Back to Basics: Metering modes & Exposure compensation

Okay, so there is one more pretty technical piece that we need to cover in the Back to Basics series before moving on to the more exciting stuff. That is Metering modes & exposure compensation.

I'm going to try and make the various metering modes simple to understand using a couple of "out of the box' illustrations which I hope will resonate with some of you.

Metering

Every camera has a sensor which, when exposed to light, creates an image. The camera also has a secondary sensor which measures the light that’s coming through the lens and determines how much is needed to produce a "well-exposed" image based on the scene you're pointing your camera at. As light is reflected from a scene or subject through the lens, it hits the mirror in front of the imaging sensor and is reflected up to the camera’s focusing screen and metering sensor and the camera assigns a shutter speed (in Auto, P and AV) modes.

Cameras these days allow you to control how much of a scene is taken into account when calculating the exposure of an image through a number of metering modes and, if you'd like to find out more about that right now, you can check out this post. Otherwise, lets assume that you are using the most common of these modes, evaluative or matrix metering.

In this metering mode, readings from around 80% of a scene (with a slight bias towards your active AF point) are taken  in order to calculate the shutter speed necessary for obtaining a "correctly exposed" image.

Make sense?

Stay with me here...

Now, lets see how this works in the field.

Assume we are shooting in evaluative metering mode and we have selected the bottom centre focus points to achieve focus on our yellow and light Lioness in the bottom centre of our frame. The camera will now measure light across pretty much the entire frame with a bias towards the active focus point.

Now, this is where I'm going to try and illustrate what happens when the camera meters of the scene and gives you a shutter speed based on your desired aperture value and ISO setting. Please keep in mind that these are not actual values but I'm using this to illustrate the way evaluative metering mode works - bare with me on this one!

In order to correctly expose the darker areas of a scene the camera would need to use a slower shutter speed (1/125) in order to allow sufficient light in and correctly expose these shadows or darker areas. At the same time the camera is picking up average shutter speeds for the mid tone areas (1/250 - 1/320) and then a very fast shutter speed (1/800) for the light Lioness as well as a fast shutter speed for the bright patch of sky in the top right corner (1/1000) which takes up only a small portion of the frame.

Based on all of this information the camera then averages out the shutter speeds to keep the exposure "in the middle" and not too dark (underexposed) or too bright (overexposed). In this instance the camera may have chosen an average shutter speed of 1/320 to capture this scene. This average shutter speed would result in our Lioness being overexposed, however the darker background would be "correctly" exposed.

That is an example of a bright subject on a dark background, now let's look at the converse.

Here is a Pied Kingfisher hovering in the sky set against a bright background and, in evaluative metering mode, the camera once again measures light across pretty much the entire frame with a bias towards the active focus point.

Using my hypothetical analogy of assigning shutter speeds across the scene where the camera is metering from, the camera may have decided on something along the following lines.

Clearly the scene is dominated by faster shutter speeds (1/3200) assigned by the camera to ensure that the bright sky does not end up being over exposed. Unfortunately for us, the dark subject which occupies only a small portion of the frame has only one shutter speeds of 1/60 assigned to it. In the bigger picture (pun intended) the bright sky wins the battle and the camera would have chosen a shutter speed of 1/3200 which is too fast a shutter speed to render our subject correctly exposed.

This will explain why you will often look at an image of a scene on your camera and feel disappointed. Perhaps your subject is too dark or perhaps too bright based on what you had hoped or envisaged. It's all around how the camera doesn't actually know what creative effect you are wanting to achieve or what kind of light your subject is actually reflecting. The camera is simply trying to give you the best of both worlds and balance out the entire scene.

Not ideal though right?

Thats where exposure compensation comes into play.

If you've grasped this concept the next step in the puzzle should be pretty simple.

Exposure Compensation

Example 1: A Subject against a bright background

In evaluative metering, the camera will be giving you an exposure value based on the entire scene. In this instance the camera would balance out the bright sky and the dark subject resulting in an image which is pretty dark. Using manual exposure compensation we can dial in an exposure compensation value of +1 which, using the initial metering as a base, will force the camera to brighten (over-expose) the scene by one full stop.

Whilst the sky may not be as dark and may even be leaning towards being over-exposed, I am not phased by this as the exposure of my subject is much better with detail in the shadows and even the mid-tones being presented in a much more appealing way than the cameras initial evaluation.

Example 2: Shooting with a Wide Angle Lens

Shooting with a wide angle lens, whilst including more of a scene, often results in the camera having to deal with a greater range of light intensities across a scene. Even with a very cloudy top 2/3 of the frame, the camera's initial evaluation of the scene results in a rather dark image - especially if we are judging this by the exposure of our subjects in the bottom of the frame.

Understanding that the camera will assign more priority to the sky which takes up 2/3 of the scene we can make a manual exposure compensation adjustment which tells the camera to brighten up the scene by +2/3 and then +1 full stop which results in a better overall exposure.

Again, in aperture priority this is achieved by a change (reduction) in the shutter speed which allows more light to fall onto the sensor. Sometimes the exposure compensation adjustments may result in a less than optimal shutter speed (eg less than 1.focal length) which may then in turn require you to increase your ISO.

It all comes back to Aperture, Shutter Speed and ISO.

Example 3: Creating Mood in an Image

One of the reasons that photographers love that early morning and later afternoon light is the fact that it creates depth in a scene. It is often the keylighting that falls on a subject whilst the rest of the frame is dominated by the long shadows that adds that little something extra to a scene.

Evaluative metering, by virtue of the fact that the entire scene is evaluated, often ends up washing out these kinds of scenes as the camera tries to bring back details into the shadows. The example of this Hippo photographed in the early morning light is a great example of how some sort of manual exposure compensation is needed in order to preserve and enhance the mood of the scene.

Underexposing by 1 full stop here darkens all of the shadows in the and really makes our subject pop with the golden light coming in from behind. Without exposure compensation the shadows become a washed out grey and the bright light falling onto the Hippo almost results in our subject being a bit overexposed.

Example 4: Sunrise & Sunset

Again, looking at how the camera will evaluate this scene we have a ball of bright orange and a predominantly dark landscape. The camera, in evaluative metering mode, will try to find an exposure which copes with the bright sun whilst trying to expose/preserve some of the details in the shadows. The result, a washed out scene with an overexposed sun.

Underexposing by a full stop (-1EV) preserves more of the highlights in the sun and preserves more of the yellow, orange and red tones. Underexposing by two full stops (-2EV) does so to an even greater extent. Perhaps too much so.

Which is correct?

That's up to you and what you are wanting to capture and create.

In Summary...

Evaluative metering does a pretty good job 90% of the time but being aware of whether you want to make the scene and your subject darker (underexposing) or brighter (over exposing) can make a massive difference to your final image.

In the way that I analyse a scene and capture an image, I am constantly aware of the light within the scene and will more often than not, make some sort of manual exposure compensation adjustment which uses the cameras initial evaluation of the scene (based on evaluative metering mode) as a starting point before being adjusted to either underexpose (darken, faster shutter speed) or overexpose (brighten, slower shutter speed) a scene.