Being relatively new to photography I want to try some macro photography of flowers in my garden. I've bought a set of macro extension tubes and a ring flash (both unbranded and cheap to start with). I'm using a Canon 50D camera, the few test pictures I've taken so far have all been totally burnt out (white).
I assume the issue is that the camera is metering on ambient light, is there a way to calculate the correct exposure settings so that the exposure is correct or do I just have to use trial and error? I have been using the macro setting on the camera, but think I may have to use manual. The flash unit instructions are externally brief and poorly translated from Chinese but include guide number but no units.