Bit-Depth

This Won’t Hurt a Bit

Many photographers new to the concepts of digital image processing, when they encounter the phrasebit depth their eyes glaze over. They know that it has something to do with image quality, and that more bits are somehow better, but that’s about it. Let’s look at this relatively simple but often misunderstood topic and see if we can make sense of it.

We should get some basics out of the way first. Abitis the smallest unit of data. It can be1or0,blackorwhite,onoroff. 8 bits comprise abyte. A byte (or 8 bits) can therefore represent 256 different states; 2 ^8th power.

Most of the digital world operates on8 bitimages. This includes your inkjet printer and usually your monitor. What this means is that 8 bits of information are used to create the image that you usually see. 

A Short Digression

This is why you don’t want (for example) to print a B&W image on an inkjet printer using just black ink. The printer would only be able to lay down 256 shades of gray, from black to white and these are not nearly enough for a decent image. Instead you should print using colour inks as well, which means that all three primary colours ( Red , Blue and Green ) will be mixed together to create16 millionshades of gray (256X256X256). More than enough.

8 Bit or 24 Bit, Which is It?

What gets confusing at times is that while we speak of an8 bit image, we’re usually really talking about one that consists of3colours and therefore it is also referred to as a24 bit image(3X8). 

High Bit Rates 

Instead of using just8 bitsto represent a single colour we can instead sometimes use12or16bits. A16 bitimage can handle65,536discrete levels of information instead of the256levels that an8 bitimage can. As you can imagine the increased degree of subtlety that this makes available is dramatic. And, just as an8 bitimage is actually24 bitwhen we’re dealing with colour, a16 bitimage is actually called48 bit(16X3) when speaking of a 3 colour composite. A48 bitimage is capable ofbillionsof colours. 

So, interesting theory, but why is this important to photographers?

Scanners and Imaging Chips

All imaging chips in scanners and digital cameras are capable of producing24 bit(8X3) images. Some are capable of36 bit(12X3) capture, and some high end scanners and cameras can even capture48 bit(16X3) images. 

Why would you want to, and what are the problems and limitations, if any?

A Problem Image & Missing Functions

Storm Over Mt. Hillers, Utah.  April, 2001

The key advantage of using a high bit image is that when you apply aLevelsorCurvesadjustment inPhotoshopyou are doing so to much more data than when you work on a low bit image. Changing the tonal range of a high bit file that has 65,536 levels Vs. a low bit file with only 256 levels means that when the data is compressed or stretched by usingLevelsorCurvesthere is more data to work with. A low bit image simply leaves gaps (the toothcomb effect as seen below) and this leads toposterization. Posterization manifests itself as abrupt jumps in colour or brightness level. 

The above photographprovides a good illustration of this. The contrast range was huge, from the almost burned-out area near the sun to the deep shadow area of the base of the mountain. There was no time for fancy footwork. From the time we crested a hill and saw this scene till it it disappeared was no more than 2-3 minutes. I had time to shoot about a half dozen frames with myMamiya 7 IIand the first one was the best compositionally.

I scanned this transparency on myImacon Photoscanner in48 bitmode because I wanted to gather as much data to work with as possible. Once I finished adjustingCurvesandColour BalanceI then converted to24 bitfor printing and web display. Below is what the Histogram looks like after all the adjustments had been made.

The first aspect of thedark sideof doing a high-bit scan is that the file size is doubled. Even cropped by some 30% before scanning, this particular scan of a 6X7cm transparency was more than 300 megabytes in size at 3200 dpi. This means that even myDual Processor 533 Mhz Mac G4with 1 Gigabyte of RAM faces a serious challenge working on this file inPhotoshop

The second downside is thatPhotoshopis limited in what functions are available when working with32or48 bitimages. The most important of these limitations is the inability to utilizeAdjustment Layers. If you’ve read my tutorialInstant Photoshopyou know that I regardAdjustment Layersas vital to effective workflow. You should useLevelsorCurveson your high-bit images and then when these basic adjustments are done convert the file to8 bitusingImage / Mode / 8 bits/channeland do the rest of your image processing in that mode.

What I do is first save my hi-bit scan with a name that includes the notation that it’s RAW, or 48 bit. I’ll put a couple of these on a CD-R and then salt them away. Since a CD-R costs less than $1 and takes up little physical space this is a simple and easy way to avoid having to retrieve the original transparency or negative and do another 30 minute scan if you ever want to rework the image in the future.

Is it Worth It?

Since high-bit images are very large, slow to scan and limited in what one can do with them, I don’t do all of my scanning in this mode. I only do so when a look at the transparency shows that I’m going to have some problems with the shadows. When dealing with digital camera files (such as those from theCanon EOS D30) I usuallydoimport48 bit RAWimages, since I want to maximize my advantages in every way possible. The choice is yours.

This tutorial joins previous ones on this series, includingUnderstanding ResolutionandUnderstanding Sharpness.

A few weeks after this article was first published my colleagueIan LyonsofComputer Darkroompublished a related article on his site calledEditing Hi-Bit Images. I highly recommend it to anyone interested in learning more about this subject.