High Speed Flash Sync

High Speed Sync (HSS)

High speed flash sync isn’t included on all speedlites as standard. Fully manual speedlites usually don’t include it and if you tend to shoot solely in a studio environment then you probably don’t need it. However if you plan to shoot outdoors in bright sunny conditions and plan to use shutter speeds above 1/250 second (or above your camera’s sync speed) then high speed sync is not just a useful feature it’s a necessity.

Let me explain what it is and how to use it. I’ll give the simple explanation first but in order to understand it properly you’ll need to carry on reading below for a more detailed explanation.

High Speed Sync (made simple)

Every camera has a property known as its sync speed. The sync speed is the fastest shutter speed that can be used when working with a speedlite in normal firing mode (non high speed sync). For most cameras this shutter speed is around 1/250 second but it can differ slightly depending on the camera make and model so it’s best to check in the specification section of your camera’s manual or just do a simple search online or check our list of Canon Camera Sync Speeds

For example the maximum sync speed for the Canon EOS 80D camera is 1/250 second. For any shutter speeds below this the speedlite with work fine. Usually when I’m using a speedlite in the studio I’ll put my camera in manual mode and set my camera at f/8 with a shutter speed of 1/125 second and an ISO 100 as a good starting point. I can always change my aperture and ISO to give me the creative result that I want but I usually keep my shutter speed at 1/125 sec which is below the camera’s sync speed. I could set it at 1/200 sec or even at the sync speed itself 1/250 sec and it would be perfectly fine. But if I set the shutter speed above the sync speed, say 1/500 second then things start to go wrong. Photos taken above (faster than) the sync speed will show a black band across the bottom of the image. This is caused by the shutter curtain being in the way.

When you use a speedlite on-camera the camera detects that a speedlite is located in the hotshoe and it automatically prevents your camera from using shutter speeds above the camera’s maximum sync speed. Most of the time you don’t have to worry about it, it just works.

High Speed Sync (HSS) Icon
High Speed Sync (HSS) Icon

So What is High Speed Sync?

If your speedlite doesn’t have high speed sync then you have to make sure that your maximum shutter speed doesn’t exceed your camera’s maximum sync speed and you’ll be ok. If you use faster shutter speeds you’ll start to see a black band at the bottom of the image and you obviously don’t want to see this.

If your speedlite has the high speed sync function then you just turn it on and then, if you need to, you can shoot above the camera’s maximum sync speed only now you won’t see that horrible black band ruining the bottom of the image – amazing, it’s like magic. So what’s the drawback, there’s always a drawback. Well as soon as you start to use higher shutter speeds the speedlites power output rapidly drops. By increasing your shutter speed to 1/500 second your speedlites power output may drop by half (the guide number of the speedlite is less than half it’s normal rating) and this can drop to a quarter at 1/2000 second.

Shooting Outside with Flash in Bright Sunshine

When you are working outside in bright sunshine your camera settings can start to get a bit tricky when try to keep the shutter speed below 1/250 sec and this is when High Speed Sync can come in really useful.

Let’s consider a photoshoot with a model at the beach on a nice bright sunny day. This lighting condition creates hard shadows on the ground as well as some contrasty ugly shadows on the face. We want to use the speedlite to help fill in some of these shadows making them softer and more flattering. A typical exposure for the scene is aperture f/16, shutter speed 1/125 sec and ISO 100 (Sunny 16 Rule)

I’ve included equivalent exposures below.

Aperture f/Shutter Speed

An equivalent exposure setting will be f/11, 1/250 sec and ISO 100. This brings me up to the camera’s max sync speed. I can shoot using my speedlite at f/11 @ 1/250 no problem. If the sun is lighting the model from off to the side of the camera (say at a 45 degree angle to the model) the model doesn’t need to be squinting but there will be hard shadows across the face and possibly darks shadows around the eyes. The speedlite will be used to reduce and soften these shadows to look more attractive. To work out the exact speedlite power setting we’ll need to look at the speedlites guide number and we’ll need to know the distance of the model from the flash.

Full f-stops: f/16, f/11, f/8, f/5.6, f/4, f/2.8, f/2
[see table of full f-stops at 1/3 stop increments]

Let’s use a Canon 600EX ii-RT out on a sunny beach with a model at 4m away and I’ll be using a 70-200mm lens.

The speedlite has a guide number rating at full power of
60m at 200mm at ISO 100
57m at 135mm at ISO 100
54m at 105mm at ISO 100
46m at 70mm at ISO 100
(info taken from the back of the speedlite manual)

Guide number (m) = f-stop x distance

which can be rewritten as

f-stop = guide number / distance

therefore my maximum f-stop at full power
at 200mm = 60 / 4 = f/15 (f/16 is the closest)
at 135mm = 57/4 = f/14
at 105mm = 54/4 = f/13.5 (f/13 is the closet)
at 70mm = 46/4 = f/11

Since the direct sun is already giving us f/11 you can see that the speedlite is more than powerful enough at all focal lengths to be used at 4m away at full power. To fill in the shadows on the face we would only need a flash that could deliver one stop less than the sun i.e f/8.
With the lens at 200mm and the subject at 4m we’d have to reduce the flash power by 2 stops to bring it from f/16 to f/8. i.e we can use the flash at 1/4 power to add a fill light.
When the lens is zoomed out to 70mm and the model at 4m we’d have to reduce the flash power by 1 stop to bring it from f/11 to f/8 i.e we can use the flash at 1/2 power to add a fill light.

Shooting at Large Apertures (f/2.8) in Bright Sun

In the previous example we were shooting with an ambient exposure of f/11 @ 1/250 sec and using a fill light of f/8 . The exposure and speedlite setting worked well for fill light but at f/11 the background is also going to be in focus. It’s therefore a good idea to try and use a plain background such as all sky or all sand to avoid any distractions. A better idea would be to use a large aperture such as f/2.8 to put the background really out of focus, giving us a nice soft blurry backdrop that makes the subject stand out.

From the equivalent exposure table above we can see that using an aperture of f/2.8 at 1/4000 sec at ISO 100 will achieve a correct ambient exposure (but with some hard shadows that we want to try to reduce using fill in flash).

at 200mm
Camera setting for ambient light f/11 @ 1/250 sec with a 1/4 power flash for fill (f/8)

If we set the same ambient light at f/8 @ 1/500 sec we need to engage high speed sync. We are still looking for a fill light of 1 stop less than ambient which will now be f/5.6. Canon says that at 1/500 sec the guide number can drop by more than a half (4 stops less). So at 200mm let’s use a guide number of 30m. Correct exposure would be 30/4 = f/7.5

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