To write with light i.e. photography, sufficient light energy (photons) must reach the sensor. As the light can vary e.g. outdoors or indoors, cloudy or sunny day, sunrise & sunset, or midday one, must be able to adjust the camera settings to capture enough light in different r i.e. obtain the correct exposure. This is done by altering one or more of the following three settings: aperture size, shutter speed, and the ISO value (sensor sensitivity). The interaction of these settings is often explained using the bucket model.
The site referred to is fantastic and I have learnt a great deal on all aspects of photography there, but I feel the bucket model is inadequate to explain exposure clearly. It explains two variables aperture diameter and shutter speed simply and clearly but the role of the ISO is not clear in the bucket model.
We modestly suggest that a better model is “watering the garden lawn” as that allows the porosity of the ground to be used as an analogy for sensor sensitivity. This makes changes in the ISO value easier to understand. At least I hope so. This will be discussed in the next blogs.