14-bit A/D Conversion in DIGIC 4 Cameras

March 28, 2011

14-bit A/D Conversion in DIGIC 4 Cameras
To the critical user, the immediate benefits of 14-bit vs. 12-bit A/D conversion include the fact that there’s more brightness and color information in each RAW file

Canon’s state-of-the-art DIGIC 4 image processor, found in both the 21.1MP EOS 5D Mark II and 15.1MP EOS 50D digital SLRs, increases the speed and performance of both cameras, and enables other advanced features such as Face Detection, High ISO Noise Reduction, Peripheral Illumination Correction, Auto Lighting Optimizer functions, and 1080p Full HD movie recording (on the EOS 5D Mark II). These enhancements are relatively easy to demonstrate, and they improve a photographer’s chances of capturing high quality images in difficult lighting or environmental conditions. Equally important are the image quality benefits provided by the DIGIC 4’s integrated 14-bit Analog-to-Digital (A/D) converter. But what is an A/D converter, and in what ways does a higher bit-depth conversion improve image quality?

A/D Conversion Explained:

Simply put, an A/D converter samples the analog electric signals from the camera’s CMOS or CCD image sensor, which vary in intensity based on the number of photons captured in each pixel, and converts them into digital data consisting of 0’s and 1’s. In the case of the EOS 5D Mark II and 50D cameras, the raw digital data produced by the A/D converter is fed directly to the DIGIC 4 image processing circuit, which does the mathematical “heavy lifting” of converting the data to a usable image.

Here is a very simple analogy: The original analog signals are like an old-fashioned clockface -- the moving hands progress through the second markers, indicating every instant of passing time: In other words, the signal is stepless. For a digital camera to function, these analog ('stepless') signals must be converted into digital signals with definite, distinct steps. Using the clock analogy again: This digital signal acts more like a modern digital clockface, with a numeric display changing to indicate the passage of time, but without continuous movement between seconds.

Several factors may affect the strength or purity of the analog signals reaching the A/D converter, such as electronic noise generated by the image sensor or its readout circuitry, or electronic noise generated by the signal amplification which occurs at high ISO speed settings. These factors primarily affect the dynamic range of the image, and will be explored later.

A major characteristic of any A/D converter is its resolution or “bit depth,” in other words the number of discrete digital values it can produce based on the range of analog values it receives. Each discrete digital value is called a bit. The higher the resolution of the A/D converter, the greater its bit depth. In the case of a digital camera, greater bit depth means finer tonal gradations between pure black and pure white. This is clearly important for digital photography, since most pictorial images are intended to be as lifelike as possible.

A [hypothetical] basic 1-bit A/D converter acts like a simple light switch, and allows for only 2 conversion states, either off or on. Image data is recorded as either 0 (off) or 1 (on), and in imaging terms describes either a black (off) or white (on) pixel. Therefore a monochrome camera or scanner with a 1-bit A/D converter could only produce an image with two shades (2x1=2)—either pure white or pure black. Good examples of real-world devices that can perform 1-bit A/D conversion would include fax machines, scanners and copiers, which are typically set to 1-bit mode when copying simple text-only documents.

A 2-bit A/D converter allows for up to 4 states (2x2=4): (0,0) off or black; (0,1) dark gray; (1,0) light gray, and (1,1) white. If a camera or scanner had a 2-bit A/D converter, it would only be able to capture images with 4 tonal gradations. As you may have deduced, the bit-depth rating is logarithmic and scales by the power of 2, so the number of total shade combinations (X) = 2Y (where Y equals the bit depth rating).

 

 

Bit Depth

Shades of Gray (Total)

1

2

2

4

4

16

6

64

8

256

10

1,024

12

4,096

14

16,384

 

Most early color digital cameras included 8-bit A/D converters, which allowed for up to 256 shades (28=256) in each of three color channels (typically red, green and blue, or RGB), totaling up to approximately 16.7 million color combinations (2563 ≈ 16.7 million).  However, those early cameras only stored image data in JPEG format, which not only has an 8-bit limit, but also uses a lossy compression algorithm that permanently throws away some image information  in the process. As a result, the 256 shades per color channel created by the 8-bit converter were downsized to make room for more photos in memory. In the early days of digital photography, displayed or printed JPEGs typically wound up with only 6-bits of data or less per channel, leading to color banding and other JPEG compression artifacts.

RAW data files are “containers” that aren’t limited to holding 8-bits of data per pixel, and can store whatever the A/D converter is capable of producing. However, unlike a JPEG image, the brightness data from the sensor and A/D converter is not processed into a finished image.  Instead, it is losslessly compressed, and then written to the camera’s memory card. To generate a usable image, the RAW  data must be processed with special software in your computer. RAW image data takes up more space than a JPEG but less space than an uncompressed TIFF file, and contains much more image information than either. 

