There are 2 important facts you need to know when working with light meters.

1. Your light meter only measures the light reflected back from a scene, not the light on it.

2. Your light meter assume all scenes have an even distribution oh highlights, midtones and shadows.

Therefore the reflectivity of your subject strongly influences the accuracy of the final reading. In the same lighting conditions, white or shiny surfaces will reflect more light than dark surfaces.

Second, when your meter assumes even distribution of highlights, midtones and shadows, the overall value of these tones blended together reflects the same amount of light as a spectrally mid-tone grey with 18% reflectance. What this means is that your meter is at risk of under or over exposing a scene that is predominantly very light or very dark because it ASSUMES they all adds up to grey.

To prevent your camera from doing this, you will need to learn how you can utilize you camera's metering systems...

There are generally 4 types of metering systems. The center weighted, spot, partial/selective area and multi-zone/matrix/evaluative. Each is used in certain conditions.

The simplest of them all being center-weighted. If you know nothing about metering, this is probably what your camera is set to. What it does is averages the exposure for the whole scene and assign a greater weighting to the center portion, which is about 40-75% of the frame. The layout vary depending on the camera and some put more weighting to the bottom half of the frame to reduce influence of skies. This is easy to use but easily fooled and user intervention is often required.

Partial metering is almost same as center-weighted, only restricts metering to a smaller central portion, about 6-15%. It is therefore ideal for backlit or high contrast subjects. Place your subject in the center if it isn't already there and lock on the exposure and recompose.

Spot metering is the most accurate way to calculating exposure. It meters light falling on a very small specific area in the center, about 1-3.5%. If you choose to you this, choose your metering spot very carefully. A minor error can result in big differences. Spot metering is often used for high contrast scenes and determining exposure of a very specific subject area.

Multi zone/Matrix?Evaluative metering is the most complex to use. The frame is divided into a honeycomb patter of between 5-270 segments. Lights falling into each of these segments and exposure is calculated with a very complex algorithm. SLR matrix is very very accurate even in scenarios where other metering systems fail. This is because exposure is calculated with much more data than other metering systems, including focusing point, size, brightness, color, backlighting and size of main subject. Some advanced SLRs even have a database of thousands of images it can use to compare with the result from each segment for even greater precision.

If you're really serious about photography, you'd want to use an external light meter. Here's why:

If you really want accurate exposure, you'll need to use spot metering to specifically target an area u(nless of course you're a seasoned pro). However, you'll find that many times the camera's in-built spot metering is not small enough to do that! An external light meter, most of the time, offers smaller spot reading than your camera can, thus producing better exposure.

An external meter can enable you to walk up to the subject to your subject and take a reading without having to carry your camera and other equipment around, resetting tripod etc. This is more important especially when you have spent 10mins or so setting up your equipment to perfectly align for the shot. You wouldn't want to move it around to take about 50 spot meter readings.

Most important though is that a light meter lets you take incident readings. Incident readings measures the amount of light that is falling on the subject and ignore the backfround tonal influence, meter patterns you are using etc. What this means is that a subject 15 meters away will appear the same as one that is near. It doesn't matter what is the color of the background, incident reading allows you to get exposure of your subject accurately.

If you try using your in-built camera's light metering patterns, and shoot a white, black and grey paper, all will appear as grey. You'll need to intervene in these scenarios but that is time you often cannot afford! An external light meter allows you to quickly adjust for this exposure and produce white, black and grey!