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Metering modes on canon DSLR with examples

Metering is a process in which a DSLR camera measures the amount of light entering the camera depending on the sensitivity of the sensor (ISO). Once the light is measured, it informs the user to adjust either shutter speed or aperture to get a properly exposed image. Older cameras never had a lightmeter preview, so an external light meter was a necessity. The same external light meter logic applied to film cameras as well. 
In this age of DSLR cameras, there are few quirks about various metering modes available in almost every brand. A metering mode is a way for the camera to determine the correct exposure settings. Your DSLR then tells you to adjust the settings, so that the meter needle sticks to zero. This can be achieved by changing the shutter speed, aperture or ISO settings. Let's see how does metering mode work with examples. 4 images below are taken from a kit lens using Canon 40D DSLR with same shutter speed, ISO and aperture settings. All of them are exactly similar which is expected as no setting was changed except the metering mode dial. 

Evaluative:
                                               F5.6, 1/25 second, 100 ISO. Evaluative 


Partial:
                                               F5.6, 1/25 second, 100 ISO. Partial 

Spot:
                                                  F5.6, 1/25 second, 100 ISO. Spot 

Center-weighted average:
                             F5.6, 1/25 second, 100 ISO. Center-weighted average 

Above 4 images clearly explain that the image exposure would not change just by changing the metering mode dial, settings need to be varied. Look at the 4 images below now, exposure settings were changed for every mode in order to keep the metering needle at centre. The Same spot was used to focus on centre cross point for all the images. Here are the settings for individual shots:

1) F5.6, 1/25 second. 100 ISO. Evaluative

2) F5.6, 1/20 seconds. 100 ISO. Partial.

3) F5.6, 1/15 seconds. 100 ISO. Spot. 

4) F5.6, 1/20 seconds. 100 ISO. Center-weighted average. 


                                               F5.6, 1/25 second, 100 ISO. Evaluative 


                                            F5.6, 1/20 second, 100 ISO. Partial


                               F5.6, 1/15 second, 100 ISO. Spot metering 


                                 F5.6, 1/25 second, 100 ISO. Center-weighted average 

My personal favourite out of the above 4 would be the first image, for the fact that it has somewhat properly exposed background. 

How does it work?
As soon as the in-camera meter is balanced (any mode), the metered area records at 18% gray tone of a gray card. Under normal circumstances, metering would yield the expected results. Most commonly used or default option: Evaluative metering mode (matrix in Nikon). 
Evaluative/Matrix: Takes the whole scene into consideration. Works for 90% of the cases, will make another post on where it fails and the possible workarounds. 
Spot metering: Only considers the AF point exposure which is roughly 2-3 % of the total scenery. Let's say you focus on the subject's forehead while taking portraits, spot metering will ignore the background ( Example: Sun) in this case and expose for the forehead only. 
Partial: Quite similar to spot metering except for the fact that it takes into consideration approx 6% of the total scenery. 
Center-weighted average: Extra focus on the centre while ignoring the corners. It does take into account 75% of the scene. 

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