Bit Depth vs. Dynamic Range in Layman’s Terms

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Hi friends! A reader recently sent me an e-mail asking me to explain the difference between bit depth and dynamic range, so I thought I would post the explanation here for everyone’s benefit.

Bit Depth

A bit is defined as the smallest unit of data. It’s binary – it can be either 1 or 0, “on” or “off”. Eight bits equals one byte, a thousand bytes equals one kilobyte (KB, or K), a thousand kilobytes equals one megabyte (MB), and a thousand megabytes equals a gigabyte (GB).

- An 8 bit image uses 8 bits of information to represent a single color.
- A 12 bit image uses 12 bits of information to represent a single color.
- A 24 bit image uses 24 bits of information to represent a single color.
- A 32 bit image uses 32 bits of information to represent a single color.

So what on Earth does all of THAT mean? It means that the higher the bit depth, the more color nuances are present in the image.

Let’s put it in layman’s terms in a purely illustrative fashion.

- An 8 bit image will be comprised of red, orange, yellow, green, blue, purple, black, and white.

- That same image as 16 bits will be comprised of red, orange, yellow, green, blue, purple, black, and white… but ALSO ecru, goldenrod, lemon, teal, azure, orchid, gray, and cream.

- The same image as 24 bits will be comprised of all of the colors of the 16 bit image PLUS crimson, tangerine, amber, mint, cornflower, lavender, dark gray, and vanilla.

- And so on.

Because an image of high bit depth uses so many bits of information to comprise a single color, the higher the bit depth the larger the image file size.

Dynamic Range

Dynamic Range refers to the ratio between maximum light intensities and minimum light intensities (lights and darks, highlights and shadows) that your camera can capture. A camera with a large sensor captures more of the dynamic range (you can see more of the shadows and highlights) than smaller sensors. So, a photograph will have a high dynamic range if there is a lot of variation in the lights and darks present in the image (lots of different colors, spots with reflected light, areas in the shade), and a low dynamic range if there is little variation between the lights and darks (monochromatic or following a single color scheme).

To achieve an image with a high dynamic range without buying a very expensive large sensor camera, create HDR images by taking several photos of the same scene at different exposure levels, then combine them in post-processing (this is why you have to convert to a higher bit depth when merging HDR photos – more shades of light also means more nuances of color). The dynamic range can also be compressed by adding light to dark areas (i.e. use a flash to pull a dark subject out of the background). Filters can also be used to darken portions of the scene, which would also compress the dynamic range.

Your camera’s dynamic range capabilities aren’t anywhere near the capabilities of the human eye. That’s why so often times the photo you take isn’t exactly what you saw “in real life”. The camera can only capture a certain range of light intensities. To once again illustrate in layman’s terms, think of dynamic range as a ruler. Your eyes can see everything from 1 to 12. But some cameras can only see 3 to 10, or 4 to 10, or 2 to 9. Click to enlarge:

Two Sentence Summary

To put it in even more basic terms, bit depth is related to color and dynamic range is related to light. So while they effect one another, they are not the same thing.

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  • Renan Le Caro

    We can simplify the question with a b&w picture, it can be ldr (8 bits / pixel) or hdr (ie 32bits / pixel).

    If we then take the histogram of the 2 pictures, they are the same, except that the 8 bit histogram has 256 columns and the 32 bits histogram has 4294967296 columns.

    It means that you can manipulate that 32 bit histogram (level, curves, local adaptation) without getting the noise you would get with the 8 bit image.

    Local adaptation is a way to make a 32bits picture become an 8bit one based on the fact that some quite large parts of the image are dark, and some are bright. It makes the dark and bright areas essentially the same average tone, but uses some visual effects to trick our eyes and make us understand which parts are the bright ones.

    This visual effect is why some hdr pictures (actually ldr gotten from hdr by local adaptation) look artificial.

  • Anonymous

    Thanks for the additional information, Renan!  It was very helpful.

  • Harry Lime

    “To put it in even more basic terms, bit depth is related to color and dynamic range is related to light. So while they effect one another, they are not the same thing.” Is English your first language? If so, back to school, please; bit depth and dynamic range don’t “effect” one another, they “affect” one another…..a lesson from back in, oh, say 6th grade.

  • Fuzzy Navel

    Affect and Effect – No one should get that wrong in 2012. But that doesn’t make you any less of an unwashed dick for bringing it up you tool, Harry Lime.