Metering
For the T90, Canon developed their most sophisticated light-metering system yet. Although it introduced no novel metering techniques, it assembled the majority of the metering techniques then developed into one easy-to-use system. First, it took the metering options from the New F-1—center-weighted average metering, partial area metering, and spot metering—and makes them available with a press of a button and a turn of the command dial. The New F-1 requires a focusing screen change to switch metering patterns. On the T90, partial area metering uses the center 13% of the picture area, while spot metering uses the center 2.7%.
To these, Canon copied the metering options found on Olympus' OM-4. Multi-spot metering allows the photographer to average up to eight spot meter readings from different parts of the scene. In another feature borrowed from Olympus, separate Highlight and Shadow spot readings could be taken. These adjust the camera's metering decisions to ensure extremes of tonal range are not muted and grey in the final exposure.
Two built-in sensors are used to implement all these metering options. Center-weighted and partial area metering are performed by a double-area silicon photocell (SPC) in Canon's standard location above the eyepiece, while spot metering is performed by another SPC located at the bottom of the mirror box. Light reaches that sensor via a half-silvered area of the main mirror and a secondary mirror located beneath it. The spot metering cell also allows for automatic TTL "off-the-film" flash metering, again borrowed from Olympus.
Notably lacking is what is now the most common metering mode on SLR cameras, matrix metering. Nikon had introduced this in the FA in 1983, but Canon did not follow suit until 1987's EOS 650.
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