We often hear about flash sync speed and how we normally cannot use a flash to sync at extremely fast shutter speeds. A flash is an intensely powerful beam of light that is fired instantaneously. It can go as fast as 1/40000 sec. Shutter speeds on the other hand are also pretty fast, but not quite as fast as the speed at which a flash can fire. It can go as fast as 1/8000 sec on most professional cameras.
The question is if a flash can fire at such high speeds and a shutter speed is much slower, then why can’t we fire a flash and properly expose an image at say 1/2000 of a second? Why is that most cameras have a flash sync speed limitation of only 1/200 – 1/250 of a sec?
How Does a Modern DSLR Shutter Curtain Work?
To find the answer we need to look at how shutter curtains in modern cameras work. Shutter curtain refers to a set of two thin surfaces located behind the flipping mirror of your DSLR camera. There are normally two curtains – front and rear or as some prefer to refer them as – the first and the second (curtains). There is a reason for them to be referred as such.
When you press down the shutter release button the first or the front curtain starts to move. When it is completely moved away it exposes the sensor behind it to light. At that point the second curtain starts to move and cover up the sensor and the exposure is shut.
At Slower Shutter Speeds
At slower shutter speed the first curtain opens completely and closes down before the second curtain starts to open and move. That means for some moments the sensor behind the curtain is completely laid barren with no obstructions whatsoever.
At Faster Shutter Speeds
However, when you increases the shutter speed gradually something peculiar happens. For some time the shutter curtain mechanism adapts to the increasingly higher shutter speed by simply hastening up the opening and closing movements. After a while the mechanism simply switches gear. It starts to open the second curtain even before the first curtain has opened up completely. What happens as a result is that a thin slice of the sensor is exposed to oncoming light as the two curtains travel across the surface in a cat and mouse chase.
When You Add a Flash to the Scheme of Things
At slower shutter speeds, when you fire a flash it has enough time to completely expose the sensor and freeze the moment. The limitations of flash sync speed does not step in here. This results in perfectly exposed images. When the shutter speed increases and the flash is fired the whole of the sensor is not exposed to the light. A thin slice of the sensor is and the remaining sensor is simply blocked off by the shutter curtains.
You may have noticed something like a black band on your photos when using a flash at extremely high speed. This happens because only a small portion of the sensor was exposed to light at the instant the exposure was made. Normally, when using built-in / pop-up flashes or external speedlights that are synced to your camera, you would be limited to the maximum sync speed that your camera supports (1/250 or 1/200). So, you wouldn’t be able to see this problem of black band appearing in your images. In some rare cases, however, this problem crops up from time to time.
Let’s say that you use a flash with a camera that are not TTL compatible. You may be able to expose for a scene but the flash will not be bound by the sync-speed limitations. For experimentation purposes you can use such a set-up to check the results. You would find, no sooner you have crossed a reasonable shutter speed, your images will start to have a black band.
Is there a way to counter this Flash Sync Speed Limitation?
There is. One solution is to use a technique known as high-speed flash sync. For this you will require a flash that can be set to high speed sync mode. Note, not all flash units are capable of doing this. In this mode the flash does not fire a single burst of light, rather a series of light bursts to beat the flash sync speed limit. This properly exposes the subject / scene and capture it even with a fast shutter speed. The way they achieve it is by firing a series of light beams as the thin slice of opening traverses across the sensor surface. This is a true mechanical marvel because the syncing is flawless with no areas of the image appearing to be either over or under-exposed.
The other option is to use a camera that does not have a focal place shutter. Now, focal plane shutter is the normal shutter mechanism that modern DSLRs have. Medium format cameras have a shutter mechanism that is a leaf like structure. It operates in a different way when compared to normal focal plane shutters. It resembles the mechanism of an aperture diaphragm. Thus, it can open with tremendous speed, and sync with flash bursts at a higher speed.
The third option is to use a laser trigger. One of the best laser triggers in the market is the MIOPS trigger. It allows you to sync your flash to fire at the instant a sound is heard. You can even set the trigger to fire when a laser line is broken. There are multiple other options. You can even adjust the flash delay, the sensitivity and other settings. It is really a very versatile item.
Medium format cameras are extremely pricey. Not many photographers can afford them and even if they do they are either very well to do with a dedicated clientele and big contracts or are heavily funded. The MIOPS trigger setup is comparatively a lot cheaper but takes time to setup and perfect. The easiest option, thus, is to do high speed sync.