Thursday, November 19, 2009

Light Metering

Metering Zones
It's fall, and it's time for the back to basics style. What I'll be posting next are a few sections on the basics of your SLR camera, and how to use them. My thoughts on this are that I'm not the only one that would rather play with a camera than read the manual. In most cases, knowing the intricacies of your camera's settings will allow you to consistently capture the best shot in a situation.

Functionally speaking, one of the most basic aspects of photography is correctly metering the light in a scene. Although modern cameras do really well at this, knowing your metering modes is still important and will keep pictures looking great.

First, lets look at how metering works on a fundamental level, and then we can dive into each mode to see how they can be used in specific lighting situations you will encounter.

Metering Zones

Before we can discuss the fundamentals of metering, what is a metering zone? To answer this simply, the modern cameras will break the viewfinder into a number of metering zones as seen in the sample picture above. Generally more metering zones will allow the camera to give more accurate results.

Fundamentals of Metering

One of the easiest ways to think of this is to look at the sample picture as the camera would meter it; it looks at the image in the viewfinder as all gray scale. Each of the zones gets all of their values averaged together, and in the end has one value per zone.

Here is an example of how the camera might "see" a picture when metering with 49 zones.
Mouseover to see the average values for each zone!

In order to properly expose the picture, the camera adjusts the exposure so that metered zones are equal to what is called "middle gray". This value is effectively what the camera's metering mode attempts to achieve, a middle gray in the area that is being metered.

So if you were to take a picture of a completely white sheet of paper, your camera would expose it as "middle gray". Similarly, a picture of a completely black sheet of paper would also appear as "middle gray".

The reason for this is that the universally accepted method for properly exposing using shutter speed, f-stop, and ISO is designed to render subjects as "middle gray". Anything that is "middle gray" will reflect around 18% of the light that hits them, and cameras then assume that in any given scene, the average should reflect 18% of the light.

Taking a picture of something that is all white (like a picture of snow) will then come out underexposed because the picture is going to reflect more than 18% of the light that hits your subject (the snow).

Given this, when we describe a metering mode that only meters 9.4% of the picture in the center, this means the camera will average the zones in that area together, and then attempt to make the grayscale version of that specific portion of the image "middle gray" after exposing.

Middle Gray

Surely by now you're wondering "What is middle gray?! And why does this guy keep putting it in quotes?!" Well, I was putting it in quotes because the actual value of it will depend on the white balance you have chosen. Perfect middle gray is 100G-100R-100B. However, if you have set your white balance to a slightly warm value, you'll see more red and less blue in the middle gray.

Finally, Metering Modes

Now that you are a certified genius in relation to metering, lets discuss what each of the modes on your camera does.

Evaluative Metering

Spot Metering

Partial Metering

Center-Weighted Metering

Visual examples of the different metering modes in a Canon EOS camera with the green focal box in focus.

  1. Evaluative Metering (aka Matrix Metering)

    This is the Auto Mode of metering - where the camera uses some sort of advanced algorithm to determine the lighting of the scene and adjust appropriately. Different manufacturers will implement this different ways, but it can be as complex as actually having a library of typical "scenes" that it will compare the image lighting pattern to in order to appropriately expose your scene.

    Canon's specific implementation of this will link the metering directly to the active Autofocus point and compare the light values from that point with surrounding metering zones to attempt and provide accurate exposure. This works really well when taking a picture of a subject, but may not work well for large objects that are majority black or white (because they take a large area of the viewfinder and do not reflect 18% of the light).

  2. Spot metering

    This mode uses just the center of the image in the viewfinder, and does the averaging only in that zone.

  3. Partial Metering

    Similar to Spot metering, this mode will cover an area that is slightly larger, 9.4% of the total viewfinder area.

  4. Center Weighted Metering

    Using a weighted average, this mode will assign a heavier weight to the zones in the center of the image, but will take into account all zones in the viewfinder..

And Then?

You should have a slight idea of when each of these would be useful, but lets go over a few situations and see which metering mode would be the best suited.

1. A person standing in the snow or another very bright background.
As long as the person is the active focus point, evaluative metering will do a decent job. However, I recommend you choose spot metering for this; it will ensure that the person is properly exposed. If the background is blown out, then try using a flash for fill lighting on the person, and switch to partial metering so that the entire image isn't over exposed.

2. A black car that takes up most of the viewfinder.
Since the subject is not reflecting the expected amount of light, any of your modes that only meter the black subject will not look great. Try using Center Weighted Metering, and then you may still need to adjust the image to under-expose an f-stop or two.

3. Inside of a house, taking a picture of the view through a small window.
So here, we get into the problem of dynamic range of the image. Either you'll have the window's details properly exposed, or inside of the house properly exposed (unless you do some dynamic range photography).

Choose partial or spot metering, and using your camera's exposure lock, you can get the proper exposure of the window or the inside of the house, and then adjust the framing of the shot.

If you try to use exposure lock with evaluative metering, it may still over expose if the zones being evaluated have too much wall in them, or under-expose if they have too much window in them.

4. General Purpose
For general purpose, you should just use the Evaluative Metering, and then if you're using Canon, be aware of the focus point - that's what is going to be properly exposed so long as there's not a large area around it that's a deep dark or light color.

Your mission, should you choose to accept it.

Take a picture of a backlit subject, and change the metering modes to see what the differences are. Don't use a flash for fill light on this exercise.

Can you produce an image where the subject is properly exposed? What about one where the backlight is properly exposed, and the subject is just a silhouette?

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