When taking a photo using ambient light only, the exposure is controlled via a combination of three elements
(for a more detailed explanation about digital photography exposure click here)
The range of shutter speeds available is typically from as fast as 1/8000th second to as long as 30 seconds.
However if you need to supplement the ambient light with flash (either using the pop up flash or dedicated speedlight flash) then you will find that the shutter speed is limited by the max sync speed of your camera* which is usually around 1/250th of a second.
Why is this? and when might this limitation to shutter speed be an issue? Read on to find out more.....
nb *Some speedlights offer a High Speed Sync (HSS) mode which overcomes this limitation - for reasons which will be explained later in this article
The reason for the max sync speed relates to the way the shutter mechanism operates on most Digital SLR cameras.
Normally mechanical focal plane shutters are used.
When you take a shot, first the mirror (which is reflecting the image through the viewfinder) lifts upwards out of the way - this is why the viewfinder goes dark for a fraction of a second.
Then the shutter curtains which are preventing light from reaching the image sensor move out of the way.
There are two curtains which both move downwards from top to bottom.
The first curtain drops followed by the second. It is the time lag between the two curtains that controls the exposure duration - which is what we refer to as "shutter speed"
There are physical engineering limitations to just how fast the mechanically operated curtains can move. The time it takes for a shutter curtain to actually move the entire distance across the sensor is what determines the "max sync speed" So if your camera has a sync speed of 1/250th second then this process takes 1/250th second.
This operation is most clearly explained and demonstrated in the following excellent youtube video create by "The Slow Mo Guys"
So (by watching the above video) it can be seen that to achieve exposure durations shorter than the max sync speed - the second curtain follows the first curtain with such a brief time lag that the full area of the image sensor is never completely exposed at any one moment in time - rather a moving "letterbox" of exposure passes across the sensor.
So despite the fact that the entire sensor area isn't exposed simultaneously, each individual pixel does in fact get the correct exposure - thus the final image exposure is correct.
So, why is this an issue when using flash?
The reason this is an issue when using flash is due to the extremely short duration of "flash" exposure.
When you fire your flash, a pulse of light is produced in the flash head. The duration of this pulse (rather than it's intensity) is controlled to effect the amount of flash exposure (either manually by the power setting of the flash or by the cameras metering system if using ETTL flash exposure).
The duration for the light pulse can be very short - much faster in fact than the maximum shutter speed of the camera.
For example typical speedlights operate in the range of 1/15000s (lowest power) to 1/250s (max power)
This is why if you exposure a photo where the subject is effectively lit entirely by flash (rather than ambient light) then the subject motion is "frozen" irrespective of actual shutter speed.
The following diagrams will help explain this more clearly
The above diagram shows the situation BEFORE taking the shot.
Note the subjects in the image scene are a little dark - so they will need lighting with flash exposure.
Before activating the shutter button the image light from the scene is prevented from reaching the sensor by the raised shutter curtains - so the image is yet to be exposed.
The above diagram shows what happens with exposure settings of
When the shutter is pressed the first curtain drops fully uncovering the sensor. At this moment the flash fires so the image is exposed across the entire sensor. The second curtain then drops to end the exposure.
Note the image subjects are now correctly exposed by the flash, whilst the brightness of the image background remains unchanged
In the final image we can see what would happen if it were technically possible to set a shutter speed in excess of the max sync speed (most dslrs automatically prevent this from happening - unless high speed sync is activated.)
In this shot the to keep the background exposure the same the settings would be
Here we can see that at the moment the flash fires - both shutter curtains are still scrolling across the image sensor leaving a "letterbox" slit through which the flash exposure can pass. Thus only this part of the image scene will benefit from flash exposure - leaving dark bands of under exposure at the top and bottom of the frame.
In the examples above it can be seen that it wouldn't be possible to choose an aperture of F5.6 as this in turn would require a faster shutter speed to avoid overexposing the background (the non flash lit parts of the scene).
The max sync speed therefore can limit depth of field control (this being dependant upon size of aperture)
For example wide apertures are often preferred with portraits as the resultant shallow depth of field gives a pleasing defocussed look to the background and helps make the subject stand out.
When shooting in bright daylight , flash can be useful as "fill" light to open up shadows areas, especially when subjects are backlit. But in this instance at the 1/250th second max sync speed may require extremely small apertures - which aside from potentially undesirably deep depth of field could result in loss of sharpness due to diffraction limitations or create undesired "starburst" effects from any point light sources (e.g. the sun)
Finally when shooting fast moving subjects that require some fill flash (outdoor sports such as mountain biking for example) the subjects will still be lit by the ambient exposure - so at shutter speeds of 1/250th sec even tho' the flash exposure will freeze motion , the motion blur from the ambient exposure will still be evident.
So, what is the solution? Read on to find out
As explained earlier in this article, the reason that flash exposure is a problem is that the flash pulse duration is considerable less than the max sync speed - meaning that the flash will not fully expose the entire area of the image sensor due to the "letterbox" effect as the two shutter curtains scroll across the image sensor when shutter speeds hight than the max sync speed are used.
High Speed Sync (HSS) overcomes this issue by changing the way in which the flash head works.
When high speed sync is activated - instead of firing a single high energy pulse which is terminated to give amount of flash exposure needed, the flash instead fires a rapid sequence of consecutive pulses. These pulses are so rapid they effectively merge to a single pulse of protracted duration - lasting at least as long as the max sync speed. This effectively means that as the "letterbox" slit between the two shutter curtains scrolls passed the image sensor - there is always light from the flash available to expose the sensor.
The limitation of high speed sync is only that the flash's capability to fire the rapid burst of flash pulses does significantly reduce maximum available flash power (so for example the distance at which a subject can be lit is dramatically reduced - and further more flash units capable of high speed sync tend to be considerably more expensive than more simple speedlights.
One other possible alternative to using high speed sync - especially if the objective is primarily to use a large aperture in order to get a shallower depth of field - is to use an ND filter to restrict the light entering the lens ( and thus necessitate faster shutter speeds)
For example, let us suppose that you want to use flash outdoors in relatively bright conditions (perhaps to overcome dappled shadows on your subject for example) and ambient exposure calls for 1/250th sec at F11 and ISO 100.
Let us suppose you ideally want to use F4 for a shallower depth of field.
This would mean that you would be 3 stops overexposed (F11 to F-8 to F5.6 to F4 = +3 stops) unless you also reduced shutter speed by 3 stops - so 1/250th to 1/500th to 1/1000th to 1/2000th = -3 stops.
But this would now mean that you are well above the max sync speed!
However the shutter speed can be maintained at 1/250th second by reducing the light by a 3 stop ND filter (often labelled as ND8 as 3 stops = 8 times the amount of light). The only downside of this is that composing and focusing can be significantly more challenging when viewing your scene through such a dark ND filter. Note also that the effectiveness of the flash exposure will also be significantly reduced as the flash has to work hard to overcome the darkening of the filter. However if you are relatively close and are using ETTL flash exposure the cameras metering should automatically boost the flash power to the desired amount for correct exposure.
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