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Tangents

max flash sync speed

December 13, 2008

settings: 1/250th @ f2.8 @ 400 ISO, with +1.0 flash exp comp

maximum flash sync speed

In previous postings I have mentioned that the specific settings for a photo aren’t often of direct value to us in figuring out the method of exposure. But the one setting that is of huge importance when using flash, is your camera’s maximum flash sync speed.

When the ambient light levels are low, then your shutter speed can vary appropriately, depending on what you want to achieve. But once you work in bright conditions, or have your subject against a bright background, then most often it just makes the best sense to work at maximum flash sync speed, as in that image above.

There is something very sweet happening at that point, and I believe it is imperative for every photographer that use flash, to know what is happening, and why.  I would venture as far as saying that this blog posting contains some of the most essential information I can give you about flash photography.

So let’s work through some of the basics …

Let’s work through a hypothetical scenario where our subject is in shade, and our background is much brighter.

For the following tutorial, get your camera and lens out, with a speedlight so that we can go through some of the settings.

Let’s say our background exposure is 1/60th @ f11 @ 200 ISO
Now, it should be obvious that the following are all the same exposures:

  • 1/60th @ f11 @ 200 ISO
  • 1/125th @ f8 @ 200 ISO
  • 1/250th @ f5.6 @ 200 ISO
  • 1/500th @ f4.0 @ 200 ISO
  • 1/1000th @ f2.8 @ 200 ISO

If this isn’t immediately clear, then  check out this posting on Things You Need To Know.

Let’s go ahead:

Step 1.
Switch your Speedlight on, and take it out of bounce position.  The reason for this is that I want you to be able to see the distance scale on the LCD display. Put your flash-head directly forward.  The distance scale disappears in the bounce position, since your camera and flash then have no idea of the distance you’re going to bounce your flash off.

The Nikon SB-800 scale looks like this:

.
.. and the Canon 580EX scale looks like this:

.
The distance scale shown on the back of the speedlight, shows the range in which the Speedlight can give correct exposure.  The maximum distance is how far your camera has calculated you will still get correct exposure for the ISO and aperture settings you’ve chosen.

Some speedlights like the Nikon Sb-600 and Canon 430EX don’t show the distance scale.  Sorry, you’re out of luck there, but please follow the rest of the explanation anyway.  There is a certain logic and consistency here, and hopefully it will all make sense in the end.

Step 2:
Switch on your camera and set your camera to manual exposure mode.
If you are using a Nikon D-SLR, set your exposure compensation on your camera body to zero.

Step 3:
Set your flash to TTL exposure, or whatever flavour of TTL your camera spouts, whether E-TTL or E-TTL2 or i-TTL or D-TTL.  It’s all pretty much the same.

Step4:
Set your flash exposure compensation on the camera body and on the back of the speedlight to zero.

Step 5:
For the purposes of this explanation, disable high-speed sync on your camera / flash.
If you don’t know what is meant by this step, then good.  Hang in there.  This will make sense in the end. :)

For Canon users, High-Speed Sync (HSS) is set on the back of the Speedlight.  It is that little H button with the lightning symbol. Push it so that HSS is disabled, ie no H and arrow shows on the LCD display.

For Nikon users, this is usually custom function E1.  Set it the default, ie, not on Auto FP.

Step 6:
Not so much a step, but a reminder that shutter speed has no (direct) effect on flash exposure.   This little snippet of info is hugely important.  If you’re not sure of this, or haven’t seen this in practice yet, just make a mental bookmark .. we’ll come back to it again.  But for now, just remember, shutter speed has NO effect on FLASH exposure.

Step 7:
Let’s see what the distance scale says on the back of our speedlight for that  series of settings mentioned earlier:

  • 1/60th @ f11 @ 200 ISO
  • 1/125th @ f8 @ 200 ISO
  • 1/250th @ f5.6 @ 200 ISO
  • 1/500th @ f4.0 @ 200 ISO
  • 1/1000th @ f2.8 @ 200 ISO

Now, as much as we had said that shutter speed has no effect on flash exposure, for this example, let’s just change the shutter speed settings as well.  For ambient light, there is a direct correlation between shutter speed and aperture.  They work in tandem.  In that sense, shutter speed has an indirect effect on flash exposure, since it affects our aperture .. and aperture (and ISO) has direct effect on flash exposure.  (Check this post if you’re unsure about this – TTL flash vs manual flash.)