Some readers may wonder about in-camera TIFF files, especially as an alternative to unprocessed RAW files.  After all, most RGB TIFF files are uncompressed, finished, and ready-to-use.  However, there are several downsides to the idea of in-camera TIFFs. For one thing, standard in-camera TIFF files are typically limited to 8 bits of information per pixel instead of 12 or 14 bits, so they are inferior to RAW image data in terms of tonal gradation. Another downside to uncompressed TIFF files is file size: With a 12 million pixel camera, an 8-bit RGB TIFF image would produce a file size of about 36MB on the memory card for every shot — with none of the added flexibility or additional tonal information available in even a 12-bit RAW file. If the camera offered the option to record 16-bit TIFF files, the situation would be even worse because the file size would double to 72MB per image! These huge files would drastically impair camera burst performance and squander memory card storage capacity, with less image quality compared to RAW files.  As a result, TIFF recording is rarely an option in modern digital cameras. However, TIFF is still the preferred recording format for storing post-processed images.

The DIGIC 4 image processor’s 14-bit converter (with 16,384 shades per channel) is capable of higher precision when describing the tonal gradations in a scene than less sophisticated 12-bit converters (with only 4,096 shades per channel). This extra image information might be overkill if it weren’t for the advanced CMOS image sensors found on both the EOS 50D and EOS 5D Mark II. The pixels on both of these sensors have improved fill factors and lower read noise than those in previous Canon sensors in their class, with the larger-sized pixels on the EOS 5D Mark II providing low-noise performance that’s a notch better than the EOS 50D at every comparable ISO setting. In combination with the camera’s lower levels of background noise and amplifier noise, the EOS 5D Mark II’s 14-bit A/D converter helps to provide smoother tonal gradations, along with the potential to increase detail in bright highlights with the Highlight Tone Priority option.  The EOS 5D Mark II also maintains a slightly higher dynamic range at high ISOs than the EOS 50D and the older EOS 5D it replaces.

The expanded dynamic range provides increased detail in deep shadow areas and a superior ability to open up shadows in post processing without banding and excessive noise. Detail is also improved in highlight regions, but the difference is less noticeable as a result of inherently lower noise levels in bright tones and highlight areas. However, having extra bit depth and detail in highlight areas allows a photographer to adjust exposure and contrast in post processing to reduce blown out areas without blocking up shadow areas.

The Benefits of High Bit Depth in Digital Cameras:

To the critical user, the immediate benefits of 14-bit vs. 12-bit A/D conversion include the fact that there’s more brightness and color information in each RAW file (and initially, in the first steps of JPEG conversion).  With this added information come some important benefits.

One, the potential for smoother tonal transitions, was just mentioned above.  Any time you have a scene with a single “color” that moves from light to dark or vice versa, this is a benefit — whether it’s a wide-angle shot of a clear blue sky or a shot of a car with shiny highlights blending into darker tones along its painted body.  Skin tones in portraits, especially with “hard” light sources like direct sun or on-camera flash, are another area where 14-bit conversion can mean smoother changes from specular highlights to diffused highlight skin tones.

The increased information in each file also allows cameras like the EOS 5D Mark II and EOS 50D to incorporate Canon’s Highlight Tone Priority option.  With much more tonal data, Highlight Tone Priority leverages this information and preserves anywhere from 1/3 to a full stop of additional detail in bright highlights.  It can make the difference between washed-out highlights vs. controlled highlights with visible texture and detail.  And unlike simply changing exposure to darken an image, Highlight Tone Priority does not change the brightness level of mid-tones or shadows.  There’s a chance of a small increase in digital noise in darker areas, but even at high ISOs, it’s a minor difference, and a small price to pay for the potential of added  detail in bright highlight areas. 

With the expanded tonal information provided by the 14-bit per channel conversion, it’s possible for the camera to make tone curve adjustments on a case-by-case basis. In addition to toning down highlight areas, underexposed areas can be lightened. This is the function of the Auto Lighting Optimizer, a new feature found on the EOS 5D Mark II, EOS 50D, and also on the EOS Rebel XSi and EOS 40D cameras. JPEG images can be fine-tuned as they’re processed and recorded; RAW images from these cameras can have tonal adjustments applied when they’re processed in Canon’s Digital Photo Professional software. Users are free to disable this feature, or set Low, Standard, or Strong levels in the EOS 50D and 5D Mark II.

