Understanding Metering in DSLR

hensil

Guru
Understanding Metering in DSLR

Most people when they buy their first DSLR gets frustrated when some images would come out too bright.

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or too dark

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Until they have learned about the metering system in their camera.

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Every DSLR has something called “Metering Mode”, also known as, “Exposure Metering” or simply “Metering”. Knowing how metering works and what each of the metering modes does is important in photography, because it helps photographers control their exposure with minimum effort and take better pictures in unusual lighting situations. Let find out how this metering works.

What is Metering?

Metering is how your camera determines what the correct shutter speed and aperture should be, depending on the amount of light that goes into the camera and the sensitivity of the sensor. Every DSLR is equipped with a light meter, which is a separate sensor that measures the amount and intensity of light. This is not the same sensor that captures the image.
 

hensil

Guru
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This integrated light meter automatically measures the reflected light from the subject/scene and determines the optimal exposure. The most common metering modes in digital cameras today are:
1. Matrix Metering (Nikon), also known as Evaluative Metering (Canon)
2. Center-weighted Metering
3. Spot Metering (Nikon), also known as Partial Metering (Canon)


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You can see the camera meter in action when you shoot in Manual Mode – look inside the viewfinder


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or the rear LCD

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and you will see bars going left or right, with a zero in the middle,

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If you point your camera at a very bright area, the bars will go to “+” side, indicating that there is too much light for the current exposure settings. If you point your camera at a very dark area, the bars will go to the “-” side, indicating that there is not enough light. You would then need to increase or decrease your shutter speed to get to “0″, which is the optimal exposure, according to your camera meter.
A camera meter is not only useful for just the Manual Mode – when you choose another mode such as Aperture Priority, Shutter Priority or Program Mode, the camera automatically adjusts the settings based on what it reads from the meter.
 

hensil

Guru
Problems with Metering in extreme lighting

Camera meters work great when the scene is lit evenly. However, it gets problematic and challenging for light meters to determine the exposure, when there are objects with different light levels and intensities. This is because DSLR have six stops of Dynamic Range unlike human eyes that has approximately 14 stops dynamic range. For example, if you are taking a picture of the blue sky with no clouds or sun in the frame, the image will be correctly exposed, because there is just one light level to deal with. The job gets a little harder if you add a few clouds into the image – the meter now needs to evaluate the brightness of the clouds versus the brightness of the sky and try to determine the optimal exposure. As a result, the camera meter might brighten up the sky a little bit in order to properly expose the white clouds – otherwise, the clouds would look too white or “overexposed”.
What would happen if you added a big mountain into the scene? Now the camera meter would see that there is a large object that is much darker (relative to the clouds and the sky), and it would try to come up with something in the middle, so that the mountain is properly exposed as well. By default, the camera meter looks at the light levels in the entire frame and tries to come up with an exposure that balances the bright and the dark areas of the image.

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Now imagine a tree in this scene and a person standing under the tree. This is a tricky situation. The shade of the tree renders the person dark. The camera has under exposed this person in the image. This happens because the dynamic range of the scene has exceeded the dynamic range of the DSLR that is 6 stops.

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hensil

Guru
Matrix Metering


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Matrix Metering or Evaluative Metering mode is the default metering mode on most DSLRs. It works by dividing the entire frame into multiple “zones”, which are then all analyzed on individual basis for light and dark tones.

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After reading information from all individual zones, the metering system evaluates the exposure and compares the image data to a database of thousands of pictures and applies that setting.

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You should use this mode for most of your photography, since it will generally do a pretty good job in determining the correct exposure.
 

hensil

Guru
Center-weighted Metering


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Using the whole frame for determining the correct exposure is not always desirable. Now here is the scene explained above where a person is standing under the tree. This is where center-weighted metering comes in handy. Center-weighted Metering evaluates the light in the middle of the frame and its surroundings and ignores the corners.

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Compared to Matrix Metering, Center-weighted Metering does not evaluate the whole scene but only evaluates the middle area of the image.

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Use this mode when you want the camera to prioritize the middle of the frame, which works great for close-up portraits and relatively large subjects that are in the middle of the frame. For example, if you want the person to be exposed properly with bright sky at the back, position the person in the center of the frame take the reading, recompose and shoot.
Or you can shoot a silhouette in a scene like this

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where the sky/background is very bright compared to the building/foreground.

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hensil

Guru
Spot Metering

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Spot Metering only evaluates the light around your focus point and ignores everything else. It evaluates a single zone/cell and calculates exposure based on that single area, nothing else. You can use this mode for bird photography if you need to make sure the bird is exposed properly,

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whether the background is bright or dark, because the birds mostly occupy a small area of the frame.

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One more example

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Also, if you were taking a picture of a person with the sun behind but they occupied a small part of the frame, it is best to use the spot metering mode instead.

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When your subjects do not take much of the space, using Matrix or Center-weighted metering modes would most likely result in improper exposure, if the subject was back-lit. Spot metering works great for back-lit subjects like that.

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Another example to shoot silhouette

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hensil

Guru
How to change camera metering mode

Unfortunately, this varies not only from manufacturer to manufacturer, but also from model to model. On Nikon D5000, for example, it is done through the menu setting (Info button), while on D90 there is a dedicated button on the bottom left side of the shutter. On professional cameras such as Nikon D300s, Nikon D700 and Nikon D3s, there is a separate switch for camera metering.

All the best.

Henry
 

arindamdas

Active Member
Excellent tutorial, Henry. I'm probably nitpicking here, but for Canon cameras, spot and partial are two different metering modes. Here's an excerpt from

Canon DLC: Article: Quick Tip: Photographing Snow

----- BEGIN QUOTE ----------

Spot: This metering mode gets exposure information only from the single exposure zone in the center of the frame (approximately 3% of the total picture area).

Partial: This metering mode is similar to Spot Metering, but covers a slightly larger area, reading only the cross-shaped central five metering zones (approximately 10% of the total picture area).

------ END QUOTE -----------
 
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