Today I wanted to explain what flash sync speed is. In short the Flash sync speed is the fastest shutter speed at which you can use your camera and fire flash to get a properly exposed image.
The sync speed is a limitation of your camera and how the shutter inside of your camera works. What happens when you take an image is you press your shutter button and inside of the camera, there is a shutter that is blocking the image sensor from light, that shutter will begin to open to take your photo allowing the image to be exposed.
Now one of the important things to understand is that as that shutters moving upward, it is covering part of the sensor and leaving some of that sensor exposed. And what that means is there’s more light at the bottom of the image sensor than there is at the top of the image sensor.
Now this gets balance out with what happens at the end of the photo because at the end of a photo, a second curtain starts to close behind that first one and that second curtain also exposed as part of the sensor while blocking some of the sensor.
And what it does is it evens out the exposure. Everything is timed perfectly so that your image is evenly bright from top to bottom.
So that’s basically how the shutter in your camera works, and the sync speed is the fastest shutter speed at which you can take a photo where the entire first curtain opens all of the way before the second curtain has to start closing behind it in order to maintain and even exposure across the entire image.
When you set a shutter speed that is faster than your sync speed. What happens is that first curtain starts to open, and before it’s gone all the way up to the top to finish taking the image, it opens and then the second curtain starts to come up behind it and they move together across the image sensor to even out the exposure and make sure that your picture looks the way you expect it to look.
This is a limitation for flash photography because your flash fires once and it fires for a very brief period of time, so the entire image sensor has to be open and exposed to the flash when it fires.
For every shutter speed that is your sync speed or slower, the image sensor will be fully exposed for a brief moment when that first curtain is fully open and before that second curtain starts closing. And when you’re using a flash, that’s when your flash fires.
If you try to use a flash and you’re using a faster shutter speed, say you’re seeing speed is one to fifth of a second and you’re using 1/500th of a second then what will happen is your flash will fire with part of the image sensor being blocked by the top or the bottom curtain and you’ll get an image with a black band across it. That black band is a picture of the curtain in the camera blocking the image sensor. So what this means is two things
Number one, when using Flash, if you put a flash on your camera, you may find that you cannot go above a certain shutter speed automatically. The camera will not let you do it. It depends upon your camera. But common sync speeds are 1/180th, 1/200th and 1/250th of a second.
However, if you’re not using a flash that communicates with the camera, your camera may not recognize that there’s actually a flash there and it might let you set your shutter speed at something that is faster than your sync speed. If that is the case, then you’re going to get that image with the black bar, and what that means is when you see this, you need to remember that you’ve set your shutter speed faster than your sync speed and bring it down to at least your sync speed.
It’s a very simple concept, but it’s one that’s very important to understand and remember as you’re learning and using flash photography.
I used to be scared of my flash, and I understand how intimidating flash photography can be, but a flash is just a tool. Once you understand how that tool works you’ll be using your flash to create photos that used to seem impossible.
If you want to master your flash and take those amazing photos, check out my Understanding Flash Photography Video Course.