Another technology that these new cameras utilize is built-in correction for lens vignetting, called Peripheral Illumination Correction. Since tones in sky or other light-colored outer areas of images have a lot more tonal information and much finer steps between them, the camera can automatically apply correction to lighten these areas without risking tonal “skipping”, banding, or noticeable increases in digital noise. Both Highlight Tone Priority and Peripheral Illumination Correction can be applied to JPEG as well as to RAW images.

Higher Bit Depth Equals More Colors?

It was mentioned earlier that a digital camera with an 8-bit A/D converter is theoretically capable of capturing images containing up to 16.7 million color combinations (remember, R:256 x G:256 x B:256≈  16.7 million), and storing all those colors in either RAW or JPEG files. In reality, other factors contribute to most JPEG and even RAW images containing less than 16.7 million colors. In addition to noise and data compression issues previously mentioned, the inherent sensitivity of the imaging sensor, the accuracy of the camera white balance system, the power and precision of the A/D converter, and a wide variety of image quality settings play a significant role in the color accuracy and capability of a camera.

But let’s assume most of those factors are normalized, with a sensor capable of recording all colors in the visible spectrum, combined with no background or random noise throughout the system, a perfect white balance mechanism, and standardized image quality settings. Would a camera using a 12-bit A/D converter be capable of capturing an image containing 68.7 billion colors (R:4096 x G:4096 x B:4096= 68.7 billion)? Or better yet, would a camera with a 14-bit A/D converter be able to capture an image with trillions of colors (you do the math!)? Not necessarily. All of these A/D converters might be fairly equal when it comes to reproducing the maximum number of visible colors in a scene from the RAW data. Why? According to the color experts, a human with excellent color vision is only capable of identifying between 10 and 12 million colors—even less than the 16.7 million color combinations captured in an 8-bit system or displayable on a high quality color monitor.

Because of our visual limitations, no imaging device, camera, or scanner can accurately claim to capture or display more than 16.7 million colors, let alone billions of colors. But many products make this claim. The confusion stems from using the word “color”, which can only be accurately defined as a human visual response to light reflected or transmitted by an object, instead of the term “color data combination” or “color combination”. These last two terms describe the ability of an imaging system to produce the same color visual sensation using one or more combinations of Red, Green, and Blue data.

It’s a fact that different color dye or light combinations can produce similar visual sensations, which is why we are able to produce accurate colors in print, even though printing inks differ in their density and color, and printing methods range from silver halide to CMYK printing presses. The same holds true for different display technologies, paints, and clothing dyes. Metameric color pairs are those that appear exactly the same under one light source but different under another, proving that the color sensation we see is being formed by color components with different spectral properties.

A 14-bit A/D converter has the ability to map the 10-12 million true colors captured by the camera into trillions of color data combinations. This may provide a benefit down the road in a color-managed system when converting a RAW file or 16-bit TIFF file for viewing on wide variety of color displays or printing on wide gamut color printers. While a 14-bit A/D converter might not be able to produce any more “visual” colors from RAW data than a 8 or 12-bit A/D converter, its increased precision allows for higher color accuracy and smoother gradations between colors, plus additional details in dark, saturated colors that can be attributed to the increased dynamic range.

The increased precision of the 14-bit A/D converter in combination with the  computational speed and  power of the DIGIC 4 processor also helps images stored in the finest quality JPEG format within the camera, although those improvements are more subtle. When using the RAW+JPEG option, photographers have the best of both worlds. JPEGs can be viewed and e-mailed quickly, while top selections stored in RAW will benefit from the advanced RAW conversion algorithms found Canon’s Digital Photo Professional software.

This is where access to the additional gradations and dynamic range gives serious photographers the ability to adjust shadow and highlight details that would have otherwise been tossed away, or create smooth gradations that might have posterized during the in-camera conversion to an 8-bit JPEG. When satisfied, the higher bit-depth image can either be stored in a non-destructive file format such as a 16-bit TIFF, or output directly to Canon imagePROGRAF ink jet printers using Canon’s advanced 16-bit export modules for Photoshop and Digital Photo Professional software.

As technology progresses, more and more display devices and printers will be compatible with the higher bit-depth image data captured by the EOS 50D, and EOS 5D Mark II, and similar cameras, and use it to automatically fine tune image quality while taking advantage of expanded dynamic range these cameras offer.

 

Tip written by Michael J. McNamara for the CDLC
Photos by Michael J. McNamara, © 2009
Mr. McNamara is the former Executive Technical Editor of Popular Photography Magazine, and the Editor-in-Chief of The McNamara Report.

The CDLC contributors are compensated spokespersons and actual users of the Canon products that they promote.

All images are copyright Mike McNamara

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