I remember that Roberto Valenzuela mentions in one of its courses that for educational purposes it is a good idea to buy a light meter and study light properties in different circumstances, locations. Digital cameras also have built in light meters. Light meters are lighter and easier to use, but is there any other benefit for me in buying a $200 Sekonic or just use the built in light meter?
The keys to this kingdom we call photography are subject matter, composition, and exposure control. When it comes to exposure control, we are talking about the ability to pre-conceive what the final image will look like and then adjust the camera to achieve that objective. Not just once but repeatedly hitting the mark.
It is true, todays cameras incorporate a built-in light meter. This meter is coupled with computer logic that takes the drudgery away. You have at your disposal, umpteen scene types that assure a good outcome. However, is that setting yours, or is it the handwork of some unseen engineer?
Don’t get me wrong, nothing is wrong with automation! On the other hand, nothing is wrong with learning the basics and then relating exposure readings to camera settings. All this -- to achieve an image that is faithful to the picture that was in your mind’s eye when you composed the picture.
The hand-held light meter is just a tool. Serious photographers keep a toolkit of goodies. For me, the hand-held light meter is indispensable. Get one that reads, reflected and incident plus flash.
There is more to it. Our camera already has a very good reflected meter. The purpose of getting a hand held light meter would likely be that it can be an Incident meter then. The meters in cameras are reflected meters.
A reflected meter meters at the camera, aimed at subject, and reads the light reflected from the subject. An incident meter meters at the subject (in their light), aimed at camera, and directly reads the light incident upon the subject, independent of subjects colors. This is wonderful in the studio, but metering at the subject is generally awkward out in the field.
Example: A white dress reflects a lot of light. It meters high, and a reflected meter sees that reflected light as bright, and then sets an exposure to make it be around middle gray (underexposing).
A black dress reflects little light. It meters low, and a reflected meter sees that reflected light as dim, and then sets an exposure to make it be around middle gray (overexposing).
Many subjects are a mix of "average" reflectivity, which often works OK. Exceptions don't. That is simply how reflected meters necessarily work. The meter does not know what the subject is, so result (average of any metered area) is put around middle gray (14%), not too bright, not too dark.
A Spot meter is a reflected meter, so we better know and consider the reflectivity of that spot.
An incident meter reads the actual light directly, light on the subject at the subjects location but independent of subject, and either dress likely comes out exposed about right.
The camera meter is a very good reflected meter, surely best choice in a reflected meter.
But an incident meter is more often correct and it also reads flash.
An external light meter (or a flash meter, which is in practice the same thing) is good to have when you are balancing multiple light sources - e.g. a studio setting involving a couple flashes and an open window to boot.
Outside of a studio it is often easier to apply the brute force approach: take a number of shots with different exposure settings (exposure bracketing - expose 1 EV less, on spot and 1 EV more than measured by your camera) and pick the one you like in post process. Most cameras have a dial for this.
Card space is cheap, and this approach allows you to focus more on the artistic aspects of your photo and less on the technical ones (of course, it is a legitimate course of action to fetishize light, and worry endlessly about N+1 and N-2 development and the Zone system, and then you badly need a light meter)
Light meter is a black and white camera of 1 pixel resolution, coming from the times when it was the only way to immediately measure the light intensity, in order to learn the correct exposure settings for a film camera. Being only single pixel, it relied on clever techniques, personal experience and careful planning.
Today your digital camera has millions of light meters, which makes it not just infinitely more precise, but in fact, perfect. No, the camera will not always select the best exposure (and there is even no such a thing). I mean that the photo itself is the light measurement, the best and the only relevant light measurement for the given circumstances.
Just use your camera, or even your smartphone, take photos and you will learn from the results more than from a light meter.