Double & half

ISO 400 is twice as sensitive to light as ISO 200, therefore it is 4 times as sensitive as ISO 100, and half as sensitive as ISO 800.

Although the f-stop seems irregular, f8 is double the size of f11 and therefore allows in twice the amount of light. Conversely f16 lets in half the amount of light as f11.

1/500 is twice as fast as 1/250 and therefore halves the duration that the film is exposed to light. Vice versa, 1/125 is half as fast as 1/250 and therefore doubles the duration that the film is exposed to light.

This is essential knowledge for calculating exposure mentally. For example, if we study the LV tables we know that using ISO 100 film, f16 at 1/125 gives the correct exposure in daylight (Sunny 16). Given that all factors either double or halve we can calculate which settings we need to get the correct exposure at ISO 200. ISO 200 is twice as sensitive to light as ISO 100 so we need half the amount of light. We have two options to halve the amount of light; either halve the shutter speed to 1/250 and stay at f16, or keep the shutter speed at 1/125 and stop down to f22.