When we talk about light, remember that there are three aspects of light that define the overall quality. Today we focus on only one facet of light and that is brightness. This is the first facet of light that any photographer learns to deal with. It is the brightness of the light that drives our exposure settings so let's look closer at how we measure the brightness of light.
If you think about what's happening in front of your camera you begin to realize that there are two basic ways to measure the light's brightness. We can either measure the light before it strikes our subject or after it's already reflected off the surface of our subject. The light meter in our camera is a great example of the later. When we use our camera's light meter, the light has already been reflected off our subject and has bounced into the camera.
Some photographers like to use hand held light meters with those little white domes on them. In this case, they are measuring the light before it hits our subject. So why, you may ask, do you use one over the other? The answer is simply control.
Reflective Light Meter
When you use a light meter that measures the light reflected off your subject, you have to make some significant assumptions. When light strikes a surface, not all the light is reflected. A white surface reflects a great deal more light that a dark charcoal toned surface. The meter does not know what your subject looks like. Instead it assumes that the subject is an average middle toned subject.
Now, and here is the important part. The reflected light meter does NOT tell you the correct exposure. It shows you what settings to use in order to record the subject as a middle tone subject. This is the very reason why your camera has an exposure compensation dial strategically located for quick adjustment. If your subject is a white washed building in the sun on Santorini, then you'll most likely need to set exposure compensation plus a stop or so.
Incident Light Meter
An incident light meter measures the light before it strikes your subject. It doesn't care if the subject is a Black Lab dog or a white rose. These measurements tend to be very accurate and because of this many professional like to use a handheld light meter when photographing important work.
One of the most frequently asked questions is where to you point the light meter? At the camera? Think about it. We are measuring the brightness of light that is coming from a specific light source. The camera is NOT our light source. The light only strikes the camera after it's already been reflected off our subject. To accurately measure the brightness of the light falling on our subject, we point the light meter at our light source.
In an earlier post we discussed how to Calibrate Your Light Meter.
Incident Vs. Ambient Light
Some refer to a handheld light meter as an ambient light meter. (As opposed to a reflected light meter.) This is not really a correct way to classify these meters. The term ambient means “of or relating to the immediate surroundings of something.” Photographically this means the light that is present in a scene prior to adding any supplemental light, like studio strobes.
If you use any type of flash, when the shutter is open, your camera is seeing a scene illuminated by ambient light and the light from your flash. Incident is an adjective that is defined as “falling on or striking something.” Photographically this is all the light that is falling upon the subject at the time the photograph is taken. Hence, we use an Incident meter to measure the light.
So What About Measuring Flash Exposures?
When you use a flash, you are mixing ambient light with the flash. You can actually measure these exposures with either a reflective or incident light meter. The trick is synchronizing the meter so that it know when to measure the light. For measuring flash exposures you have to have a light meter that is designed to measure flash and ambient together.
Obviously your camera's internal meter can do this when you use a flash designed to work with your camera model. In fact, the flash metering systems on cameras today are nothing short of phenomenal. But if you have studio flash units, very few can be measured by the camera's metering system. (A notable exception are the Profoto B1 and B2 Off Camera Flash systems).
To properly measure studio flash units you need a light meter that will handle flash exposures. One difference in the features on these light meters are the supported synchronizing systems. Some handle only cable attached metering, other can handle a timed flash where you press the button and then have so many seconds to set off the flash. When it sees the flash it will calculate the exposure. Other higher end units will support wireless triggering devices like Pocket Wizard products.
Yes, buy one. Which one is up to you.
Rob and Tony both use the Sekonic L-758. We've had them for many years and they are work horses. In addition to all the bells and whistles this meter also has a reflective spot meter mode which can be very handy. And yes, we know it's expensive. You'll most likely pay in the neighborhood of $600 for one but this device will be in your camera bag many many years from now.
Another choice to look at is the Sekonic L478DR. It's a newer model and one of the first touch-screen meters. It doesn't have the spot meter function but very robust in other ways. My recommendation is that you get your hands on one and play with it. Take it outside and make sure you can easily read the touch-screen panel on a bright sunny day.
Personally I like the touch of the real buttons on the 758 but many, many photographers love the 478 series. And, it's a bit smaller and will fit in your pocket - something the 758 doesn't do well.
A Final Thought
The light meter is a tool. It doesn't define your work or you. It's up to you to master this tool. Take time to practice with your light meter so you know how to use all the functions and can recognize problems displayed on the screen.
Sekonic has a very nice classroom section on their website where you'll find a lot of helpful articles and videos.