Maximum Flash Synchronisation (Sync) Speed Explained

The history, the evolution and the facts behind max flash sync speed:

Maximum Flash Sync Speed; As it is a complex topic and one worthy of a detailed explanation I think to start this off we’ll go back in to the photographic history books for a minute and then talk about how it has evolved and then on to how it actually works in the real world. Finally we will discuss the effects it has on your flash photography and how operating at maximum flash sync speed is the most efficient way to use your flash. For fans of flash photography the concept of maximum flash sync speed is absolutely critical to understand as it could be the difference between an awesome photo and one for the bin. As a word of forewarning however, if you have not encountered either Maximum Flash Sync Speed or Flash High Speed Sync before then this article may be a bit much in terms of theory, but for those that are aware of the terms then I will try and keep the explanation as simple and practical as possible. I’ve included photo sequences to explain how it operates in real life. Also, this article does go hand in hand with our other article on Flash High Speed Sync so once you’ve had a read of this then take a look at our article on Flash High Speed Synchronisation.

The History:

In the age of the modern DSLR, flashes are electronically controlled and pulse almost instantly once given the command. However back in the good old days, flashes consisted of a bulb in a housing, electrically linked to a contact on the camera's shutter. The bulbs themselves had different ratings in terms of reaching peak illumination and this rating was governed by a letter or series of letters such as M (Medium), F (Fast), X (Instant) and FP (Flat Peak). Class M bulbs reached max intensity around 25 milliseconds after being given the command, F bulbs around 5 milliseconds and X is more or less instant. The last Class, known as Flat Peak is an important one as it is what Nikon uses to brand its version of High Speed Sync.

Each class of bulb required a different method of synchronisation; after all, it is the camera that needed to tell the bulb to fire and since each bulb class handled differently in terms of reaching max illumination then this time needed to be considered when operating the shutter. So for Class M bulbs a type M-Sync shutter mechanism was used, for F bulbs, F-Sync was used and so on. The differences in sync only related back to how the shutter operated within the camera. For M bulbs, the shutter contact activated (signalling the flash to fire) around 20 milliseconds before the shutter completely opened. This ensured that the bulb would be up to full intensity once the shutter was completely open. The same applied for Class F Bulbs: for these, the shutter contact activated (signalling the flash to fire) around 5 milliseconds before the shutter was completely open again ensuring the bulb was up to full power once the shutter was fully open. In terms of X type (Xenon) bulbs similar to the bulbs we have in our modern flash guns, the shutter contact activates immediately the shutter is fully open and because everything is electronic, the flash fires instantaneously, with almost no delay. So really, in terms of technology the only real advancement in terms of triggering the flash has been the delay time. The delay time can be considered to be effectively zero for the purposes of modern day DSLR flash (Speedlight) systems. For the remainder of this article we will be discussing the X-Sync system only as this is what most modern DSLR/flash combinations use. In fact, you don’t need to remember M, F or X systems, I only included them as it should give you a better understanding as to how flash has evolved over the decades.

The Focal Plane Shutter Mechanism:

In terms of the camera itself the key component to consider is the shutter mechanism and as is always the case with these things there are a number of different shutter types. For the purposes of this article we will be considering the 'focal plane shutter' only. Other shutter mechanisms such as electronic shutters or leaf shutters we will leave aside as they are only used on a handful of cameras unlike the focal plane mechanisms which are used on the vast majority of DSLRs on the market today.

For focal plane shutter mechanisms, the film or sensor is exposed using an opening between two curtains, the front curtain and rear curtain. Depending on the shutter speed, the time between the front curtain fully opening and the rear curtain closing will either be long or short. So for slower shutter speeds such as 1/30, 1/60 etc the front curtain will in most cases be completely open before the rear curtain starts to close meaning the opening to the film or sensor is unobstructed, or fully open. Whereas at shutter speeds of 1/4000 or 1/8000, the rear curtain will start to close almost as soon as the front curtain has opened, meaning the sensor/film is exposed through a small opening or slit between the front and rear curtains of the shutter mechanism that travels the width the sensor, uniformly exposing the scene. This is demonstrated in the diagram to the right.

