Practical Artistry: Light & Exposure for Digital Photographers

106 ISO and Noise

Understanding Light Sensitivity

Understanding Light Sensitivity

Photographers coming to digital from film are astound-

ed at digital’s powerful and flexible ability to “dial in”

light sensitivity (called ISO). With a film camera, you

are limited by the sensitivity to light of the film in the

camera. Not so with digital!

It seems that it should be possible to tell a digital cam-

era

how sensitive to light its sensor should be by setting

a menu item or twisting a dial. In fact, using a higher

ISO doesn’t actually increase a sensor’s inherent sensi

-

t

ivity to light. When you increase the ISO, you are am-

p

lifying the signal captured by the sensor. All captures

are subject to some noise r

elated to heat created by the

sensor and the theoretical limitations of electronic

circuitry. Amplifying the signal increases the baseline

noise present in all photos.

So what, exactly, is noise? As a general matter, from

the viewpoint of information theory as applied to com-

p

uting and communications, noise is data not used to

transmit a signal. In other words, digital noise is sim-

p

ly an inherent (and unwanted) effect.

With film, the higher the sensitivity rating of the film,

the more “grain”—visible particles that are residues

o

f the silver halide chemistry that makes film sensitive

to light. With digital, as sensitivity is increased, noise

(rather than grain) also increases. ...