Here’s One You Need To Know. What External Camera Flash Sync Speed Means

so..2 flashes walk into a bar

First time using a flash

I avoided using a flash a lot of times. Not because I was able to take a photo without it, but because I couldn’t fully understand how to use them and in which conditions.

I remember once trying to take a photo. The windows in the space I was were big and very bright so everything inside looked underexposed. If I exposed correctly whatever was inside, then the view outside the window was overexposed. So naturally, I figured I should use a flash to maintain a good exposure of whatever was inside the room. I didn’t care much about shades. It was an office space.

So I set up my flash and took a photo. When I looked at it, half of the picture was dark. I figured that the light bounced weirdly so I gave it another try. Same problem. And no matter what settings I used on the flash, half of the picture was dark. I started thinking that it must be something I do.

And it was. I checked all my settings and played around with them, but I didn’t correct one. And that one was my shutter speed. And here is where flash speed sync comes in.

What is flash speed sync?

Each camera brand has a flash sync speed. This means that there is a maximum shutter speed at which you can use the flash. For example different manufacturers have different flash sync speeds: of 1/60 sec, 1/200 sec, 1/250 sec, 1/320 sec.

This means that if you use a shutter speed above that, you will notice that your picture is darker on the bottom.

I will show an example of how a picture might look at different shutter speeds.

Example 1
Example 2
Example 3

For the sake of the examples, I dramatized a bit the black strip.

You see that dark area on the bottom? This is what happens if you go above the flash sync speed. Hold on, I will explain why.

Why is the image getting dark using higher shutter speed than flash sync speed?

It is actually very simple. You need to know what exactly shutter speed is. Shutter is actually made of two curtains which slide from the top to the bottom of the sensor. First curtain goes and then the second one follows. The shutter speed is the speed with which those curtains travel. Most sensors are entirely exposed to light at speeds lower than 1/200 (could differ from camera to camera or brand to brand). Which means that the second curtain will follow much later, leaving the entire sensor exposed at once. If the shutter speed is faster it means that the second curtain follows the first one very fast, not leaving the entire sensor exposed at the same time. For example the second curtain could follow the first one, when the first one is still at half way. Both curtains always travel at the same speed.

And here lies the issue. If you use a faster shutter speed than the flash speed sync recommended, the flash will influence only the part of the sensor not covered by the curtain. And because the second curtain follows the first one very fast, it means that not the entire sensor is exposed at the same time. And the part of the sensor covered by the curtain will not be “touched” by the flash.

The flash triggers when the first curtain finished sliding. At that time the second curtain is already in motion, covering a part of the sensor. And that part is the darker area in the examples above.

You said top to bottom. Why is the dark area at the bottom then?

You are right. I did say that the curtains travel from top to bottom on the sensor and the images I used as examples, show the darker area at the bottom, implying that the curtain traveled from bottom up.

This is because the image is projected on the sensor upside down and backwards, but when you see it on your camera, it is side up and the right way around because of the prism built into your digital camera.

Is there a way to go around this?

Not really. Some manufactures added an option of higher flash speed sync. This could be the answer of maybe getting rid of the flash limitations on DSLR. Medium format cameras have a higher speed because they use leaf shutters instead of focal plane shutters (the one a described above). If you run around the internet, you will find some hacks that could allow you to use higher speed sync, but those hacks are not quite the answers we are looking for.

Until then, we have to wait and hope that with next dslr generations maybe we could get a higher sync speed.