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DSLR shutters, flash sync speeds and the second curtain

by Philippe Dame on October 21st, 2010

One thing I didn’t know much about when I got started with my DSLR was the concept of flash sync speed (also known as x-sync speed). I also didn’t understand what high-speed sync was and where exactly the second curtain was found inside my camera. And finally, I couldn’t figure out why my camera would use a slow shutter speed even when my flash was ready to fire.

Luckily I figured it all out so I thought I’d share these tid bits with you in this post.

There is already plenty of help out there if you know where to look. There is a video I’ve linked to before which is the Adorama TV Digital Photography 1 on 1 series. See episode 17 on flash sync speed and flash duration. Another resource I just found is this blog post by Dan Edwards (has some useful illustrations).

Photo credit: Derek Miller and Canon

The important things to know are…

  • Your camera has two shutters, not one.
  • The first shutter is called the first curtain and it lets light reach the image sensor.
  • The second shutter is called the second curtain and it blocks light from the sensor.
  • At fast shutter speeds, the second shutter follows the first one so quickly that the entire sensor is not exposed at once. It’s just a sliver of light travelling across the sensor from top to bottom.
  • Your flash fires VERY quickly (e.g. 1/10000th of second) which is faster than your shutter. This means you only have a very brief moment to expose the entire sensor.
  • If the second shutter closes before the first one is fully open, your sensor can’t get evenly exposed to the light blast produced by your flash. You end up with a dark band at the bottom of your photo (think of it as the first shutter’s shadow).
  • A relatively slow shutter speed is required for the sensor to be fully exposed, this is called the flash sync speed. While using a flash, your camera usually won’t let you set a faster shutter speed. That speed varies per camera. For the Canon 60D and 7D it’s 1/250th of a second, for the Canon T2i it’s 1/200th of a second.

If you need your flash while using a faster shutter speed (e.g. fill flash while outdoors on a bright day), you must enable the high-speed sync setting on your flash unit. It effectively slows down the flash burst so the light is on while the shutter exposes the sensor quickly but progressively. The trade off is that the flash power is significantly decreased.

If instead you will want a long exposure, you can have the flash fire immediately after the sensor is exposed (this is normal) or just before the sensor is about to get covered up again. Doing the latter requires you to choose an option called second-curtain flash. It allows the scene to get exposed as if you were not going to use a flash and then, bang, the flash hits at the very end. See this example of second curtain flash. If the flash was fired at the beginning of the exposure, the playing card would have been seen at the top of the frame, not the bottom.

Got all that? If not, just refer to the video and blog post I linked above.

Flash Sync Speed in Aperture Priority Mode

Once I did get my head around all this, I had one more issue to solve with my camera. I’d put the flash on, choose aperture priority (Av) mode and then shoot. My camera was left to pick the right shutter speed but it kept selecting very slow shutter speeds when I was in a dimly lit room. The shutter was way to slow to freeze any action, even with a tripod. I thought to myself, “I have the flash on, doesn’t it know I want freeze the action?”. It made me think something was wrong with my camera or the flash unit.

It tuns out that in Canon cameras (perhaps Nikon as well) are setup to treat aperture priority mode differently than other modes. If you put it in aperture priority mode, the camera doesn’t bother to take into account that the flash is on. Instead, it sets the shutter speed to expose the scene as if the flash is always off. This “feature” ensures the ambient light in the scene gets exposed perfectly but the what you end up with us unusable, blurry photos.

One solution is to use Manual mode on your camera and set both the aperture and shutter values. This is the quickest solution. If you plan to hand hold your camera, set the shutter to 1/60th of a second to start. If you get some blur, make it a bit faster.

If you still want to use aperture priority mode, I found a custom function that’s available in Canon cameras which instructs the camera to shoot no slower than 1/60th of a second while in aperture priority mode with the flash enabled. For the Canon 60D and 7D it’s in “Custom Function I: Exposure” and is setting number 7: “Flash sync. speed in Av mode”. On the T2i, it’s Custom Function 1-3 instead of 1-7. See below.

The default setting for the custom function is “0: Auto”. This allows the shutter speed to be very slow or very fast. Setting “1: 1/250 – 1/60sec. auto” is perhaps the most useful option. It will ensure the slowest speed is 1/60th. The downside here is it also limits the top speed to 1/250th. If you want to use high-speed sync now, you’ll probably need to undo this selection.

To avoid all these custom function hassles, I simply recommend you don’t use aperture priority mode and your flash together while in a dim room. Simply switch to manual mode and set both the aperture and shutter values in one step. Your ISO value can be set as well but it can remain on auto. Don’t worry, you’ll still get a good exposure in most cases as the flash can adequately compensate by varying the amount of flash power used (your flash effectively replaces the shutter speed since you’re using TTL flash, it’s still like being on automatic).

Give it a try as it takes practice to get these concepts cemented in your mind. I still have lots of experimentation to do. For more information on custom functions, see this useful guide that describes the Canon 7D custom functions in detail.

Have you got any tips or resources to share on this topic?

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