Today’s Question: The in-camera light meter tries to take everything middle gray so if you take a picture of a white wall in auto mode the wall will appear gray, not white. So you would have to overexpose by 2-3 stops to get the wall white as it should be. What happens when you meter with a external light meter such as Sekonic. The same?
Tim’s Quick Answer: A hand-held light meter provides the same basic function as the meter in your camera, though the specifics of how you deal with the information provided will vary depending on the type of meter.
More Detail: An exposure meter enables you to measure the light level illuminating a scene and determine appropriate camera settings that will ensure a good exposure for that scene. The general way these light meters are described is that they are trying to determine the average light level illuminating the scene and determine an exposure that will ensure that a middle gray card in the scene will appear as middle gray in the final exposure.
Of course, you can meter off something other than a middle gray card, which means your metered exposure won’t necessarily provide an optimal final exposure. For example, if you meter off snow on a bright sunny day, the meter will attempt to set an exposure where the snow will appear gray. You’ll generally need to compensate by increasing the exposure by about two stops to ensure the snow will appear white rather than gray.
There are some differences with accessory versus in-camera light meters in terms of the options available. In general, both will offer options for the size of the area being metered, such as spot metering versus evaluating a larger area of the scene. In addition, handheld meters (unlike in-camera meters) may offer the ability to measure incident light rather than reflected light.
In other words, with some handheld exposure meters you can position the meter in the area where the subject you’re photographing is located and therefore illuminated by the light, to measure the light actually reaching the subject. Other meters, including those in the camera, will measure light reflected by the scene rather than being emitted toward the scene.
The overall concept, however, is the same in terms of a light meter only measuring light based on the settings you’ve established and the method for that measurement. Regardless of the type of meter being used, you will generally need to consider how you may need to compensate for the meter reading you obtain. This is less true for an incident meter, since an incident meter is actually measuring how much light is reaching the subject as opposed to measuring how that light is reflected off the subject. But some degree you’ll need to compensate for the meter reading regardless of the particulars.
Most photographers will be able to get by perfectly well with their in-camera exposure meter, taking into account how you may need to compensate for the area of the scene you’re metering off of. In some cases, however, it can be helpful to use a handheld meter, and especially an incident meter, such as when photographing a subject that is illuminated by a series of strobes rather than natural light.