Metering
While the Nikon D90’s 3D Colour Matrix Metering II system uses data from its Scene Recognition system and a single RGB sensor to determine exposure settings, the 63-zone Focus Colour Luminance (iFCL) metering system in the EOS 550D uses two sensors to gather luminance and colour information, as well as subject distance information from the autofocus system.
One of the Canon sensors is sensitive to red/green light, while the other is sensitive to green/blue light. This is designed to make the metering system less influenced by red light in tricky lighting conditions.
In addition, both cameras have centreweighted and spot metering options, with the D90 providing much more control over the centreweighted mode than the EOS 550D, as users can specify whether to give 75% weighting to a 6mm, 8mm or 10mm circle in the centre of the imaging frame. The D90’s spot metering option also offers a greater level of precision as it covers just 2% of the frame (a 3.5mm circle), whereas the EOS 550D offers 4% spot or 9% partial metering.
Both cameras’ general-purpose evaluative metering systems have the ability to surprise (generally in a good way) by not being unduly phased by an especially bright or dark object in the scene.
However, it is still advisable to keep an eye on the histogram views when there are highlights that need to be preserved or shadows that may become too dark. It pays to be especially careful with the EOS 550D when the subject under the active AF point is lighter or darker than a midtone.
