The Beam Breaker User Manual


Receiver front and right hand side
Receiver front and left hand side


The Laser Transmitter

An Alternative Transmitter

Receiver Controls and Indicators

Note: all instructions will reference the items on the boxes from left to right and top to bottom, as shown in the photos.
Right Hand Side: READY LED (green) and Photo Sensor
Left Hand Side: Connector to connect to the camera, via a stereo lead to the camera remote switch lead.
The switches are:
SW1: POWER OFF/ON
SW2: EXPOSURE METER/AUTO FOCUS
SW3: FIRE SHUTTER
SW2 and SW3 have 3 positions:

  1. UP is spring loaded and is for checking the connections or using the box as a remote. You need to hold both in the UP position in sequence.

  2. Centre is OFF

  3. Down is OPERATE

Transmitter Controls and Indicators

There is a simple slide switch to turn the power ON. It may require the use of a fingernail to operate; it can stick if you push it with your finger! Always check the operation by pointing the transmitter at a wall or the floor; never look directly at the laser yourself, nor point it at anyone’s face. For the alternative transmitter, a cut down laser spirit level, the switch is push for ON, push for OFF.

Set Up

With SW1 OFF, check that all the connections are correct by pushing SW2 UP – the camera should auto-focus with an audible beep. Holding SW2 UP, also push SW3 UP and the shutter (and flash, if connected) should operate.

Make sure both the transmitter and receiver boxes are firmly fixed to suitable solid supports, such as a table or bench, or use retort stands, either side of where you want to create the beam which will be broken. With SW1 ON, point the laser transmitter beam, with laser light ON, at the photo sensor. The laser beam makes it glow by reflected light and the green LED should light.

Turn SW1 and the laser light OFF except when ready to take a photo to conserve battery power.

Arrange your subjects; manually focus the camera with the falling object in the expected capture position and you should be ready to go! Make sure the camera and flash are turned on and are ready to fire. You should have the shutter set to single shot. If you have it set to multiple shot, the camera shutter will operate every time the beam is broken, which can be a nuisance.

Operate

Turn the laser light ON; turn SW1 to ON, SW2 to OPERATE and then SW3 to OPERATE. At this point any interruption, intended or otherwise, to the beam will cause the LED to go out for the duration of the interruption and the shutter and the connected flashes to operate. After the shutter has fired put SW3 and then SW2 into the centre OFF position. You should not be able to fire the shutter again nor will you be able to operate the camera manually until you have completed these last two steps. If you are using a laptop to capture your images, it will not record them until the two switches are OFF. This can be frustrating but is a feature of the way the remote switch works! You can take further shots by setting SW2 to OPERATE and then SW3 to OPERATE and breaking the beam. Reset SW3 and then SW2 to OFF after each shot.

Understanding Timing

The shutter and flash operate at the same time but at a fixed time after the beam is broken. This time is a function of the camera, driven by its’ own internal processor and differs from model to model. This needs to be taken into account when planning your shoot. You need to calculate where the object you want to capture will be in relation to the beam when the shutter operates.

I measured the camera delay by measuring the height a nut has to fall from a stationary start, breaking the beam at the start of the drop. In the photo below, the laser was aligned with the bottom of the tape at 14½ inches. The drop I recorded for the 300D, 10D and 20D were all between 4½ and 4¾ inches. This equates to a delay of roughly 150ms.


Measuring Camera Delay

Some Maths

Distance = D feet
Time = T seconds
Acceleration due to gravity = g (32ft/s/s)
Speed at impact = Vi ft/s (“impact” is when the shutter fires)
Speed at start = Vs ft/s
Average speed = Va

Now: Va = (Vi- Vs)/2     (1)

But if the beam is aligned with bottom of the nut before it falls,
Vs is zero, so Va = Vi/2     (1a)

Trust me, Newton’s Laws say:
D = Va *T     (2)
Vi = g*T     (3)

So substituting (1a) and (3) in (2):
We get D = (g*T/2)*T = g*T2/2     (4)

Or T = (2D/g)1/2     (5)

Using equation (5) for the dropping nut, with the shutter firing after a drop of 4 ½ inches:
T = (2*(4.5/12)/32)1/2 = 0.153s = 153ms

The Turkey Baster Shot

So, if you want to catch a water drop as it hits the surface of the water, you need the turkey baster about 4 ½ inches above the surface of the water, assuming you align the beam with the bottom of the baster. Actually, in order to catch the splash, you need it a bit higher, to give the splash time to form. For the Déjà Vu shot, I had the turkey baster 6 inches above the water and the beam breaker 2 ¾ inches above the water, the two heights achieved by trial and error.

Update (2008) - EOS 50D

The arrival of my new camera, a Canon EOS 50D, was a good reason to revisit the exercise above and measure the shutter delay.  Using the same equipment, I dropped an eraser, the bottom of which was aligned with the 250mm mark on the ruler.  The shutter caught the image when the eraser had travelled 35mm (1.38 inches).

 
35mm drop

Using the equation (5) above, where g is 9.806m/s/s we get the shutter delay:
Tr = (2*0.035/9.806)1/2 = 0.0844s = 84ms  Apologies for going metric but the ruler was only visible that side!

It seemed like a good idea to try using a water drop instead of the eraser because it was difficult to let go of it without the eraser turning a little.  The image above was the only one I captured that was straight.  However, a disadvantage of using a water drop is identifying the start point.  In the image below you can see that surface tension hangs on to the drop for as long as it can!  This image suggests the start point is about 13mm (about half an inch) below the bottom of the baster.

 
Where is the start point?

Using a water drop takes away the uncertainty of the twisting eraser and allows for a greater drop between laser and the capture point because the drop is travelling faster when the laser beam is broken.  In the images below the laser is lined up with the bottom of the tape at 24.4 cm.  The distance from the start point to laser is  (244-189-13) = 42mm and the distance from beam break to shutter release is (350-244) = 106 mm (just over 4 inches).  The total distance between start point and shutter release is therefore 42+106 = 148mm.

           

Drop forming and drop captured after release and beam break

Using the equation (5) above, we can calculate both the time from start to beam break (Tb) and the time from start to shutter release (Ts).

Tb = (2*(0.042/9.806))1/2= 0.0925s = 92.5ms

Ts = (2*(0.0148/9.806))1/2 = 0.1737s = 173.7ms

The shutter release time Tr = Ts-Tr = (173.7-92.5) = 81.2ms

This is acceptably close to the figure using the eraser method (3%).

It is interesting to calculate the speed of the drop as it is caught by the shutter release.  Using equation (3)

Vs = g*Ts = 9.806*0.1737 = 1.7m/s or 1.7 mm/ms

This means that if the shutter speed is set to 1/1000s, the drop will move 1.7mm while the shutter is open.  This is why the shutter speed was set to the minimum, 1/8000s, reducing the movement to 0.2mm, a modest amount of blurring!