Do You Need a Light Meter? If so, which one?
A recent thread brings up the question: does a video shooter need a light meter? My answer is, provided her/his work is run-and-gun, No. But once you get involved with setting up multiple lights, my answer is a strong Yes.
Why? It's not so you can determine the f/stop, though if you know what you are doing you can use the meter for that purpose too. The real usefulness lies in quickly assessing and adjusting the intensity of each of those lights. Yes, with a good monitor, you can make that assessment, but that method is cumbersome.
So, which meter? I suggest the first meter to get is a good incident meter. (I am in awe of Todd Terry starting off with the Pentax digital spot. More on that below.) Properly set, its the easiest to use. The standard method to determine exposure (f/stop) is to stand in the place of the actor and point the meter's dome toward the lens. If the actor moves, move the meter along the path she/he will take and observe the changes. If the actor will step into an area you consider to be too dark, your meter will tell you right away how much "too dark" that area is. (There are some tricks to using any incident meter well to determine exposure -- back/side/overhead lights may throw your reading off; you need to shield these from the meter's dome.)
For the video shooter, a huge benefit of using an incident meter is it's a great way to check/adjust the intensity of each of your key lights. For this purpose I point the meter directly toward each light. Your meter will tell you right away how many stops too dark or too bright each light is in relation to what you've decided is your f/stop. (Be sure to shield from the light dome any back/edge light that might throw off your reading.)
And, the incident meter I currently recommend is the Sekonic L-308DC. (Do NOT get the cheaper S model -- it's for flash photography only.) The meter is amazingly rugged. It has 3 modes: movie/film, stills, and video. (It is, alas, a bit tricky to set up correctly -- it's too easy to change the frame rate or other variable without meaning to.) I have gone through many incident meters. These days the only one I take with me is this little Sekonic. My Spectra Pro stays at home. Yes, the dome is too small, and getting the settings right is a small pain. But it fits neatly in my shirt pocket and stays true and my readings are always good, film or video.
If I were restricted to only one single meter in life, it might be the Pentax digital spot equipped with a Zone System sticker. I don't need a gray card to determine exposure because I know that if I point the meter at a specific spot between my thumb and first finger and set the EV number on the line between Zones V and Vl, I get a very good exposure. I love that meter because it allows me to determine in what Zone each element of the scene falls, once I've decided on my f/stop. The big defect: it's fragile and easily goes out of true calibration. And, it's big. These days, too, this one stays at home. I only take the Sekonic.
MFA/BFA Lighting and Camera Instructor Academy of Art University
San Francisco Bay Area
Personally, I only use a meter in a couple of circumstances, but in those circumstances, the meter is critical. They are:
1. To give instructions to my crew on what stop to set additional lighting to once I've determined the stop I'm shooting at. It's much faster to say "I'm shooting at a 2.8/4, so light this area two stops under that and this area 1.5 stops over that", instead of playing around with each individual light at the monitor ("Eh, too bright. Nope, too dark. OK A/B it, no, split the diff....").
2. To have accurate measures of light intensity to re-create the setup on another day. Someitmes I'm doing the exact same setup for the same shoot/client in multiple locations on different days or maybe it's a particular "look" that I use for certain common situations for different shoots. Either way, you will make the next time you use certain setups go MUCH more smoothly if you keep an accurate record of the intensity of all the lights.
I will agree with Rick in that the Sekonic 308 is quite good, as is the Sekonic L-508.