Lots of photographers who are in weddings, photojournalism or even just starting out prefer to use TTL flashes and metering over manual settings. But when they become more advanced, they start to do things manually. Why? Because just like automatic metering, they don’t always trust it. Automatic metering (which is essentially what TTL is) will give you what the flash system thinks you want based on a certain set of parameters.
But like normal exposure settings: metering is metering is metering. F2.8 at ISO 100 and 1/50th is going to soak up the same amount of light no matter what if all given situations are the same. So with that said, why isn’t there a universal standard for TTL flashes yet?
TTL stands for through the lens metering. It takes into account a couple of factors such as your ISO and aperture settings. That’s why lenses being used without providing aperture readout to the camera won’t necessarily work.
Try a flash like the Vivitar 285HV: it’ll teach you all about TTL by showing you what settings your flash should be at based on the aperture and ISO that your camera is set to. And for the most part it’ll be spot on.
So that further begs a question of why there isn’t a universal standard. Just for sales so that companies can say “You should only be using our flashes and our system?” That makes no sense considering that no one is breaking the laws of metering so to speak.
A big question that many people ask to start out with is “Why can’t I use a Canon Flash on a Nikon camera?” And in truth, you probably can but you won’t get TTL metering. You’ll need to do everything manually–which is tough in some ways in certain situations like weddings. Instead, to get TTL metering with Canon cameras you’ll need to use their own branded flashes or something else like Profoto, Phoenix, Yongnuo, etc. Then when you go about switching camera systems, you’ll need new flashes unless you work with all manual to begin with.
From a business standpoint it makes a bit of sense as does lens mount choice. However, lenses have more to do with an actual design of the camera body itself and don’t necessarily affect metering unless you’re talking about what an imaging circle offers. Flash metering on the other hand doesn’t change.
To that end, getting companies to agree and come out with a standard on hot shoes may be tougher. They’re all around the same size overall; but the communication may vary.