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
- Nikon D810
- Sigma 50mm f/1.4 DG (for Nikon) / Sigma 50mm F1.4 DG (for Canon)
- Profoto B2 To-Go Kit
- Profoto transmitters
- Westcott Rapid Box 36″ Octabox (B&H / Amazon)
- tall Manfrotto 1004BAC light-stand
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
- Nikon D3
- Nikon 70-200mm f2.8 AF-S VR II / Canon EF 70-200mm f/2.8L IS II … used at 200mm
- Lastolite EZYBOX 24×24″ softbox
- Nikon SB-910 Speedlight controlled by PocketWizard FlexTT5 Transceiver & AC3 Controller
or alternately, the Canon 600EX-RT Speedlite controlled by Canon ST-E3 Transmitter
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
- review: Profoto B2 Off-Camera Flash – photo shoot
- Tutorial: high-speed flash sync (HSS)
- Will flash freeze motion at slow shutter speeds?
- more articles on Off-camera flash photography
- Shallow depth-of-field does not mean good bokeh
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.
Food for thought. Though one more thing you need is a HSS trigger.
I believe Neil would have used the Profoto AirRemote is used to trigger the B2, which has HSS capability.
For speedlights I have used the Phottix Strato TTL.
Thanks for the very informative post, as always. The best way I understand the subject of overpowering the sun is as follow. The flash has a maximum output. When inside the softbox, you lose a stop or two. Lets say that this maximum output at iso 100 is f/11. You are under the sun at midday, so the sunny 16 rule applies. Hence the sun is at f/16. So even with HSS there is a canyon of 1 stop you never can go over. So the only solution to overpower the sun is: use more light or wait for lower ambient light at sunset or in the shade. If there is too much light at midday sun, then the flash becomes more of a fill light than the main light.
That being said, I guess my question is, at what point can you reasonably overpower the main light outside with 1 speedlite and a softbox like the Wescott Rapid box for instance. On an overcast day is it feasible? in the shade of a tree at midday sun?
Thanks a lot :)
For most of the large speedlights, the guide number (in ft, for 100 ISO), is about 110.
That equals having the speedlight at 10ft for f/11 (with the head zoomed to 35mm)
Now, you mention the Sunny 16 Rule and that you end up at f/16 … and therefore that there is a “canyon of 1 stop”. Not really. The Sunny 16 Rule is at 1/100 so if you bump your shutter speed up to max sync speed, you get f/11.
Now couple that with the GN giving you f/11 at 10 feet … and there you have an easy magic set of numbers which will help you with using your speedlight in bright sun.
This is explained carefully here: applying the Sunny 16 Rule, and the Guide Nr of your flash.
So that is for bare flash. If you add a softbox, you lose … let’s say about 2 stops of light. So now you’re down to shooting at around f/5.6 … and this will dictate in what kind of light you can use that softbox with a speedlight.
Thanks Neil for the thoughtful explanation. It helps giving a ballpark idea of how powerful a single speedlight can be.
Hi Neil,
Thank you for the interesting read, apologies if my question is foolish but I am wondering if I would not get the same results setting flash compensation to -1.3 and shutter speed to 1/250 ? keeping aperture at 2.8 and ISO 200.
That would result in the same *flash* exposure, but the ambient light exposure would be 1.3 stops higher. That would mean that the background would be blown out, and the nice glow on the hair would become a nuclear-strength light source, likely flaring out the main image altogether.
But let’s assume for a moment that you *want* to completely nuke the background and could get a recognizable image of the model, and think about what you’d be left with. Raising the ambient exposure by 1.3 stops would also mean that the modelling — the pattern of light and shadow — on the model’s face would be quite a bit different, coming largely from an uncontrolled overhead light source (the sky).
The flash would become fill light, or at best a competing source with the sky, causing things like ugly crossed double shadows. Either way, it’s the opposite of taking control of the light: as fill, it would be slightly taming “wild” light that’s mostly coming from an unflattering angle; as a second light source of competing strength it will create ugly shapes and contouring that would never happen with a “main plus fill” lighting environment.
It’s (almost) never about “enough light”; it’s about the direction and quality of that light.
Thank you for the detailed explanation!
Just to confirm that Stan nailed it.
So perhaps this is just a restatement of the second reason to use HHS… to maintain the background (keep it from blowing out and still lighting the subject with flash as in an outdoor portrait. If I’m shooting OCF speedlight, I might need to keep my f stop at f5.6 to get the flash lighting I desire. At my native sync speed (1/200) the background blows out. Moving to HHS, allows me up to my shutter to tame the ambient while still maintaining the desired f5.6 for the proper amount of flash for my subject.
adding…. and I would have 1) open up the aperture; 2) power up my flash; or 3) move the light it in closer, to account for the decreased flash output in HSS.