Understanding Metering - Abhi Madangeri

Digital cameras have different modes that facilitate control of exposure by allowing users to play around with the basic aspects of exposure like Shutter Speed, ISO and Aperture. The different camera modes apart from the auto modes (which takes away control from the photographer and makes creativity camera dependent) are Aperture Priority (A/Av), Shutter Priority (P/Tv) and Manual (M) mode.

Let us look at these modes on a high level before we get to the main topic of discussion - metering. 

Aperture Priority: The camera decides the shutter speed but user can control aperture value (Av) and ISO.
Shutter Priority: The camera decides the aperture but user can control shutter speed (Time Value Tv) and ISO.
Manual: User has complete control over shutter speed, aperture and ISO.

So on what basis does the camera control these settings?
Metering is the technique used by the camera to measure light and derive the settings for optimal exposure. Cameras have an integrated light meter which measure light and this value is used by the camera to set exposure values. To accomplish this, cameras have metering modes which work on different algorithms to calculate exposure.
Camera always tries to expose for the middle tone (18% Grey) and in doing so, it tends to underexpose white and overexpose black rendering them as a middle tone (grey).

What are the different metering modes?
The common metering modes which can be set by the user are:
Evaluative Metering (Matrix Metering): In this mode, the camera measures light across the frame, but calculates exposure by prioritizing the active focus point area.
Center Weighted Metering: Measures light throughout the frame like evaluative mode, but prioritizes light reflected off the center of the frame.
Partial Metering: The camera measures light on a small circular area around the center point (area little larger than center focus point) to calculate exposure.
Spot Metering: Here the light at a point in the center of the frame (center focus point) is used to measure exposure.

Based on the above explanation, try to identify the metering mode in the below illustration.


Now, if you are wondering how does metering affect exposure values or if you are one of those who never used metering modes but only played around with Shutter Speed and Aperture, here is a simple experiment with metering modes which can be useful to clear those doubts.
I took pictures of a green object on black and white sheets with spot and evaluative metering modes by metering some for the sheet and some for the green object. In the below image which is the darkest and brightest block?


Here are a series of images and a table for comparison:



In both spot and evaluative when I expose for the background, the shutter I get is almost the same. Camera overexposes dark BG and underexposes bright BG to get an almost middle tone.
Now, when I expose for the subject using spot metering, camera meters on the subject and doesn't take the background into account, so the background is much closer to white and black tone.
But the same setup using evaluative metering brings only a slight change in exposure (similar to exposing on BG), because the camera exposes for the complete frame and also takes the focus point (subject) to calculate the exposure.
The darkest and brightest block above was the background when spot metered on subject.



This small experiment gives a fair idea to beginners on how metering works and understanding it will allow you to select metering as per your needs and situation. Spending some time on understanding metering modes and its impact on final result will help in analyzing the lighting conditions and choosing the appropriate metering mode. You can try a similar experiment on different metering modes and lighting conditions to understand the metering modes and its impact on exposure values.

Many such discussions happen on Bracketing Circle Facebook group. All those interested can join the group and be part of the knowledge sharing experience.



0 #1 Prajnyan Mishra 2016-03-24 17:25
I had a lot doubt on metering. Why do we need? why people use extra metering device in fashion photography?Goo gled a lot.people simply answered to find correct exposour. That is fine, but never got the logic. And this topic gives the logic and I found it mathematical like f stop and diameter and focal length formula.
Thanks Abhi Madangeri​ for this beautiful explnation. I feel like you are an optics master.
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