7 things you should know about camera flash light
1. A FLASHGUN’S POWER IS EXPLAINED BY ITS GUIDE NUMBER If you’re in the market for a new flashgun, you’ll naturally want to know how powerful it is. The way manufacturers measure this is with a guide number (GN), which unfortunately is the least intuitive system ever conceived, and also the most exploited. Here’s the principle behind it. GN is calculated by multiplying the maximum flash-to-subject distance (in metres) at which a certain aperture will correctly expose the subject, assuming the ISO is 100. Confused? Let’s check out an example. Imagine you’re shooting a portrait at f/4 and ISO 100. If your flash has a guide number of 40, this means you can be up to 10m away (as 4x10=40), as any further will underexpose the subject. Some flashguns zoom in by narrowing the throw of light, which changes the GN, so bear this in mind when researching spec. You’ll also need to check a manufacturer isn’t measuring in feet rather than metres, or at ISO 200 rather than 100, as these, too,