As discussed in part 1 of the blog on exposure in digital photography I feel the widely used “bucket” analogy does not explain the ISO setting clearly. We will here via three figures gradually expand the object bucket to use “watering the lawn” to explain exposure. watering the lawn entails providing enough water by increasing the duration or increasing the diameter of the water spray head. This is similar to the variables in filling a bucket. But the amount of water on the lawn that will actually penetrate to the roots of the grass will depend on the porosity of the soil i.e. spiking the lawn will increase penetration. So the soil is then more “sensitive”to the water analogous to the sensor becoming more sensitive to light as the ISO increases. So now we have an analogy with the three settings needed to understand the “exposure triangle” of aperture, shutter speed, and ISO values.
Finally a brief discussion on how ISO is physically altered in your camera. With the camera sensor sensitivity is modified by amplification of the available light signal to a greater electrical signal. This amplification is similar to that in a HIFI system: the audio amplifier will increase the sound level. But amplification of sound or light will potentially also increase the “noise”. This is termed the signal/noise ratio. With sound you hear an increased hissing noise relative to the music. With light, increasing the amplification i.e. higher ISO more noise is produced i.e. a scattering of light (“speckling”) called luminance noise or blocks of colour called chrominance noise.
Please refer to the excellent website “Cambridge in colour” for further detailed explanations of all aspects of photography