How Max Flash Sync Speed operates:

So where does maximum flash sync speed fit in? Well, given focal plane shutter mechanisms consist of these two curtains as explained, the maximum flash sync speed is the maximum shutter speed you can use on that camera model where neither the front curtain nor the rear curtain obstruct the camera's sensor or film. So in layman’s terms, this means the front curtain has fully opened and the rear curtain has not started to close yet, leaving the sensor wide open to a pulse of light from a flash unit. Let’s think of it another way, what if you have a maximum flash sync speed on your camera of 1/200s and you decide you want to shoot at 1/250s, what will physically occur in the camera is the following:

1. The front curtain will open and begin to traverse the width of the camera sensor.
2. Before the front curtain fully opens the rear curtain will begin to close and obscure a section of the camera sensor.
3. As soon as the front curtain fully opens the flash fires and illuminates the scene
4. The portion of the sensor exposed to this light will record the pulse of light, but because the rear curtain has begun to close, a portion of the sensor will be obscured resulting in no recorded pulse of flash light and only the ambient light recorded before the rear curtain started to close. This will appear as a black band in the photo such as that shown in the second series of photos below.
5. This black band is the rear curtain obscuring the sensor/film as the flash fired.

Non-flash photography does not have this problem because ambient light is usually constant, there is no timing factor involved, each portion of the sensor is exposed for the same amount of time and if the ambient light is constant then the exposure will be uniform. But where flash is involved, timing is extra important and to ensure uniform exposure, the pulse of light emitted from the flash or flashes must trigger when the shutter is completely open and the sensor is unobstructed, otherwise bands or non-uniform exposures will occur.

Have a look at the first series of images. The first 7 photos were taken with the shutter speed below or equal to the max sync speed of 1/250s. The final image was taken at 1/320s. This is above our maximum flash sync speed and as a result the flash will operate in high speed sync mode. Notice the huge drop in flash exposure, around 2 to 3 stops of flash exposure is what will occur when you move from normal x-sync to high speed sync. In all the below photos the flash was mounted on camera. You can easily see how each of the first seven photos are identical. This is becuase up to the maximum flash sync speed of 1/250s there is absolutely no difference in flash exposure (as flash is not affected by shutter speed - once operating below your maximum sync speed). But as soon as you go above the magical maximum flash sync speed (last photo), things change dramatically. The only variable that changed in the below series of photos was the shutter speed. ISO, aperture, flash and camera position, flash head zoom position, flash power, everything else in fact remained constant:

So carrying on with the above sequence, what happens when you go above your maximum sync speed (in this case it is 1/250s) and do not have high speed sync enabled or do not have a high speed sync capable flash unit or TTL intelligent triggers? As soon as you go above maximum sync speed the appearance of black bands starts to occur and the further you go above the maximum sync speed of your camera the more apparent the black bands become. Again, in this sequence of images, the only variable that changed was the shutter speed:

The above photos were taken with a camera with a max sync speed of 1/250s using the flash mounted off camera. There is a bit of a story behind the last sequence of images in fact. Using the Nikon D800 and SB910 with Pocketwizard Flex TT5 and Mini TT1 units I can tell you it is absolutely impossible to replicate the banding seen in the above photos. I spent the best part of 3 days trying to figure out a way to do it. I tried everything, using the popup flash as a commander, using a second SB910 flash in a master/slave setup, I converted the Mini TT1 to a basic trigger using the Pocketwizard Utility but that did not work either as it wouldn't trigger the off-camera flash (apparently with the TTL channel turned off, the Mini TT1 will not convert to a basic trigger unit on Canon or Nikon compatible cameras). I even tried covering the TTL hot shoe contact points with tape such that they would not communicate, but this was annoyingly delicate so I gave up. If I had the basic Pocketwizard Plus II triggers it would have worked, but with the clever Mini and Flex triggers it seems it is impossible to simulate. So in the end I gave up and a friend John (whose Flickr work is here) provided the images. John used a D700 with basic triggers and becuase there is no TTL or HSS information being transmitted, the slave flash is completely oblivious to the shutter speed so will fire in standard x-sync mode and will not use high speed sync (branded HSS for Canon or Auto FP for Nikon). So for owners of the Flex TT5's and Mini TT1's, if you are using a high end Canon or Nikon camera and a high speed sync compatible flash then rest well knowing you will never be able to get the hideous bands, once the nightmare of all flash photographers. If you do happen to get a band it means your flash misfired or the timing was off (such as that to the right). In this case I had 3 remote flashes, all on Flex TT5's, controlled using the Mini TT1 and AC3 zone controller on camera. You can see in the photo that the background flash (with a salmon coloured filter on it) fired correctly, the hair light flash fired correctly but the main light never fired until the shutter was half closed. And this was operating at 1/250s with the maximum flash sync speed set to 1/250 Auto FP on the D800. This was just a single occurrence so I put it down to luck to be honest and not the other off-camera flash bug called propagation delay which we will talk about now.

