Flash Sync and High-Speed Flash Sync
So you finally bought yourself an external flash this Christmas having listened to advice that an off-camera flash unit can help you take your photography to the next level.
But there is a problem. You can’t seem to make good use of it!
When using the flash via a remote trigger, or via an e-TTL cable, your camera does not go faster than a shutter speed of 1/200 (or 1/250) of a second.
When attempting to use a shutter speed faster than that you get a black band in your images, as if there is an opaque object in front of the camera.
That opaque object is actually the shutter curtain (most certainly the rear one of two that’s there in a DSLR).
Additionally, there may be a situation when you simply cannot use the flash with the desired results in bright sunny conditions and or backlit situations.
We shall be discussing these problems one by one in this article and share a few tips to get better images using your external flash.
Operation of a focal plane shutter
Before we understand how to overcome the dreaded black bands in your images let’s understand how a typical focal plane shutter works.
A focal plane shutter has two curtains, which opens and closes vertically, to expose or block light coming through a lens.
These are called the first and the second (or rear) curtain. The first curtain opens to expose the sensor and then the rear curtain moves in to block it.
At slower shutter speeds, these curtains open and close with sufficient pause between each movement so that a flash fired in between can expose a scene correctly.
At faster shutter speeds the curtains move at such high speeds that it’s just a thin slice of opening which travels across the sensor.
When the flash fires it is just an intense burst of light and invariably catches the curtains as they are moving. This is the mystery behind the black bands.
Flash sync speed
To counter this problem camera manufacturers have programmed a maximum sync speed of 1/250 or 1/200 of a second.
You cannot set a shutter speed faster than that when using an on-camera flash or when using an external flash connected (or when remotely triggered) to a camera via a TTL chord.
That way the camera kind of guarantees that you won’t get any surprises in your images.
Using High-speed flash sync
The maximum flash sync speed, however, does have a drawback and that obviously, is that you are limited in terms of the shutter speed that you can use for a scene.
You must be thinking, “How can I use a shutter of 1/1000 of a second in broad daylight with flash on?” Well there is an answer and the answer is using high-speed flash sync.
High-speed flash sync is a kind of cheat for the maximum flash sync of your camera. To turn it on you will need to check the manual for the exact steps applicable for your camera.
For Nikon systems, simply go to ‘Custom Setting Menu’ > ‘Bracketing / Flash’ > ‘Flash Sync Speed’.
Here you have multiple choices to set the maximum sync speed from 1/60 of a second all the way up to 1/250 of a second. There is also an option for setting 1/320 of a second. Select this. Now your camera will be able to sync beyond 1/250 of a second.
How it works?
We just discussed in the previous paragraph that flash cannot expose an image with the shutter curtains only allowing a little bit of window through which to allow light to reach the sensor.
So, how does high-speed flash sync work? Actually, high speed flash works not by sending a single blinding pulse of light, but by sending down pulses of light at short intervals while the exposure is being made.
This way the entire frame is covered by the flash as the curtains moves across the sensor.
Using high-speed flash sync in bright outdoor conditions
High-speed flash sync is useful in situations where you want to incorporate the background but the ambience throws the metering system haywire.
For example, you are shooting at the golden hour. The sky has a magical hue to it and you want to capture that in the background. However, there is a problem.
If you meter for the sky the face of the subject will become dark and if you meter for the subject’s face you lose all that detail in the sky.
This is where you may want to use a flash set to high speed sync. But first you will need to pull down the exposure. You could do that by using a faster shutter speed or by using a bigger f-number.
Pulling down the exposure will slightly underexpose the scene and make the details in the sky visible in the final image.
Next, you set the camera to high speed flash sync and make an exposure. You will a have perfectly exposed subject with a background that is also well exposed.
What are the drawbacks to high-speed flash sync?
If you are using an external flash (speedlight) the flash does not fire at its optimum capability. The lower intensity of the light allows the flash to fire several pulses at such close intervals without dying.
But the same lower intensity makes the light incapable of illuminating a bigger scene or a subject standing further away from the camera.
If you are going to use high-speed sync, it is better to use strobes which are more powerful than speedlights.