Aperature mode and fill flash

[WayneF]

The important thing is shutter doesn't affect flash exposure, only flash sync. Dialing flash power has no effect on shutter speed.

True EXCEPT FOR HSS. Shutter speed is all important to HSS flash. The HSS flash does fast rapid pulses, all run together at a high KHz rate, becoming continuous light, same as the sun is continuous light. There is no concept of sync with continuous light, so HSS works easily at any shutter speed (same as does sunlight and incandescent lights, etc). However, HSS has to run at low power to be able to be continuous, so its around -2.3 EV down from what speedlght flash mode can do. Which is about 20% output, and distance range is rather reduced. But HSS can do the so-called sync (it works at any shutter speed).

Of course fast shutter speed decimates continuous light, so not much HSS light gets through a a 1/4000 second shutter. But.... same as continuous sunlight (and unlike regular flash), now the concept of a equivalent exposures is valid again, and we simply open aperture to compensate for the shutter speed loss. HSS flash exposure is Equal at any Equivalent Exposure (same as continuous sunlight). Being able to use a wide aperture in bright sun is the only valid advantage of HSS, otherwise it is all downside, and regular speedlight flash runs circles around it (power, range, speed, etc). Use of HSS would be dumb indoors where ambient is not significant. The speedlight mode can be much faster and more powerful.

And HSS is only "high speed SYNC", meaning, there is no sync restriction. It is anything but fast flash. Continuous light has no speed, all we have is the shutter speed to help. Instead, speedlights are called speedlights because they can be fast (very short flash duration). But HSS is continuous, on before shutter opens, until after shutter closes. Continuous with respect to the shutter.

Four Flash Photography Basics we must know - Auto FP and HSS