Monday, June 2, 2008

The Light Meter and Digital Video: the Basics

For those of you with a background in still photography, or who may have gotten the opportunity to shoot motion picture projects on actual film, you may have some familiarity with an amazingly helpful tool: the light meter। As digital video cameras continue to develop, many shooters have moved away from using a light meter. Especially as the new generation of videographers develop their skills without ever touching a celluloid camera, a light meter can be a rather overlooked tool. I know several excellent videographers who often shoot without the use of a light meter. But this doesn’t mean it is an outdated device that is no longer applicable to the new workflow of digital cinematography. Quite the opposite! Whether or not to use a light meter comes down to two things: (1) your own working preference, and (2) the type of project you’re shooting. What I’d like to discuss here are the occasions where I do choose the use a light meter (even though I am shooting digitally), why I use it, and how I use it.

Mistrusting Your LCD

A lesson I learned the hard way, after I fist purchased my Sony HDR-FX1 camera, was that the on-camera LCD screen alone is not worthy of my trust when it comes to gauging exposure. This may vary to some degree from one camera model to another, but I eventually figured out that things can look great in my LCD screen while shooting. Later, however, I have found shots to be quite underexposed and noisy. In my entry about making my short film, “Cellar Door,” I talked a little about how HDV is a highly compressed format, and underexposing footage can mean that color correction becomes problematic and introduces all kinds of artifacts and noise when you try to brighten the footage in post. Along those lines, an underexposed shot can still look great on your LCD screen (I know they sure do on my FX1). The reason for this is because the signal being sent to my LCD screen is actually an uncompressed HD signal. Also, LCD screens tend to have pretty vibrant colors. Combine that with their small size, and it really is hard to justify trusting those little suckers. It’s difficult to see noise on a small LCD. I once worked with someone who would happily boost the gain on the FX1 instead of trying to place more light on the subject when a shot was obviously too dark. He claimed he couldn’t see any difference in image quality (but only had the on-camera LCD screen to judge this by, not to mention the fact that we had no light meter available at the time and this particular person had not the slightest idea how to set exposure according to the in-camera zebra stripes and refused to use them). I wonder how he feels now that he’s editing that footage and seeing it on a much larger screen. As for me, been there, done that, not making that mistake again.

With highly compressed formats like MiniDV, HDV, and XDCAM, having precise exposure is quite important. As with most any video format, overexposing can cause clipping in your highlights. This means that the brightest areas in your frame simply become solid white. The details in those areas are simply gone at that point, and no amount of color correction will ever change that. This means, there’s an acceptable window for exposure. Most video shooters get quite used to using their in-camera zebra stripes to help them know what areas of their frame are brightest. Zebra stripes are a great tool, and I pretty much have them on all the time on my camera. I have mine set to 80%, and have gotten quite accustomed to them. These are extremely helpful in run-and-gun shooting situation where stopping to take light measurements is not practical or even possible at times. I can quickly gauge what areas of my frame are leaning to the brighter side, and adjust exposure accordingly.

Why a Light Meter?

In college, I had the opportunity to attend a semester-long program called the Los Angeles Film Studies Center. This was back in 2003, and at the time the LAFSC was still shooting an awful lot on Super 8 film (much to my great excitement!). I shot every one of my class projects on Super 8 as a conscious effort to learn more about shooting and lighting for film. It was through this experience that I was first exposed to the incident light meter. Since then, I’ve actually purchased one of the very light meters I used that semester at the LAFSC.

There are several advantages to the use of an incident light meter like the one I own. Lets take a look at them:

(1) The obvious is that with the proper settings on a light meter, you can take a measurement with it and have a pretty exact idea of what F-stop you should set your camera to. However, while a great start, that’s not the only good use of a light meter.

(2) Keep in mind that in most situations you do not want more than a four F-stop different between your lit and dark areas in your frame (unless you’re going for a high contrast Noir look). With a light meter you are able to measure and compare dark to lit areas of your set. This way, you can mathematically ensure that you are within an acceptable range of exposure when it comes to shadows and highlights. By doing this, you can guarantee that there will detail in your shadows, not just total darkness. And of course, the opposite is true as well. If you want shots with high contract, you can mathematically make sure that your shadows will be thoroughly underexposed.