Now set your camera to 1/60th @ f11 @ 200 ISO with your speedlight on, and pointing directly forward.  Your distance scale should read something along the lines of having a maximum distance of 20 ft. Within this range, your speedlight should give you correct exposure for your subject.  (And here we are working with the hypothetical scenario where our subject is in deep shade against a background that is 1/60th @ f11 bright.

Step 8:
Now change your camera settings to 1/125th @ f8 and notice how your speedlight’s range increases as you open your aperture from f11 to f8

Step 9:
For the next step, set 1/250th @ f5.6 .. but note that some cameras will be limited to 1/200th or 1/180th.  The Canon 5D for example won’t go higher than 1/200th now with the speedlight on.  (Hang on there, we’ll get to the explanation of why.)

Step 10:
Set your camera to 1/500th @ f4 … and note that the moment you touch your shutter button and activate your camera’s meter, the shutter speed limits again to 1/250th (or 1/200th or such)

Here’s why you can’t set your camera’s shutter speed over a certain value when you have your speedlight switched on (and have HSS disabled):

As can be seen in this explanatory diagram, the light from your flashgun  is dissipated as an instantaneous burst of light.  It is somewhere in the order of 1/2000th of a second. Fast! It is hugely important to realise here that the light from your speedlight  isn’t continuous light, but instantaneous.  (Or rather, near-instantaneous, for the pedantic ones reading this.)

Your shutter consists of two curtains that travel across your sensor (or film gate). So for flash to be exposed over the entire frame, your shutter speed needs to be low enough that the first curtain has completely cleared the sensor area (ie, the sensor is fully open), BEFORE the second curtain starts moving. In that moment, the flash is discharged, and the entire frame is exposed.

If we work in a dark enough area such as a studio that is completely dark, then we can get correct flash exposure at shutter speeds like 1/8th or 1/15th or 1/60th … the shutter speed has no effect on flash exposure … while we remain below maximum sync speed.

So maximum sync speed, is the highest shutter speed at which the entire frame is still open for flash to expose for the entire frame (whether a digital sensor or piece of film).

If we go over maximum sync speed, we’ll get one of the shutter curtains blocking the flash exposure, such as in this example:

The image on the left had the shutter speed at 1/60th, and the image on the right had the shutter speed at 1/320th. That dark area on the right is the shadow of the one shutter curtain obscuring the light from the flash.

Step 11:
So we’ve now hit a ceiling – the highest shutter speed that we can set on our cameras, with the speedlights switched on.  This is also the highest shutter speed at which you could work with studio lights.  But for studio work, you would most often use a shutter speed lower than maximum sync speed.  This is because there is often propagation delay, causing the flash to be triggered a touch later, and you end up seeing the edge of the shutter curtain.  So most wise studio photographers will work at a shutter speed like 1/125th, which is well below maximum sync speed, avoiding propagation delay.

However, we’re working with on-camera speedlights, and can comfortably work at maximum sync speed.  But … we’ve hit that ceiling.  We can’t use flash at shutter speeds higher than max sync speed.  However, this is only true for ’old school technology’.

High-Speed Sync Flash

So with the older flash technology, flash is dissipated as that high-energy burst of light .. but camera manufacturers came up with the stunning adaptation of that technology, where they dissipate the energy from the flash as rapidly pulsed light.  In effect, the flash now becomes continuous light over a very short period.  The light from the flash is now dissipated even as the shutter curtains move across the frame.  As that window between the two curtains move across the frame, the light from the camera’s speedlight is dissipated … exposing correctly for the entire frame.  Remarkable technology!

But .. and yes, there is always a but … this comes at a price.

Instead of the energy from the flash being dissipated now as a high-energy burst, the light from the flash is now dissipated over a longer period.  This means the effective power from our speedlights is reduced when we switch to high-speed sync mode, instead of the old-school way of triggering our flash as that high-energy burst of light.

