Editing Exposure value (section)

=== Relationship of camera settings to luminous exposure ===

The image-plane illuminance is directly proportional to the area of the aperture, and hence inversely proportional to the square of the lens f-number; thus

:<math>H \propto \frac {t} {N^2}\,;</math>

for constant lighting conditions, the exposure is constant as long as the ratio ''t''/''N''<sup>2</sup> is constant. If, for example, the f-number is changed, an equivalent exposure time can be determined from

:<math>\frac {t_2} {t_1} = \frac {N_2^2} {N_1^2}\,.</math>

Performing this calculation mentally is tedious for most photographers, but the equation is easily solved with a calculator dial on an exposure meter ([[#CITEREFRay2000|Ray 2000]], 318) or a similar dial on a standalone calculator. If the camera controls have detents, constant exposure can be maintained by counting the steps as one control is adjusted and counting an equivalent number of steps when adjusting the other control.