This will give you a brief introduction to using your Photon+ as well as the use of incident light meters in general.
When photographing something (we'll call that something the subject) there are two types of light metering:
- Incident metering, measuring the light falling on the subject
- Reflective metering, measuring the light reflecting off the subject
A question you need to ask yourself when using reflective metering is "what the light is reflecting off?". A lump of coal under bright light will reflect more light than a snowball under dim light. Unless you factor in what the subject is to your choice of aperture/shutterspeed then you may run into issues with your exposure.
Incident metering isn't always better, but if you can physically get to where your subject is to take a measurement, and the light conditions are relatively constant (eg portraiture), then it's pretty foolproof.