Light Meter Basics

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One of the most essential items of gear that a photographer must have in his or her arsenal is a light meter (even though Steve didn’t mention it in his last article, and yes I jump at the chance to tease him). Some would argue that the light meter is just as important as the lens or the camera itself! But WHY is this little tool so important? Why isn’t the camera’s built-in meter enough to achieve photographs that really stand out from the crowd?

Well, it all has to do with how a light meter measures light.

The Short Version

When using a light meter, the photographer provides two of the elements of the exposure triangle and the light meter provides the third. For example, if you know the shutter speed and ISO that you want to use, you would input that into the light meter. The light meter will then provide you with the aperture value you should use to correctly expose your shot.

So, say I’m in the studio with a model. I would input an ISO of 100 and a shutter speed of 1/200, which is my Canon 7D’s flash sync speed. I would hold the meter directly under the model’s chin, with the sensor aimed at my camera’s lens, and trigger the strobes (many light meters are compatible with strobes that have transmitters/transceivers). The meter would then output the aperture I would need to use to achieve the correct exposure. Adjusting the power of the strobes will effect the light meter’s result, so if I wanted a different aperture I would increase or decrease the power of the lights accordingly (or move them closer to or further away from the model).

On the other hand, if I were outside trying to take a photo of a model using natural light, I would input the ISO and the aperture, and allow the light meter to tell me the shutter speed. Again, adding additional light sources or reflectors would effect the meter’s result, so if the shutter speed were too slow I would bring some light into the scene (or increase the ISO).

Based on which kind of light you are trying to measure (light from a flash or ambient light), you will use either reflective metering or incidence metering mode on the light meter.

Reflective Metering

Reflective metering mode is most typically used to meter continuous or ambient light. The light meter uses spot metering to measure light that bounces off of (or reflects off of) the subject, which means it is useful for landscape photography or when shooting subjects that product light. It does not directly measure the source of the light, just the light’s reflection. It includes light that hits the subject as well as the surrounding light. It is very similar to how your camera meters light through the lens (TTL), but is much more accurate.

Incident Metering

Incident metering mode is most typically used to meter a light source such as a flash or strobe. It meters the light source directly, ONLY the light falling on the subject, and not the ambient light. Sekonic has pretty much cornered the market on light meters, and the sensor (or “lumisphere”) on their meters can be used to measure light coming from a specific light source (lumisphere in the retracted or “down” position), or the light coming from all flashes or strobes together (lumisphere in the standard or “up” position). Other brands have this capability as well, though I am most familiar with the functionality of the Sekonics. This is helpful when you want to calibrate a traditional lighting set up, where the kicker light is one stop less than the key light, and the background light is one stop more than the key light. Then once all of the lights are powered according to your wishes, you would take a final reading to determine the correct aperture to use for proper exposure.

So what does all of this mean for you?

Just how, exactly, will the use of a light meter improve your photography? By taking the guess work COMPLETELY out of the equation. There will be no more trial and error with apertures and shutter speeds, and no more “expose and recompose” using the camera’s TTL metering. You will no longer struggle with the wonky shots that occur when your camera reads the light incorrectly. You will have complete control over how your subject and your background are exposed. You will become utterly comfortable and confident in using your camera on manual mode.

I have used both the Sekonic L-358 Flash Master Light Meter and the Sekonic L-758DR Light Meter, and I highly recommend them both.

Photo credits (in order of appearance):
- “Sekonic L-358″ by Yutaka Tsutano on Flickr Creative Commons.
- “Brenda” by Tiffany Joyce

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