This is all well and good for photography lit by continuous light sources like the sun, incandescent lights, LED lights, etc. What happens when you need to figure out the exposure for studio flash units that are not TTL** compatible? The meter in your camera has no concept of flash, so you need to rely on an external meter. Typically these have been hand-held devices such as the one shown here, my trusty old Sekonic L-358 (that is no longer in production). There is also a new meter, the Illuminati Meter (which I have not yet had a chance to try), that can be placed in the scene and connects with your mobile phone so you don’t have to hold it in your hand.
These meters can either be connected to your flash via a sync cable, a wireless transmitter, or they can be set to wait for a flash of light to trigger the reading. As noted, these meters are placed in the scene to read the light that is reaching them, the “incident” light. This provides a more accurate reading because it only reads the light falling on the subject and is not swayed by the brightness or reflectance of the subject or anything else in the scene. You set the ISO and shutter speed on the meter to match that on your camera, fire off the flash, and the meter tells you the aperture (F/stop) to set the lens to. For the most accurate readings the dome of the meter should be pointed at the main light illuminating the scene.
Some hand-held meters can also be set to read reflective light and/or be used as a spot meter, but then you run into the same issues that you have with your in-camera meters. And you don’t have the computer “smarts” of Evaluative or Matrix metering in the hand-held meter.
The Illuminati Meter mentioned above and another relatively new device, the Lumu meter (iPhone only at this time and also which I have not had the opportunity to work with yet) extend the function of the meter by adding in a color meter mode. Normally color meters cost around $1,000 and only do their one thing. Having this available in a light meter costing around $300 - $500 is a big deal for those who need critical control over color.
Back to the beginning
So, back to the question of “do I need an external, hand-held light meter?” The answer remains a definite “maybe.” It depends on how you work, what type of lighting you use, how comfortable and sure you are about the display on the back of your camera, how much you know about reading the histogram on the camera, and on and on. I have friends like Tony Corbell and Matthew Jordan Smith or take multiple meter readings of every scene and friends like Joel Grimes who never touches a meter. The photographer makes the image, not the tools.
Wishing you good exposures!
* Gnomicello is a birthday gift I received from artist Mike Oncley a few years ago.
** TTL means Through The Lens. Some camera and dedicated flash combinations offer a metering mode where the flash fires twice. Once at low power just before the shutter opens letting the camera meter read the flash and then again with the shutter open to make the exposure.