Seeing the light
How does your EOS camera measure the light to set the exposure for each picture you take?
EOS cameras measure the light and adjust the exposure to suit every time a picture is taken. Even in manual mode the camera measures and offers a meter reading for you to base your exposure on.
EOS cameras have up to four metering patterns; Evaluative, Partial, Spot and Centre-weighted. The default Evaluative employs clever intelligence to analyse the frame in multiple sections to determine the correct exposure. Partial and Spot metering take the measurement from the centre of the frame, around 10-15% of the frame for Partial and 2-3% for Spot. Centre-weighted metering is a throwback to the film era, and biases most of the metering in the centre, but includes some of the rest of the frame.
Cameras measure the brightness of light reflected from the subject, which can lead to under- or overexposure; polar bears in snow come out too dark, while black cats in coal cellars are too light, requiring exposure compensation. A grey card is ideal to meter from, though green grass is a fair substitute.
Metering is activated by half-pressing the shutter button, but only locks the exposure when One-shot AF and Evaluative metering is used. This allows the metering to determine the exposure before you recompose your scene. For other modes you’ll need to press Ae-lock button – the one with the * icon – to lock the exposure. Once the picture is taken, the locked reading is cancelled if you remove your finger from the shutter button.
Many cameras now include an Ae-lock With Hold function, which keeps the meter reading locked for as long as the camera is powered on. To access it, you need to assign it to one of the camera buttons with Custom Controls. I find it great for motorsports; take a Partial meter reading from the track and press the Ae-lock With Hold button before the cars come into shot, so that the sequence of pictures that follow are all with the same exposure, regardless of the colour of the cars.