Let’s confirm this:

Step 12:  (This will vary for the different makes and models of cameras)

all Canon users (except for the 5D):
Select High-Speed Sync (HSS) on your speedlight by pressing that button on the back of your speedlight.  Now set your camera to 1/250th @ f5.6 (as we had it in this example).  Now lightly touch your shutter button to activate your camera’s meter … and while watching the distance display on the back of your speedlight, change your shutter speed from 1/250th to 1/320th.  You should see an immediate drop in the effective range from your speedlight as your move from the high-energy mode of flash dissipation, over to HSS.

for Canon 5D users:
Switch HSS off.  Now set your camera to 1/200th @ f5.6 (as we had it in this example).  Now lightly touch your shutter button to activate your camera’s meter … and while watching the distance display on the back of your speedlight, hit the HSS button on the back of your speedlight.  You should see an immediate drop in the effective range from your speedlight as your move from the high-energy mode of flash dissipation, over to HSS.

for all Nikon users
Enable Auto-FP on your camera.  For most Nikon D-SLRs this is custom function E1.  Auto FP is Nikon’s version of High-Speed Sync (HSS).  For the purposes of this explanation, switch it to 1/250th Auto FP.   Now set your camera to 1/250th @ f5.6 (as we had it in this example).  Now lightly touch your shutter button to activate your camera’s meter … and while watching the distance display on the back of your speedlight, change your shutter speed from 1/250th to 1/320th.  You should see an immediate drop in the effective range from your speedlight as your move from the high-energy mode of flash dissipation, over to HSS.

** the D700 with the SB-900 doesn’t quite follow the linear change in settings as explained here.  I’m not sure why yet at the moment, but will try and figure it out.

So everyone should be able to see how switching to the High-Speed Sync mode, reduces our effective range by about half, or even more.

So where do we have the most range / effective power from our speedlights ?
At maximum sync speed!

For Canon 5D users, this would be 1/200th with HSS set off.  For all other cameras that have HSS as a feature, maximum sync speed will be 1/250th or perhaps 1/180th or such.

For non-5D users .. the moment you go over the maximum sync speed, your camera will go into HSS mode, and your flash range will be reduced.  For 5D users, the moment you set your camera to max sync speed (1/200th with the H on), your flash range will be reduced.

It becomes apparent that maximum sync speed is indeed a very sweet spot to work at when we work in bright conditions with flash.  Or if we want the most range from our flash.

So let’s state the same thing in a few different ways, and see how it makes sense:

  • At maximum sync speed I have my widest aperture (which is in fact what gives us the most range / effective power) from our flashguns … while still remaining in the non-HSS (ie, old school) way of dissipating flash.
  • If I work at maximum sync speed, I have my widest aperture while remaining in the high-energy mode of dissipating flash, and hence my speedlight will recycle faster, and my battery pack work less hard.
  • I will have the most effective power from my speedlight at maximum sync speed, because my aperture is wider.

Let’s look at the opening image again:

I bounced my flash behind me into the open church, in order to get soft light on my bride.  Since I knew I would by dumping a lot of juice from my flash in the hope that enough light reflects back to expose correctly for her .. I immediately went to maximum sync speed.

If I can’t do it at 1/250th @ f2.8 @ 400 ISO .. then I won’t be able to do it at 1/125th @ f4 .. and especially not 1/60th @ f5.6 … and indeed, in this instance, I was actually at the very edge of what my speedlight could deliver.  Hence the stained glass windows blowing out a touch.  I would’ve preferred 1/250th @ f4 to get more saturation in the windows, but my flash couldn’t deliver that with the way I was bouncing flash.  So I accepted the compromise of the stained glass windows being a little over-bright.

If I had used 1/250th @ f4 @ 800 ISO, then the balance between my ambient light (the stained glass windows behind her), my flash (the light on my bride), would’ve been the same.  Bumping up my ISO affects both my flash exposure (or range), and my ambient light.

Another example:

settings: 1/250th @ f4.5 @ 400 ISO, with +1.0 flash exp comp

Once again, with this scenario – the model against a very bright background – I immediately went to maximum sync speed.  Whatever my aperture and ISO settings are, they will see-saw around my choice of max sync speed.

Similar to our previous reasoning: 1/125th @ f6.3 would’ve given us the same ambient exposure, but my flash would’ve had to work twice as hard at 1 stop down.  My speedlight would be slower to recycle, and I would’ve had to shoot slower.  Using this smaller f-stop mean I would use up the speedlight’s batteries faster.  My speedlight would also start overheating faster.  And .. I might not even be able to push that much light from my speedlight if I am bouncing into a large room.

So it just makes sense in this kind of scenario, to immediately default to maximum sync speed.

Photographers using off-camera lighting in bright ambient light, will, for very similar reasons, work at (or very close to) maximum sync speed.

As I had mentioned earlier, there is something special happening at max sync speed, and we need to be aware of it … and use it.

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