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When going into an edit process there is much confusion about what color depth should one use. Some pieces of knowledge are more relevant than others and some are not relevant at all. Either way, the selection of color depth in which you edit will have a huge impact on the final editing result.
The purpose of this article is to try and clear up the confusion about bit depth and give you advice on what bit depth to choose when you edit and output your images.
‘BIT DEPTH’ AND ‘BIT SIZE’
A ‘bit’ is a computer term for data storage. It can only contain two values, typically 0 or 1. 8-bit simply means the data chunk is 8 bits in total (or 2 to the power of 8, as each bit can be either ‘1′ or ‘0′). This allows for numeric values ranging from 0 to 255.
Similarly 16-bit means the data size is 16 bits in total. (or 2 to the power of 16) This allows for numeric values ranging from 0 to 65535.
Sidenote: Photoshop does not seem to be using the full range of those 16-bits. If you look at the built in information panel it allows you to swap to 16-bit view and it then shows 0-32768 values. Meaning it would in fact be 15-bits +1. For the purpose of this article this is not that big of a deal though, so I am going to show the difference to 16-bits to keep things simple.
To give you a general idea, a comparison 16 bits can contain 256 times more numerical values then 8 bits. If you were to put it on a graph, this is what it would look like:
BITS PER PIXEL
Bits per channel are pretty easy to understand, it is the number of bit used to represent one of the color channels (Red, Green, Blue). But to complicate things the ‘bit depth’ setting when editing images, specifies the number of bits used for each color channel – bits per channel (BPC).
This means that the 8-bit setting (BPC) is in fact 24-bits per pixel (BPP). Meaning that each pixel can have values ranging from 0 to 16,777,215, representing about 16 million colors.
As the human eye can only discern about 10 million different colors, this sounds like a lot. But if you consider that a neutral (single color) gradient can only have 256 different values, you will quickly understand why similar tones in an 8-bit image can cause artifacts. Those artifacts are called posterization.
Similarly the 16-bit setting (BPC) would result in 48-bits per pixel (BPP). The available number of pixel values here is mind boggling (2^48). More than 16 million times more numerical values then the 8-bit setting. Again, this may seem like an overkill, but if you consider the neutral color gradient again, the maximum amount of tonal values is ‘only’ 65,536.
Note: Photoshop will often show a color value between 0 to 255 per channel regardless of what bit depth you edit in. This is purely to simplify things for the user. Behind the scenes it utilizes the full value range. So, pure green, for example, in 8-bit is {0,255,0} and in 16-bit it is {0,32768,0}.
SHOULD I ALWAYS EDIT IN 16 BITS?
With all the topics of this article you could easily think that editing in 16-bit is always best, and it is definitely not. Very much like shooting in RAW is not always best. It depends on the situation.
- 8-bit is best when you do minor editing, and computer resources is a concern.
- 16-bit is best when you do major editing, on few images, and have the latest computer hardware.
If you still not sure what to chose, then answer these questions:
- Does your computer run slow when you edit your images?
- Are your hard drives full all the time?
- Is the difference between your unedited and edited images minor? Have you used something like an image diff tool to check?
- Is your main output the web?
- Do you edit a large number of images per day?
If you answered ‘Yes’ to any of the questions above, you are most likely better off editing in 8-bit.
Still not sure?
- Do you use the gradient tool when editing your images?
- Do you paint with large soft brushes on your image?
- Is your images of similar tonality and color?
- Does your histogram display gaps in the tonal range?
If you answered ‘Yes’ now, you are actually making use of the extra bit depth, and should consider using the 16-bit color depth setting.
I personally would have liked to see Photoshop support for 10 or 12 bit. I feel 16-bit is overkill for most images.
Q&A
Q. My source image is in 8-bit. Should I convert it to 16-bit while editing it?
A. Probably not. The conversion will not help with your existing tonal graduations and color tones. Though if you are doing any editing that introduce new graduations, or very subtle color variations, you might benefit from converting.
Q. I edit in 16-bit and I still see posterization/banding on my screen?
A. This is most likely because of your display and/or color profiles. Check the section above on limitations.
Q. What about color space?
A. To put it simply, color space determines how the available tonal values are distributed. My quick recommendation is to use Adobe RGB for everything except when exporting for web. You need to convert to sRGB on web images or they will not display correctly for the vast majority of users.
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