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Part 5 : Introducing the exposure meter

*Disclaimer: Notes written by Alistair Keddie.

 

Introducing the DSLR camera exposure meter, photography notes

 

 

Part 5 : The Exposure Meter


When we take our camera off of the auto modes, one of the first things we notice is that our exposure meter comes to life. The exposure meter is an important tool for us to understand and helps us judge where to balance the exposure of our photographs. By default, the meter is set to the central 0 position but we can choose to move this up to +1 or +2 or down to -1 or -2. But why would we want to do this? What exactly is our exposure meter doing?

An example of what your exposure meter on your camera may look like.

The exposure meter is telling us how the camera is reading the light in our scene, and most of the time will get it more or less right. Our camera has a built in light meter which, though good, is not the ideal because its reading the reflected light. That is, the light that is being bounced back from our scene. In that respect, the light meter is potentially reacting to a huge variety of different tonal values depending upon the luminosity of the objects in our scene. For instance, a snow scene reflects much more light than a black car will. What our camera is trying to do is give us a balanced exposure that takes account of all the different tonal values in the scene. To do this, the light meter looks for a mid tone grey almost exactly between black, the darkest possible tone, and white, the lightest. When the camera finds this mid tone, the exposure meter is telling us that the exposure will be balanced around the point indicated on the scale, the darks below the mid point, the lights above. This then gives us a good chance of capturing all of the tones, the full dynamic range contained in the scene.

But what if there is no mid tone grey for the camera to find? In the snow scene example the camera will struggle to set the correct exposure and most probably read the white snow as the mid tone grey returning consistently dark or under exposed shots. Likewise, if the subject is a predominantly black, like the example of the car, the camera will read black as the mid tone then consistently lighten or over expose the scene. In this case, we need to be able to over ride what the camera is doing and we do this using the exposure compensation button.

Pressing this allows us to move the meter’s setting around to position it above or below the zero on the scale. The scale itself is measured in full stops with which we are already familiar from our look at the aperture (see the exposure triangle). One full stop up the scale lets in twice as much light, to over expose or brighten the scene, whilst one full stop down lets in half as much light to under expose or darken the scene. The exposure meter’s scale usually also goes in 1/3 increments so that, like our aperture, we can have quite precise measures of how much light enters the camera. So in the instance of the snow scene, we may have to over expose our shot, that is adjust the exposure meter setting up to 1 or maybe even 2 full stops. We gauge that in the field. Likewise, our black car may have to be under exposed by 1 or 2 full stops or somewhere in between.

Usually, when we see the preview of the shot on our display we’ll have a pretty good idea about whether its too light or too dark but our camera is equipped with another tool to help us decide where to set our meter. Its called the Histogram and we can turn this on when previewing our shots. Some cameras, particularly those with live view allow us to display the histogram on the screen as an overlay whilst shooting. We’ll be having a look at the histogram in the next section.

 

Metering Modes


When we take our camera off of the auto modes, one of the first things we notice is that our exposure meter comes to life. The exposure meter is an important tool for us to understand and helps us judge where to balance the exposure of our photographs. By default, the meter is set to the central 0 position but we can choose to move this up to +1 or +2 or down to -1 or -2. But why would we want to do this? What exactly is our exposure meter doing?

Different metering modes on your DSLR Camera

 

Evaluative, Matrix or Multi Mode


This mode makes a complex reading of the light in our scene with a weighting towards the centre as generally this is where our subject will be. For most purposes it performs really well, though, like the auto focus system, it can get confused in a number of situations. For instance, it will not give best results when shooting a subject against a strongly lit background as it is trying to compensate for and measure all of the light in our scene. In this instance, it is best to use one of the other modes.

 

Spot and (or) Partial Metering


These are separate modes but similar to each other in that they will make a single light reading at the centre autofocus point. This allows us to quite accurately measure our exposure against a single subject, such as a person standing in front of a bright sky. Spot metering measures an area roughly 3 - 5% of the scene whilst Partial measure 10 - 15% in the centre. Using these modes will take a single reading from our subject and can accurately measure the exposure for a single subject.

 

Centre Weighted Average


The other common mode found on most cameras is Centre Weighted Average. This mode makes quite a predictable light reading across our entire scene becoming stronger as we come towards the centre. Its a useful mode for scenes which contain a lot of contrasts across the scene but where our subject is more or less in the middle. Good uses would include wide angle landscapes where there is a lot of contrast or portraiture where we want to retain background detail.

 

A note about AE Lock


The AE Lock button allows us to quickly lock the exposure Pressing this whilst taking a light reading will lock the exposure to that setting allowing us to recompose our shot. It can be very useful in conjunction with Centre Weighted Average mode.


 


 

 


 

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