Flash High Speed Synchronisation (Sync) Explained

Flash HSS/Auto FP made easy:

Flash HSS/Auto FP. Flash, front curtain, off-camera, very close to subject to balance with backgroundThis article goes hand in hand with our other article on Maximum Flash Sync Speed so it may be worthwhile reading both together as each one covers detail on two inter-related flash topics. We will also repeat some of the more important concepts where it makes sense to repeat them.

My first experience of flash sync was many years ago when I got my hands on my first flash unit. It was one of the early Nikon TTL flashes and as basic as it was looking back now, at the time it opened up all sorts of new avenues for me. I remember one day out photographing a particular scene in Budapest of all places and I wanted to use the flash to light my subject for a nice tourist style photo on the chain bridge with a backdrop over the river out towards the Hungarian parliament building. It was a sunny day so I was using settings along the lines of 1/4000s, f/4 and ISO200 without flash. But as soon as I turned on the flash unit, the camera reverted back to 1/250s automatically. Of course the whole scene ended up overexposed. I spent about 10 minutes trying to figure out what was happening with the shutter speed but then gave up and shot at f/16. This of course meant the scene was nicely exposed but my subject was deep in shadow as f/16 was well beyond the capabilities of my all-singing, all-dancing TTL flash! I was gutted and only months later when my knowledge of photography grew did I discover the reasons. Has this ever happened to you? Were you ever baffled by your camera magically reducing your shutter speed just because you powered on your flash unit?

Obviously, in retrospect, if I had the High Speed Sync (Auto FP for Nikon) setting switched on this wouldn’t have happened but what exactly was the camera doing? Why was it limiting the shutter speed for my flash photos? Very simply, it was setting the shutter speed to the cameras maximum flash sync speed. Why was it doing this? Well, because each camera has a maximum shutter speed at which it will successfully sync with a mounted flash unit it simply set the camera to that shutter speed to allow me use the flash in normal mode (instead of High Speed Sync mode). The maximum flash sync speed is covered in fine detail in our other article so I will not reiterate the concepts here, only to say that as soon as you exceed this shutter speed on your camera the flash will automatically switch to High Speed Sync (Auto FP) mode and if you do not have this enabled, then the camera will limit the shutter speed to the cameras native sync speed.

Why would you want to shoot at speeds above the native maximum flash sync speed? Sometimes this may be unavoidable, maybe you want to shoot a nice environmental portrait in bright light with a soft, wide-aperture, out-of-focus background, or maybe you are using a telephoto lens and need a fast shutter speed to capture a crisp image, or maybe you want to capture a fast moving subject for example a bird or a sports scene. All of these require fast shutter speeds and to ensure proper flash exposure you may have no choice but to use the flash in HSS (Auto FP) mode.

Focal Plane Shutters and how flash exposure affects overall exposure.Let’s take a look at how the focal plane shutter mechanism works in relation to shutter speeds. For the purposes of flash operation we can consider the range of shutter speeds in two groups, those speeds that are under or on the maximum flash sync speed (Group A) and those that are over (faster) than this maximum speed (Group B). Considering group A (which is represented by diagrams 1 and 2 in the image to the right), for this range of shutter speeds, there will always be an instant in time after you trigger the shutter release whereby the front curtain has full opened and the rear curtain has not started closing yet. This means the sensor is fully and uniformly exposed, no obstructions. During this instant the camera can trigger the flash unit to fire and the resulting light will uniformly light what is being recorded by the entire sensor, no obstructions.

Now take Group B (represented by diagram 3 in the image above right), because this group is above the maximum sync speed, this means that the rear curtain will have started closing before the front curtain has fully opened. In practical terms this means that for all speeds in group B, there never will be an instant in time where the sensor is fully and uniformly exposed, part of it will always be obscured by the front and/or rear curtain. Think of the resulting scene being recorded on the sensor through a slit between front and rear curtains, the width of the slit getting narrower and narrower as the shutter speed increases. For all DSLRs operating at shutter speeds over the max flash sync speed, the images are recorded through this slit between both curtains, no exceptions. This has disastrous consequences on flash based photos using non HSS flash (flash in its standard mode) as detailed in our other article. You will inevitably get the banding that we demonstrate.

