Understanding metering, part one: introduction | Ming Thein
One of the more important – yet almost always overlooked – aspects of camera operation is metering. Simply put, the meter determines what your final exposure is, and how bright or dark your image looks relative to the scene. Unless you are shooting manual – and even then – the camera’s exposure is determined by the meter. Add the fact that the eyes of a viewer tend to go to the brightest and/ or highest contrast portions of an image first (i.e. this should be your subject) – and it’s clear to see why it’s absolutely critical to understand both how metering works as a fundamental concept and any camera-specific peccadilloes that might exist. The last thing you want is to find that your camera drastically underexposed a once-in-a-lifetime shot of some critically important event because you didn’t know (or forgot) that the meter was extremely affected by point light sources….
How meters work
Depending on which exposure mode your camera is in, the meter will try to find a combination of settings that creates an image that averages out to middle gray in luminance, i.e. the histogram average is around level 127 or thereabouts. There are three exposure parameters the camera can use to control the amount of light reaching the image processor – note that the sensor is also now involved in the process – shutter speed, aperture and digital gain, i.e. ISO. If you fix any one of these variables manually – say by shooting aperture priority at a set ISO – then the camera varies the remaining parameters according to a set of rules in order to achieve the ‘correct’ exposure. If the correct exposure is out of adjustment range – e.g. the required shutter speed for a given aperture is too high – then you’re going to land up with an over or underexposed image. In program mode, the camera controls both aperture and shutter values depending on its preset program; the photographer can usually shift the program to a different combination of values which still yield the same net amount of light hitting the sensor. In shutter priority, the user fixes the shutter value manually, so the camera alters the aperture. In aperture priority, it’s the other way around. In manual mode, the user fixes both values – the only thing the meter can do is display how far off the manually chosen exposure is from the correct exposure, or alter the ISO or flash. If auto-ISO is activated, then the camera will always default to the lowest possible ISO within the specified range in order to keep the shutter speed at or above a certain value – either user selected or 1/ focal length in second. (Note that for some cameras, using manual shutter and aperture values will cause the camera to shift the ISO rather than display the variance from correct exposure.)
See full article on blog.mingthein.com