Aperture controls flash, shutter speed controls ambient…this is a good mantra to have when trying to understand and control flash.
Some photographers don’t understand the relationship between flash and aperture. A lead photographer, who I was second for, would burn through batteries in their flash cause the were shooting at f/8 at 1/4000 of a second. Their flash was shooting at HSS at full power and she wondered when the images were so dark. I have had conversations (that turn into full blown arguments) with photographers wondering why their image was blurry when shooting at f/16 at 1/16 power and 1/250 of a second. They believed, as well as many others, that shutter speed controlled flash.
When in a fast-paced event like a wedding, two things that are usually in the back of my mind: How much subject isolation and how much ambient do I want in a given scene. When capturing the first dance I try to keep a balance of having my clients in-focus ,as well as taking in the venue. Shooting at f/4 I know that I will have to raise my flash power to compensate if I was previously shooting at f/2.
For first-time users, or those who have had trouble with flash, the mantra above can provide a stepping stone into better understanding how flash works, I know it has for me; eventually leading into move advanced techniques.
Matt Day recently produced a great video that quickly explains the relationship flash, ambient light, aperture and shutter speed all have with each other, and it’s worth a watch.