Michael Freeman unravels the intricacies of the ‘exposure triangle’
It’s not enough to know that there is an exposure triangle; you have to make use of it too. Three camera settings control the brightness of the image you capture (aperture, shutter speed and ISO) and together they enable you to choose a combination to get any particular exposure, hence the term ‘exposure triangle’.
Two of the settings (aperture and shutter speed) control how much light enters the camera to strike the sensor while the third, the ISO setting, amplifies the signal. Nikon’s 16-megapixel full-frame CMOS sensors are arguably the world’s top performers, offering a maximum ISO setting of 409,600 in the D4s, but most of the credit for this low-light performance goes to the way in which the signal is processed and the noise limited.
A flexible triangle
■ Aperture, shutter speed and ISO are inextricably linked in the exposure triangle. Raising one of these three means that you’ll need to lower one, or both, of the other two to ensure the same exposure. In these examples, up means more exposure (a wider aperture, longer exposure or higher ISO), while down means less exposure (so a narrower aperture, shorter exposure or lower ISO setting).
Because noise, which is the visual equivalent of audio hiss, has no redeeming qualities at all, and just degrades the image, the exposure triangle is lopsided – there are good reasons for increasing or decreasing both aperture and shutter speed, but raising the ISO is almost always a last-resort option. This makes choosing the balance between the settings interesting and, well, a skill.
Pick your priorities
Using the exposure triangle means knowing clearly what your priorities are for any given shot. Apart from controlling the light, aperture and shutter speed each do another job for the image: aperture affects depth of field, while shutter speed affects motion blur, and you may want less or more of either in any specific shooting situation.
1/125 sec at f/4, ISO1600 During a funeral in Yunnan, China, light levels in the main interior, lit only by a doorway and window, were low. I set my ISO to 1600, relying on the D4’s low-noise
capability, and that left me a 1/125 sec shutter speed to freeze any movement.