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If you've tried the Triggertrap Mobile app, you'll have spotted that it has a lag-meter built in (this refers to the 2012 version of the app). It's pretty awesome, but it does rely some pretty weird activities: You have to take a photo of your iPhone, triggered by your iPhone, and then read the number on the iPhone's screen on the LCD screen on your camera. Yes, it's pretty backwards, and not as accurate as it could be... So I decided to see if I couldn't build a lag-o-meter based on the Triggertrap Shield.

How do you know when your camera has taken a photo?

The most reliable way that I am aware of for measuring the precise time a shutter is fully open, is to rely on a feature built into all SLR cameras: The ability to fire a flash. In normal flash mode*, a flash connected to the camera is triggered as soon as the shutter is fully open. So, in theory, the time between pressing the shutter button, and the flash going off, is the shutter lag of a camera.

*) I say 'normal flash mode', because second curtain sync complicates matters a little. Luckily, unless you know what 'second curtain flash synchronisation' means, it's extremely unlikely that you have this turned on on your camera.

To measure this precisely, we need a few things...

  1. A precise way of sending a 'shutter button press' to the camera
  2. A precise way of measuring when the shutter is opened
  3. A precise way of measuring the time elapsed between the two.

Luckily, all the parts we need to do that are already built into the Triggertrap v1 and the Triggertrap Shield. (when you grab the source-code, take a look at the first 3 lines; insert the correct numbers here for your device.)

So, all you need to do is to trigger your camera with the shutter cable, and then connect the flash sync signal (either via the PC Sync socket on your camera, or via the flash hotshoe with an adapter) back to the Triggertrap's Aux port.

Now, we have a full loop: We can send a shutter signal to the camera, and start a stopwatch. Once we get a confirmation from the camera that it has triggered, it stops the stopwatch.

The software

In this version of the Arduino sketch, it runs two different programmes.

The first programme turns on the focus and shutter signals at the same time, and measures the shutter lag 10 times. It writes the results back to the computer via the Serial interface, and then calculates the average of the first 10 exposures, too.

The second programme sends a focus signal ahead of time (equivalent of keeping the shutter button half-pressed between exposures). We run this test because some cameras have significantly faster shutter lags when operating in this mode - and if you're going to use your camera for high-speed photography, it's useful to know!

Want to give it a go?

Awesome. Grab the source code from Github. Load it onto your Triggertrap Shield for Arduino or Triggertrap v1. Create the connection cable to connect the PC Sync port or hotshoe to the Aux port of your Triggertrap, and run the sketch.

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