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Bit depth

From: Photoshop CS6 Quick Start for Photographers

Video: Bit depth

I assure you there was no pun intended when I decided to use this image of a whale diving deep into the waters off Alaska to discuss the issue of bit depth. Bit depth has nothing to do with depth in the way you might normally think of it, but rather the number of colors actually available for an image. Generally speaking, you have two options for bit depth, the 8 bit per channel mode and the 16 bit per channel mode. A bit is a value used by a computer which essentially means on or off. It's often described as the ones and zeroes of data, because those values, on or off, are often described as being a one or a zero, effectively a yes or a no. If we have eight bits, and each bit can have two possible values, essentially one or zero, that means that there are 256 possible tonal values per channel. I'll go ahead and open up the channels panel, and we'll take a look for example at the red channel.

Bit depth

I assure you there was no pun intended when I decided to use this image of a whale diving deep into the waters off Alaska to discuss the issue of bit depth. Bit depth has nothing to do with depth in the way you might normally think of it, but rather the number of colors actually available for an image. Generally speaking, you have two options for bit depth, the 8 bit per channel mode and the 16 bit per channel mode. A bit is a value used by a computer which essentially means on or off. It's often described as the ones and zeroes of data, because those values, on or off, are often described as being a one or a zero, effectively a yes or a no. If we have eight bits, and each bit can have two possible values, essentially one or zero, that means that there are 256 possible tonal values per channel. I'll go ahead and open up the channels panel, and we'll take a look for example at the red channel.

There are 256 possible shades of gray here If we were working on an 8-bit-per-channel image. And that's because each bit can have two possible values. And there are 8 bits for each channel for each pixel. So 2 taken to the eighth power equals 256. 2 times 2 times 2 repeated 8 times gives you 256. But of course, we're working with an RGB image, so there are three channels, red, green, and blue. That means that we have to take the 256 tonal values available per channel and multiply it by itself, three times.

In other words. to cube that 256 value. 256 cubed equals over 16.7 million possible color values. And it just so happens that 16.7 million is the number of colors believed to be how many the human visual system can discern. And that's a lot. So why would we ever take advantage of this 16-bit per channel mode? First, let's consider how many colors we might have available in that 16-bit per channel mode. To start with, each of the individual channels in a 16-bit per channel mode can have a large number of possible tonal values.

It's two raised to the 16th powers. So two times two times two, a total of 16 times. The result is sixty five thousand five hundred and thirty six possible tonal values per channel. That's quite a lot, and those numbers get even bigger when you remember that we have three channels to contend with. So, we have to take 65,536 and cube it. When we calculate 65,536 cubed, the result is over 281 trillion possible colors.

That's a big number. A very big number. And in some ways you can appreciate, I'm sure, that that's a bit of a theoretical number. Because you're not really going to produce that many colors in most images. In fact, Photoshop generally calculates numbers based on 15 bits per channel. And furthermore, your camera is probably not recording 16-bit per channel information. Many cameras record analog to digital information at 12 bits per channel, with most current cameras converting at 14 bits per channel. Still, some cameras do indeed capture at 16-bit per channel.

But when you convert that raw capture data, you need to put it in either in an 8-bit per channel or 16-bit per channel package. So if your camera is 12-bit or 14-bit, the image is still going to be a 16-bit per channel image, by default, even though it contains less than that full range of possible values. So why would you work in the 16-bit per channel mode? First and foremost, it has to do with detail and image quality. Because we have so many more values available to us and so many more tonal and color values for our pixels, there's a broader range available. And therefore, we can apply stronger adjustments without reducing the level of detail in a photo.

Every adjustment you apply to an image causes some loss of information, you can appreciate this by thinking of a strong contrast adjustment, for example. With a strong contrast adjustment an 8-bit per channel image might start to show some posterization, whereas a 16-bit per channel image might not. Of course, in many cases, because of the way our monitor displays work, it's very difficult to see the actual difference. But I can show you what posterization looks like. I'll go ahead and add a Posterize Adjustment layer. And you can see here posterization, A very harsh transition between tonal and color values.

A photographic image generally has very smooth gradations. But as we reduce the range of color and tonal values, and apply strong adjustments, we start to see posterization, and that can obviously be a problem. Working in the 16-bit per channel mode helps preserve as much information as possible so you won't run the risk of posterization. That said, there's really no benefit to choosing Image, Mode, and then changing an 8-bit per channel image to 16-bit per channel. If you didn't have the information to start with, you're not going to get any real benefit by converting the image to 16-bit per channel.

I do recommend converting your raw captures to 16 bit per channel. And continuing to keep those images in the 16 bit per channel mode while you're working on them. It does create a larger file size, but I think it's worth it in terms of helping to make sure you're going to retain maximum detail and quality in all of your photos.

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Photoshop CS6 Quick Start for Photographers

31 video lessons · 2415 viewers

Tim Grey
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