Why can't HSS mode be automatically turned on for shutter speeds faster than the max speed sync?

Question

When you have a flash system attached to your camera, you cannot shoot faster than your maximum speed sync (which usually is 1/200s or 1/250s in most models). If you need to go faster, then you have to turn on the HSS mode on your flash.

Having a background in IT, I have always wondered why nobody has made a camera that automatically sends a signal to the flash in order to turn on the HSS when the shutter speed is faster than the camera max speed sync.

Is this a conscious decision that manufacturers have made not to include such feature, so that photographers can be aware of the HSS flash power limitations?

Answer 1

Is this a conscious decision that manufacturers have made not to include such feature, so that photographers can be aware of the HSS flash power limitations?

In my opinion, yes, I think this is why it's usually implemented the other way around: that you have to turn HSS on, and then the camera only stops using HSS if the shutter speed falls below sync speed. Otherwise, the shutter speed stops at sync speed with flash. The shooter needs to make the conscious decision to trade off a -2EV power drop for the faster shutter speeds with HSS.

Given that speedlights are the lowest-power strobes you can use, often using one effectively is about using your power as efficiently as possible. For some folks, say wedding shooters who need faster recycling, a safety rail keeping them out of the HSS "power-suck" zone may be a feature, not a bug.

Implementing it as you suggest, as an auto-switched mode based on shutter speed is likely to result in a lot of very confused shooters wondering what's wrong with their flash when they didn't change any flash settings but suddenly lost two stops of power. Without HSS set, having shutter speeds max out at the sync speed forces shooters to be aware of the sync speed "border" and to make the decision to cross it.

Answer 2

Nikon does.

Nikon speedlights automatically switch into HSS, and Godox strobes do as well with the correct TTL radio trigger. You just have to set the flash sync to 1/250* (or 1/320*) in the camera's menu. The star indicates "auto FP" which is focal plane shutter sync (HSS in Nikon language). I'm pretty sure others do/can as well.

But there is often a problem in the transition shutter speed range. It's not the actual shutter speed that matters, it's the length of time the sensor is fully uncovered - which is the shutter speed minus the shutter travel time. That's typically 1/300sec or so, meaning that in practice the x-sync window is roughly 1/500sec (at 1/200-250sec shutter speed). That mismatch commonly causes slight darkening/falloff gradient along the edge of the frame when shooting at max x-sync speed.

Often the edge gradation/falloff/loss of light isn't noticeable when photographing outdoors or when the light isn't the primary source, but it's usually there; and it can be a problem. That's one reason why it is usually preferable to use a slower shutter/sync speed than the max possible.

Of course, they could then program the switch to HSS to be earlier as well, but often the negative of the timing mismatch/gradient is much less of a problem than the loss of effective power switching into HSS is. So, ideally they should give you a lot more options... but then everyone would have to know why you have so many options and when to use them.

And in reality, it's usually just easier to do it all manually if you do know what/why/when.

Answer 3

A high-speed sync mode is in certain manner equivalent to using a neutral density filter.

Here's why.

If you use ND4 filter, you get one fourth of the light through the filter in front of the lens. This reduces all aspects of exposure, both flash exposure and ambient exposure to one fourth of what they used to be.

If you use high-speed sync with a 4 times faster shutter speed, you get due to the fast shutter speed one fourth of the ambient light. However, if the power supply of the flash is capable of delivering a certain amount of charge through the flash tube, all light produced by the flash expose only one fourth of the picture. To expose the entire picture, you essentially need four times the charge. But the amount of charge that can be put through the flash tube is limited, so you get approximately one fourth of the flash exposure as well.

To me, it seems like a useful feature to limit flash shutter speed to the sync speed, because that's the speed after which the effective power of the flash becomes to diminish. You want to know you get diminished output, just like you know when having to put an ND filter in front of the lens. And it's not a big deal to turn the HSS mode on to allow fast shutter speeds.

Of course it would be a simple firmware change to allow using HSS mode without having to turn it manually on.