-party plug-in filters can operate in higher bit-depths, which may be an issue in your workflow.
Note that Photoshop lumps all documents larger than 8-bits per color into a single category it calls 16-bit. Your digital camera may produce a 12-bit file, and if you opened this into Photoshop, it would tell you it's 16-bit for the sake of simplicity. For this reason, I prefer to use the term “high bit” to differentiate between an 8-bit file and a file that's higher in bit-depth, which may or may not be a true 16-bit file. The important consideration is that you have more than 8-bits of data with which to edit your documents so you have more editing headroom today and in the future. If you must know, Photoshop is really operating on 15-bits of data, but we don't need to go there.
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