Falsh High Speed Sync operating in normal mode via a single pulseSo how do camera/flash manufacturers get around this? If you consider flash operating in “standard mode”, it is simply a single and instantaneous pulse of light triggered by the cameras shutter mechanism electronics when the front curtain has completely opened. When I say instantaneous, I mean it can be considered far faster than the fastest shutter speeds on even the best DSLRs, i.e. around the 1/20,000s mark or faster. For the Group A shutter speeds (i.e. under or on the max sync speed), the sensor will be fully and uniformly exposed for this 1/20,000s pulse of flight but for Group B shutter speeds there will be an obstruction, in the form of the rear curtain. Camera manufacturers get around this problem by continuously pulsing the flash once the front curtain has indicated it has started opening. In this mode, the flash can be considered to be a continuous light source and not a single pulse for the duration of the exposure. This continuous series of pulses lasts much longer than 1/20,000s. In fact it will last as long as it takes the rear curtain to close. Flash operating in normal mode is demonstrated by the image Falsh High Speed Sync operating in HSS/Auto FP mode via a series of pulsesto the left. Here you can see the flash operating via a single pulse when the sensor is fully exposed. This represents Group A shutter speeds. In the image to the right we have the flash operating via a series of continuous pulses. This represents Group B shutter speeds

So why all the hype you may ask, after all, the flash is just extending the number of pulses, why the fuss? Well it is not as simple as that. If you consider a flash operating in standard mode, let's say the pulse lasts 1/20,000s (as an example). For an exposure of 1/1,000s this means that you would need (as an example) 20 standard-mode pulses to ensure even illumination, this means 20 times the number of photons and as a result 20 times the amount of energy. This is where the complications arise. A flash unit cannot output that amount of continuous light. A single pulse is no problem but a series of equally powered pulses is just too much for the flash. Think of it another way, consider a flash in manual mode on ½ power. The flash unit has no problem providing maybe 3 or 4 successive pulses at this power in normal mode, but try and do this 20 times continuously, it simply will not work, even with an external battery pack the flash will not be able to recharge fast enough and even if it could the overheat protection would most likely kick in. So this means the flash must significantly reduce the light output to ensure a continuous series of pulses that is within the limits of the flash and this reduction in output is around 2 to 3 stops of flash exposure depending on the shutter speed. The rule of thumb in HSS mode is for every stop increase in shutter speed (for example from 1/500s to 1/1000s) you will lose 1 stop of flash exposure. This information will be available in the user manuals of each type of flash unit.

So, now that we have established why your flash in HSS mode can be considered to be continuous light for the duration of the exposure, how does this impact the exposure when ISO or aperture is changed? This is an easy one to answer. Because continuous light is similar to ambient light, exposure will respond similarly when aperture or ISO is modified. Assuming the flash is in manual and not TTL (as TTL algorithms will adjust the flash exposure based on composition), changing the aperture or ISO has exactly the same effect as you would expect when making the same adjustments in normal ambient daylight or video light. It behaves linearly.

Summarising the above, we can say that:

  1. If you need to use your flash above the maximum flash sync shutter speed, this will considerably drop the power output of your flash. You will need to position the flash far closer to your subject to get a similar flash exposure as you would in normal flash mode or else use two or more flash units. Using two flashes will double the light output meaning you can move your flashes back or else lower the power output such that they recycle faster. Two flash units in manual at ½ power have the same light output as a single flash at full power but the two flash setup will recycle much faster and battery life will be greatly extended.
  2. In bright, sunlight conditions you will most likely be limited to an aperture of f/11 at a shutter speed of 1/250s (see sunny 16 rule, ISO at 100). An aperture of f/11 is possibly on the limit of your flash unit even in normal mode and it will also mean everything will be in focus. To get nice soft backgrounds there are two options, increase the shutter speed into HSS territory which will reduce your flash output considerably or alternatively use an ND filter to open up your aperture, leaving shutter speed as it is. This way, for example, with a 3 stop ND filter you can open the aperture to f/4, leave the shutter speed at 1/250s, get the most power from your flash and those soft backgrounds that are always desirable. See our summary article on the use of ND filters. There you will find how to optimise the use of ND filters with respect to both shutter speed and aperture and see how their use affects flash photography.
  3. In HSS mode, you lose the ability to flash freeze your subject as a flash in HSS mode is continuous light whereas in normal mode it is a single, instantaneous pulse with a speed much faster than your fastest shutter speed. Some types of photography prefer to use this instantaneous flash pulse to freeze motion as opposed to the shutter speed. For example a super fast moving object like a hummingbird’s wings need lightning speed to stop motion. This is best done using flash to freeze the motion and not the shutter.
  4. To get the most power out of your flash you need to stay at or below the maximum flash sync speed. This will allow you more wiggle room to correctly expose heavily backlit subjects or for example portrait shooting into the sun. You will not have this wiggle room in HSS territory as you have automatically handicapped yourself by removing 2 or more stops of flash exposure in HSS mode. So as a rule, if you are shooting into the sun, absolutely do not use your flash in HSS mode, use an ND filter instead.

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