Do I need a light meter and which one should I buy?
Internet Filmmakers' FAQ
Understanding and controlling exposure on your camera is one of the most important factors in enhancing the production values of your film. Although most modern cameras, particularly video cameras, come with automatic exposure functions, most filmmakers find it extremely useful to have a light meter on set. And if you're shooting on film, a decent light meter is essential.
The components which are used to build today's light meters are fairly expensive, and this is reflected in their price. The good news however is that for most independent filmmakers, it isn't necessary to fork out for the latest piece of kit. Basic light meter technology hasn't changed in many decades, so as long as you don't need flashy additional features, you can get just as good an exposure using an older model. And as many of these meters don't have electronic components, they don't need batteries either.
Hidden in many DPs' kit bags is a trusty Weston 5 and even older meters, such as the Russian Leningrad IV, are still just as useful today. More modern meters such as those made by Seconic (particularly L508) and Minolta (Spotmeter F Spot) can still be picked up for a reasonable price, particularly second hand. If you're considering buying a newer light meter, there is an interesting thread on cinematography.com about this very issue.
A useful trick is also to have a decent SLR stills camera handy on set at all times. Apart from taking stills for your film, you can also use it to check your light readings with its built-in spot meter. This is particularly useful if you can't get close to the subject, but you can also use it to check the calibration of your standard exposure meter as well.
Of course, having a light meter with you is only part of the equation - you also need to become well-versed in its use. Any decent book on cinematography should give you the grounding you need, and from there it's all about experimentation and experience.