Propagation Delay:

To complicate matters further, you may find that, even when shooting at maximum flash sync speed that you still have issues with the rear shutter appearing in photos. This is called propagation delay and will almost certainly occur when you are using flash off-camera at the highest flash sync speed for your particular camera. Using a flash mounted on-camera at the highest maximum sync speed is essentially using the camera at the limit of what the manufacturer intended the camera/flash pairing to be capable of doing. The time it takes from the shutter contact activating and commanding the flash to fire to the time the flash has fully illuminated the scene is deemed to occur before the rear curtain starts to close. With a flash mounted on the camera, both camera and flash become one and internal communication can be considered instant. An off camera flash introduces a lot more room for error because radio triggers or infrared must be used to trigger the external flash. The shutter contact must trigger the transmitter, the transmitter fires and the signal is picked up by the receiver, and the receiver then tells the flash to fire. This process simply cannot occur as fast as with a flash mounted on the camera itself.

On a number of high end cameras there will be two maximum sync speeds. On high end Nikons for example the two sync speeds are 1/250 Auto FP and 1/320 Auto FP. This is usually seen in Menu E1 for Nikon cameras. There are other speeds there without the Auto FP text, these are simply the non HSS shutter speeds. The camera will not allow the shutter speed go above what you select once it knows a flash is connected. The Auto FP (Auto Flat Peak/Auto Focal Plane) denotes high speed sync (it is HSS on Canon cameras) and this means that once above this shutter speed then the flash will operate in high speed sync mode. In the case of the recent Nikons, why are there two high speed sync values? Well this is an interesting one and important for photographers that use a lot of off-camera flash. In the case of 1/320s Auto FP on the Nikons, what Nikon are actually saying here is that this is the limit, the maximum at which you can expect normal flash X-sync to occur with an on-camera flash without the hideous black bands. As soon as you take the flash off camera you are not covered. Check out the photo above left. This was taken with the D800 and SB-910 with the max sync speed (Menu item E1) set to 1/320 Auto FP. The shutter speed was set to 1/320 so I was still operating the flash under normal x-sync. As you can see from the resulting image there is no sign of a shutter shadow. Good. Now check out the below right photo. The only difference is that I placed the flash off-camera and even using high end radio triggers such as the pocketwizards the sync is not fast enough and as such the shutter appears in the frame. This is the main reason why Pocketwizard themselves recommend you use the Flex TT5/Mini TT1 combination at the lesser of the two maximum sync speeds, never the higher.

So, how do you get around propagation delay? You have two options:

1. Shoot below the highest maximum flash sync speed of the camera. As a general rule it is best to shoot at 1/3 of a stop below the max flash sync speed when using off-camera flash. So instead of 1/320, use the 1/250 option instead. Or if the camera only has one max flash sync speed then shoot at 1/3 of a stop below this when using off camera flash.
2. Use the High Speed Sync function of the flash unit (if available).

High speed sync is covered in far more detail here so we will not reiterate it but the main disadvantage using this function is that the power of the flash drops by a huge amount. Do not be surprised to see flash exposure dropping by two stops or more when high speed sync is used. In certain situations it will be impossible to avoid high speed sync but from a personal perspective I prefer to avoid it almost always, I opt for the maximum flash sync speed every time and now I will explain the reasons why.

