When to use high speed flash sync (HSS)

Let’s cut straight to it – there are only two reasons you would need to use high-speed flash sync:

to have an appropriately shallow depth-of-field,

to have a sufficiently high shutter speed to freeze action.

That’s it. Just those two things. When you need shallow DoF, or a faster shutter speed than max sync speed, then you go to high-speed flash sync.

What HSS doesn’t do – it doesn’t allow you to overpower the sun. When you go to from normal flash mode (at or below max sync speed), into high-speed flash sync, then you lose approximately two stops of light. Not a good thing when you need a lot of juice from your flash to match the sunlight.

Check this tutorial on high-speed flash sync for a more thorough explanation. Note how in HSS mode, the shutter speed becomes a linear control of the flash. As you change your shutter speed up in full stops, the flash power effectively drops by a stop. But since you are opening up your aperture by a stop (for the same exposure) when you change your shutter speed, the flash output remains the same.

In other words, if you change from 1/1000 @ f/4 to 1/2000 @ f/2.8 your flash exposure remains the same. Nothing lost. Nothing gained. So really, if you want to overpower the sun with flash, you’re most likely not going to go to high-speed sync as a default. If you’re still of the belief that HSS allows you to overpower the sun, check that tutorial again, camera in hand.

While we’re dispelling mistaken beliefs, flash does not necessarily freeze the action. It depends on the speed of movement of your subject, but mostly it depends on the balance between ambient light and flash. The more you under-expose your ambient, the more likely you are to be able to freeze movement with flash. This tutorial – will flash freeze the motion at slow shutter speeds? – explains it in more detail. A ‘rockstar’ photographer mistakenly explained it recently on a live web broadcast – “the more flash you use, the better it freezes the action”. This is entirely nonsense. It has to do with the balance between flash and ambient light – not the amount of flash you use.

Let’s continue then: we use high-speed flash sync for two reasons – a faster shutter speed, and / or a wider aperture.

1.) Using HSS for a faster shutter speed, to freeze movement

Let’s look at these two photos of a model leaping, (taken during a photography workshop in New York.) The camera settings are shown for each image.

The techie details: we used a Profoto B2 flash (affiliate) with a Westcott Rapid Box 36″ Octabox (affiliate) for our lighting. The Profoto B2 allows for high-speed flash sync, and in the shaded area on this New York sidewalk, we were even able to use an octabox to diffuse the light for flattering light.

Comparing the two photos, there are 2 stops difference in the shutter speed. (The exposure is different by that third of the stop as I changed the ISO non-sequentialy, but it isn’t important here.) The shutter speed was bumped up from the max sync of 1/250 to a higher shutter speed. These two crops at 50% will tell the story of how important that bump in the shutter speed was.

Now, you may still want to ask whether flash would’ve frozen the movement here. The ambient-only exposure will show how close we were in the ambient exposure. Somewhere around two stops under-exposure. Not enough for the flash to truly be the dominant light source – and then be able to freeze the movement. Also keep in mind that her leap isn’t very fast action. You’d have to take the shutter speed higher still to freeze faster motion.

Photo gear (or equivalents) used for this photo session

Direction & Quality of Light I wanted to distill the essence of what we, as photographers, work with – light! Before we can truly grasp on-camera flash and off-camera flash, and really, any kind of photography, we have to be aware of the direction and quality of light. We need to observe the light that we have, and then decide how best to use it, or enhance it. With this book, I try my best to share those “aha!” moments with you, and I do believe this book can make a difference to your photography. The book is available on Amazon USA and Amazon UK, or can be ordered through Barnes & Nobles and other bookstores. The book is also available on the Apple iBook Store, as well as Amazon Kindle.

2.) Using HSS for a wider aperture, for shallower DoF

The shallower depth-of-field we get with a wide aperture, gives us a wonderful separation from the background. The blurred background now becomes an element in the composition. This is where high-speed flash sync comes in handy – it allows us to take our shutter speed past maximum flash sync speed. This increase in shutter speed, allows us to use a wide aperture. Beautiful.

And just before you want to throw that word ‘bokeh’ there in the mix, yes, the bokeh here looks pretty damn fine. However, always keep in mind that shallow depth-of-field does not necessarily mean good bokeh.

Camera settings & photo gear (or equivalents) used for this photo session

The image at the top was taken at a workshop in Las Vegas, using speedlights in a softbox. For that shallow depth-of-field, we went to high-speed flash sync.

1/800 @ f/2.8 @ 200 ISO

Summary

I hope this article’s simplicity really brings this element of flash photography home – high-speed flash sync allows us to use a faster shutter speed, and hence wider aperture, and this in turn gives us to freeze movement better, and to get shallowed depth-of-field. And that’s what our decision on whether to use HSS, should hinge on. An elegantly simple decision which gives us better results as we need.

Related articles

A little bit of homework

Here is a previous tutorial article if you want to work through your best camera settings when using flash in bright sunlight.