Digital Photography in Available Light

essential skills: digital photography in available light

Additional information

Distribution of data

Most digital imaging sensors capture images using 12 bits of memory dedicated to each of the three

color channels, resulting in 4096 tones between black and white. Most of the imaging sensors in

digital cameras record a subject brightness range of approximately fi ve to eight stops (fi ve to eight

f-stops between the brightest highlights with detail and the deepest shadow tones with detail).

One would think that with all of this extra data the problem of banding or image posterization due

to insuffi cient levels of data (a common problem with 8-bit image editing) would be consigned to

history. Although this is the case with midtones and highlights, shadows can still be subject to this

problem. The reason for this is that the distribution of levels assigned to recording the range of tones

in the image is far from equitable. The brightest tones of the image (the highlights) use the lion’s

share of the 4096 levels available whilst the shadows are comparatively starved of information.

Shadow management

CCD and CMOS chips are, however, linear devices. This linear process means that when the

amount of light is halved, the electrical stimulation to each photoreceptor on the sensor is also

halved. Each f-stop reduction in light intensity halves the amount of light that falls onto the receptors

compared to the previous f-stop. Fewer levels are allocated by this linear process to recording the

darker tones. Shadows are ‘level starved’ in comparison to the highlights that have a wealth of

information dedicated to the brighter end of the tonal spectrum. So rather than an equal amount

of tonal values distributed evenly across the dynamic range, we actually have the effect as shown

above. The deepest shadows rendered within the scene often have fewer than 128 allocated

levels, and when these tones are manipulated in post-production Photoshop editing there is still

the possibility of banding or posterization.

An example of posterization or banding

Distribution of levels