Metering depends on the built-in light meter of the camera. Built-in light meters use the reflected light meter technique to meter for the ambient brightness. This is not the most accurate of ways to meter for a scene. That said hand-held light meters are the most reliable. This is because these meters use incident light for metering a scene. This is a much more accurate way to meter.
Metering will also depend on the metering mode of the camera that has been selected. This will determine how much of the scene will be used for metering purposes. There are a total of three different types of metering modes – matrix, spot and center-weighted.
Canon systems have a fourth metering mode known as partial metering.
Spot metering is a type of metering system in photography which takes into account only about 1-5% of the area that you can see through the viewfinder. Usually, when you focus using the single point AF you can limit your metering to that point and the immediate vicinity. That is often only 1 – 2% of the frame.
The need to use spot metering comes from the fact that sometimes you need to limit your focus only on a small area of the image for balancing an exposure.
An example is a variable lighting situation where there are patches of light and dark areas in the scene. You can meter by placing the AF point on a neutral subject and then meter based on that. Spot metering mode is the closest to getting a level of accuracy that is generally obtained when using external light meters.
Every type of light imparts a color cast. Be it the incandescent light in your living room or the fluorescent bulb in the drawing room or even the Sun. This color cast becomes a bit of an issue when you look at them on the computer screen or make a print. Your images are either too orange or too blue.
White balance is the name of an adjustment process wherein an image is treated to display the color white as it should be, sans the color cast.
In the process, the rest of the colors are also corrected for any color cast. The process is sometimes also referred to as gray balance. This is because the process involves displaying neutral colors such as white and 18% gray as they are – neutral. 18% gray has an average reflectance and photographers routinely use an 18% gray card as a reference when shooting in changing light. This allows faster correction of the color cast when post-processing.
The manual mode is often referred to as the ultimate in creative photography. Why? Because manual mode allows you to use your intuition in order to set the aperture and shutter speed on your own. In other words, you are no longer dependent on the camera’s built-in metering system to dictate what exposure values you need to dial in (normal Auto mode).
The manual mode is necessary when you have to control both the amount of light as well as the depth of field of your shots. Let’s take a couple of examples. Let’s say that you want to shoot a seascape. You need to use a small aperture and then drag the shutter speed to produce the right exposure and composition. Similarly, if you need to use a wide open aperture in broad daylight in order to blur the background, something that the built-in meter will not allow, you can use an ND filter and open up the aperture wide to achieve that.