Guillotine Shutters

Part 1:Constructing a Simple Shutter

# © 2010 Bill Kumpf for largeformatphotography.info

I needed a shutter to test a 6 ½Ó Goerz Red Dot lens. The guillotine shutter offers a simple device to use with a barrel mount lens. It can be quickly constructed from many materials in minimum time. This is the design I chose for that lens.

Model or Hobby Shop stocks the Basswood material used in the construction. The lens barrel is inserted into the ¼Ó Baltic plywood base plate. The hole is sized to allow a layer of tape to adjust for a snug fit. The second plate hole is sized to prevent the lens sliding forward. The 1/8Ó stand offs provide clearance for the 3/32Ó blade and the front plate ties every thing together. Wood glue was used for the assembly for all except the front plate. Screws allow the plate to be removed for cleaning and lubricating.  Two pieces of Basswood serve as a blade stop. Of course every thing is painted flat black to reduce light reflections.

 Material Size Hole Size No, Base Baltic Plywood 4Ó x 4Ó x 1/4Ó 2.5Ó 1 Lens Stop Basswood 4Ó x 4Ó x 3/32Ó 2.2Ó 1 Front Basswood 4Ó x 4Ó x 3/32Ó 2.5Ó 1 Stand Off Basswood 1/8Ó x3/8Ó x 4Ó - 2 Blade Basswood 3-1/4Ó x 10Ó X 3/32Ó 3Óx 1-1/2+Ó 1 Blade Stop Basswood 1/8Ó x 1/4Ó x 3 1/2Ó - 2

The guillotine shutter uses a slot to pass across the lens. Being powered by gravity, the drop blade is constantly accelerating as its drops. The distance the edges falls determines the drop times. The leading edge of the shutter opens the lens and the trailing edge closes it. Shutter speed is the difference in time the two edges cross the lens. The size of the lens front element determines the drop of the leading edge. For this shutter, I used 2 inches.

Calculating the drop time, T = equals the square root of (2d/g) where d = drop (in), g=32.1740 feet/sec2

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Time = Ã  d x ( 2 / ((32.1740 feet/sec2) x (12 inch / foot)))

Thus for an initial drop of 2.00 inches:

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The leading edge time,  Tl = Ã (2.00 x 0.0052)     = 0.1018 seconds

The target shutter speed of 1/30 second equals 0.0333. Adding the two times, the trailing edge drop distance can be determined.

Tl= 0.1018 + 0.0333 = 0.1351 second

The trailing edge drop distance, Dt =((g* Tt^2  ) / 2)   Dt =3.5245

For the slot width = Dt – Dl    = 3.5245 – 2.0000 = 1.5245 inch

To verify the concept I measured the shutter time using a photoelectric eye and an oscilloscope. The leading edge activates the switch while the trailing edge deactivates the switch. With the oscilloscope, the activation time was measured.  The target time was 0.0333 second.

The measured time for this shutter was 0.0336 seconds. This is the average of 6 drop. Release of the blade affects the consistency. The method I found most repeatable was to apply a slight pressure to hold the blade against the front plate then quickly release the blade.

This is the shutter during final fitting. Rubber bands are used to secure it to the camera.  Final coat of flat black paint remains to be applied.

 Leading Edge

Hopefully this will help in exploring the world of barrel lens.