As with most photography, there are some variables to keep in mind. First, the sunlight must be bright, with little or no haze. If you’re shooting on sand, snow or with a bright building around, you will have to stop down. On the other hand, if you are shooting on a cloudy or overcast day, you will have to compensate by opening up. If your subject is not front lit, i.e. back lit, side lit, etc. you will also have to make exposure compensation (see the variable chart). Keep in mind your subject’s tonal brightness. A white or light colored subject requires less exposure than a dark subject. Remember that your in-camera exposure meter is calibrated for average subject tonal contrast of approximately 18% gray. When the meter reads a scene, it tells your camera to exposure the subject as an average tonal scene. You as the photographer must learn to see your subject’s brightness and expose correctly.
Once you know and understand the variables of the “sunny f 16 rule” you can begin to “out think” your photo booth rental company for trade shows camera’s built-in exposure meter. In fact, the rule comes in handy to check the accuracy of all your light meters.
I know a lot of professional photographers who don’t use light meters at all. They have the ability, in the studio or on location, to see the subject’s tones, the subject’s lighting, the subject’s size and then compute the correct exposure, given any ISO rating, in their head. This takes years of experience and begins with a thorough understanding of the “sunny f 16 rule”.