The Magic Number:

Operating at maximum flash sync speed when using flash allows you to use the widest aperture possible once ISO is kept constant, so when operating in bright sunlight this is a huge advantage. Why? As flash exposure is dependent solely on ISO and aperture this means with a wider aperture (letting in more ambient and flash light) the flash doesn’t have to work as hard to deliver the same flash exposure. Or if circumstances require, the wider aperture will give you more distance from the flash allowing the flash to be placed further from your subject, which can be very important in certain circumstances (shooting groups of people, or environmental portraits for example). The wider aperture will indicate a longer distance on the back of the flash unit if the flash has a distance scale. Either way, both scenarios are better. A flash that is further away means more diffuse light. A flash not having to work as hard means faster recycle times and faster shooting. In extreme circumstances such as bright back lighting or bright daylight where you need to balance direct sunlight with flash then you will more than likely need the most power out of your flashgun, and although flash units are small little things, they can output fairly incredible power that will match even the brightest conditions when your settings are optimised, but the key is to optimise this power.

Let’s explain using an hypothetical example: The circumstances exist such that ambient lighting conditions are what you can consider ‘sunny’, meaning ambient exposure settings of 1/100, f/16 and ISO 100 using the sunny 16 rule. An f/16 aperture is a small aperture and more than likely you may be operating outside the limits of what your flash can deliver. In other words, the ambient exposure may be correct but because the aperture is so small your subject flash exposure may be underexposed as the little speedlight may simply not be able to output enough power for the ambient settings configured. If you have a camera with a maximum flash sync speed of 1/250s, then the flash exposure using 1/250s and 1/100s will be the same (for the same aperture and ISO) because flash exposure does not depend on shutter speed. Now, if we modify our ‘sunny 16’ settings such that we use 1/250s shutter speed, this means we are using a shutter speed that is 1 and 1/3 stops faster than 1/100s. To compensate we need to open the aperture 1.3 stops which brings you to f/10 as f/10 is 1.3 f-stops wider than f/16. So in terms of ambient exposure the settings of 1/100s, ISO100, f/16 and 1/250s, ISO100, f/10 are exactly the same. Check the below tables detailing aperture and shutter speed in 1/3 increments.

Aperture (1/3 increments)

1

1.1

1.3

1.4

1.6

1.8

2

2.2

2.5

2.8

3.2

3.6

4

4.5

5

5.6

6.3

7.1

8

9

10.1

11

12.7

14.3

16

Shutter Speed (1/3 increments)

8

10

12

15

20

25

30

40

50

60

80

100

125

160

200

250

320

400

500

640

800

1000

As 1/100s, ISO100, f/16 and 1/250s, ISO100, f/10 are exactly the same, these settings will record exactly the same ambient exposure. So what is the difference? The difference is the wider f/10 aperture will allow greater flash exposure than f/16 at the same fixed flash power output. An f/10 aperture is now in or around the limits of what your flash is capable of in bright conditions. At a fixed manual flash setting, if the flash exposure at f/16 was under-exposed, then for the same manual setting the flash exposure may be correctly exposed at f/10 so to provide the same flash exposure as you had at f/16 you either need to move the flash further away, increasing distance or decrease the power of the flash unit.

If like me you are a shallow depth of field junkie and frown at apertures like f/10 then what options have you got? You have two possible options. If you are at the lowest "base" ISO of the camera and at the limit of the X-sync of your camera (either on camera or off camera) then to open the aperture you can either increase the shutter speed, which will activate high speed flash sync mode (which will reduce flash power enormously) or you can use an ND filter, my favoured option when looking for shallower depth of field in bright ambient light. Add a 4 stop (ND16x) ND filter will allow you open the aperture to f/2.5 and becuase you have cancelled out the effects of a 4-stop ND filter by opening your aperture 4 stops, your flash exposure will remain the same. In other words, the flash power does not have to change to provide the same flash exposure as it did at f/10 without an ND filter.

In summary, operating at maximum flash sync speed allows you to use the widest aperture and allows the flash to be operated most efficiently ensuring faster recycle times and quicker shooting and also allows you increase the distance between the flash and subject which has its own obvious advantages (use in an umbrella/softbox). It is the magic number. As a general rule, when shooting outdoors with flash where you need maximum flash power then always go to the maximum flash sync speed of your camera. Only when you absolutely need a higher shutter speed, such as to capture very quick motion such as sports or quick moving wildlife, should you go above this shutter speed. But be aware that using high speed sync (HSS) you may not get the flash exposure you want meaning you may need to use an additional speedlight or a more powerful flash.

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