Learning Photography Course — 2. The Exposure Triangle

Module 2 of the Learning Photography Course. Aperture, shutter and ISO are a hole, a duration and an index — three things with nothing in common until they are all measured in stops. This module builds that single unit and then uses it: why the f-number sequence runs 1.4, 2, 2.8, 4 and what the square root of two has to do with it; why a bigger f-number is a smaller hole; the two kinds of blur and why only one of them has more than one cure; what ISO actually is, according to ISO 12232:2019, and why it is not sensitivity; equivalent exposures and the two-decisions-one-consequence method; the shooting modes; what a reflected-light meter assumes and the two scenes where that assumption fails; exposure compensation, including the most common unforced error in photography; reading a histogram, and why clipped highlights are gone while clipped shadows often are not; and reciprocity — the law that makes the whole system work, and the published, measured point at which film stops obeying it.

Figures are sourced. Where a number could not be traced to a primary source — middle grey, and stabilisation stops — the module says so and declines to assert it.

Educational documentary. Not financial or investment advice.