(3) A light meter takes a measurement in a very specific area, and that can be quite handy, especially if you are shooting a rather wide shot. With wide shots, it can be quite hard to tell, even with zebra stripes, if you’ve got a good exposure. With a light meter, you can walk right up to the subject you’re shooting, take a measurement, and know with confidence how to expose your shot. It takes the guessing out of it.

(4) Another advantage to using a light meter is that it is a small piece of equipment and easy to carry with you. I don’t only use a light meter when I’m shooting a movie. In fact, when I’m hired to shoot an event or wedding, I will often show up early to the location and walk around with my light meter taking measurements. This way I can get an idea of what the light in a room is doing and how I might work with it. With weddings, I will do this during the rehearsal. This gives me the opportunity to speak with the person in charge of the lighting for the location and express (more often than not) my need for more light. Using the light meter this way means I can get a solid idea of what my exposure is going to be in my camera without having to actually bring my camera along and setting it up.

Calibrating Your Light Meter

There are several models of light meters available. Some provide you with incident as well as spot metering. Some even give you color temperature measurements. Depending on the model you get your hands on, you might be presented with rather specific options on how to set up your light meter for use with a digital video camera. My Minolta Auto Meter III doesn’t have any options quite like that. So what did I do?

I set up my FX1 under a rather flat and ideal exposure situation. Since I’m shooting with a digital camera, I have to think of my CCDs in my camera as the film stock I’m shooting on. What speed of film are my CCDs at 0 gain (that is, at their native sensitivity; boosting gain is a boost of the CCDs sensitivity to light)? Thus, having locked in all my manual settings on my camera for gain and shutter speed, I let the camera’s auto iris adjust to the exposure it thought would be best in that situation. I was sure to keep my zebra stripes on too, so I could judge exposure according to those as well. With that, I set my light meter to the same shutter speed, and took a measurement. With any light meter, you’ll able to change your ASA (which is the film stock speed, also known as the ISO) number. I changed my ASA number until I found the F-stop on my light meter that matched with the F-stop on my camera. I did this at several shutter speeds, and with different gain and neutral density filter settings, taking note of the results. I also varied the amount of light while keeping all my settings on my camera and light meter the same to see if the ideal exposure measurements between camera and light meter would drift.

The end result: I found the FX1 set to 0 gain to be a rather slow camera, being equal to shooting on 100 ASA film stock (about as slow as they come). So much for all that talk about how light sensitive this camera is (only when you crank the gain up, which introduces noise and grain). But truth be told, pretty much all prosumer HD cameras are rather slow. And that is something every shooter wanting to use a 35mm lens adaptor has to keep in mind. By the time you add an adaptor like the M2 or the Letus35 as well as a prime lens, you are going to sacrifice a stop or two of light. But that’s a discussion for another time.

With the notes I took during my light meter experiment, I created a chart. I keep this chart in my camera case for use on shoots. It tells me what settings match up on my camera and light meter. This way, I’m prepared to shoot in high-noon sunlight, normal indoor situations, and even candle-lit rooms. I know how to set up my camera and light meter so that they’re speaking the same language (F-stops that actually match up).


Now, it is by no means essential that one use a light meter when shooting digitally. But, depending on your shooting style, having a light meter handy can be quite helpful. I’ll always have my light meter with me for every shoot I do. Some shoots, I don’t break it out at all, other times, I take light measurements even between takes. It just depends of the situation. Some times, in a controlled environment, I may use my light meter during the first set up, and then never touch it again that day.

When it comes to digital cinematography, a light meter is a tool you might be able to live without if you just don’t have the money for one. But once you actually use one, it might be hard to give it up. You may find yourself at times whishing you had one handy (much like many other specialized tools shooters often use). Given the range of prices for a light meter, if you’re shooting often, spending $50 to $200 for a used light meter is not at all a bad way to go. Even at $500 for a brand new elaborate light meter with color temperature and spot measurements as well as incident readings doesn’t sound too pricy to the professional doing a lot of shooting. It can prove to be quite worth it. It comes down to what you need, and how you’ll use it. I got my hands on a used meter for very little money, and it’s been quite worth it. In fact, I’m considering an upgrade at some point in the near future.

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