Whether to use the meter provided by your camera or use another light meter is one more topic many photographers have different opinions on. Some haven't used an incidence meter in years, or really ever, and are very happy with their results. Still others swear by the incidence meter and have effective arguments as to why the camera meter is inferior.
Before we talk about why you should or shouldn't use either, we need to talk a little about each method. First, let's look at what the camera uses to determine the amount of light available for the exposure. When you use the light meter provided by your camera you are using what is known as "reflective metering". That means that the meter in your camera uses the light reflecting from the subject back to the camera for determining exposure. The primary issue with this is that
reflective meters use the light reflected back which can be very different because of the light intensity of the subject and background. What do I mean by this? What if your subject is wearing a white shirt, light pants, and is leaning on a white wall. All of that is very light intensive and will reflect back to the camera with a reading that there is a large amount of light. Conversely, in the same amount of light, if the subject was wearing dark colored shirt and pants, and leaning against a darker colored wall, the camera would reflect back there there is less light. Obviously, this is not a true measurement of the actual light in the scene. Sekonic is one of the premier makers of incidence light meters. They have a very good explanation of incidence metering
on their website.
Reflective metering in camera (image courtesy of XDA Developers)
Incidence metering, on the other hand, measures the light falling on the subject for determining exposure and there is no account of the intensity of reflective elements. The photographer stands by the subject and points the meter at the camera and presses the button to measure the light. Most of these meters have a dome that you can expose or not. Exposing the dome is when you want to measure the light in all directions. When the dome is not exposed you must point the meter in the direction you want the light metered from (usually the camera). The reason photographers like this meter is because it is an accurate reading of the light hitting the subject and not a reflection that is an estimate of the scene. For the meters I have used I just set the meter for a particular ISO and shutter speed and get the Aperture reading. Then I can toggle either the shutter speed or ISO up or down to see what measurement is required to get a particular Aperture. If I am in a studio using strobes I can power the lights up or down to get the reading I want.
Sekonic L-358 Incidence Meter (image courtesy of Amazon.com)
So when should I use the reflective meter in the camera and when does it make more sense to use the incidence meter? The following are my opinions, so take them for what they are and hopefully they will be helpful to you. As I always say, it all depends on what you are shooting. I rarely use an incidence meter except for in my studio. But in my studio I always use the incidence meter. I primarily shoot families and seniors. Some of those are shot in the studio and some are shot in urban locations such as municipal parks or public access buildings. In the studio I have complete control of the light and I usually want to shoot the main light at a particular aperture (usually F8) with background and hair lights at about a stop less. With different configurations and multiple lights it's just easier for me to use the incidence meter to make sure I have the light I'm looking for. However, when I shoot outdoors I have less control of the light and I like to shoot either in all natural light, or utilizing natural light with speed lights to compliment. When I do this I have to be aware of the intensity of reflective elements in the scene. If it is a highly light intensive shot I may take an exposure reading on something like the ground or something else that may reflect a more balanced exposure. I will also shoot at multiple apertures depending on the depth of field I am looking for. With these requirements I will always use the reflective metering in my camera. It's just easier and the differences can be handled in post.
I also shoot high school sports and landscapes. When I shoot high school sports it's typically in a setting where the light is not so good. My aperture is usually wide open, with a high ISO, and semi fast shutter. I don't really have a choice. However, even if this were college or professional sports I probably wouldn't use an incidence meter. For these events there is no control of the lights and the amount of light can change at any moment. Landscapes are a little different, but again I don't use an incidence meter for these as well. Most of the time when I am shooting these I just try to not clip any highlights or shadows. I'm also usually shooting long exposures or HDR (with bracketing) so I will have a lot of latitude for exposure when I get to Lightroom or Photoshop.
So, to sum it up, when you should use either of these depends on what you are shooting and what you are most comfortable with. One of my favorite photographers is Joel Grimes. He never uses an incidence meter. He just looks at his exposure to see if it's what he's looking for. But he's been shooting for 30+ years and knows what he's trying to achieve. He's done it hundreds if not thousands of times. Another well known portrait photographer is Tony Corbell. He always uses an incidence meter and swears by it. He also has been shooting for a long time, but knows what he's comfortable with.
To sum it up, it's all about you're comfort level. There's no rule that says you must use one or the other. For me, in the studio, I like to set the lighting in advance for what I'm trying to achieve. I do that with an incidence meter. That way when I press the shutter release I know what I'm gong to get. Of course, if you have been shooting for a long time, like Joel Grimes, you may just need a couple of test shots and you're on your way. If you're uncertain, and if you have the budget, get a cheap used incidence meter and give it a try. But if you're comfortable with the reflective metering in your camera there's no reason to make a change.