How the Minox shutter works

Minox front view on lens with shutter blades removed

The shutter is usually the most complicated part of a mechanical camera. This is particularly true with the Minox because it does not have a variable aperture. Therefore, only the shutter determines the amount of light hitting the film. The mechanical shutter of the Minox A IIIs, B and BL is also a marvel of precision engineering because all shutter speeds from 1/2 to 1/1000 s are implemented in a single mechanism.

The Minox has a unique guillotine shutter, which consists of two stainless steel blades, each of which is firmly connected to spring hooking arm via a tension spring. On the other side of the blades there are eyelets into which two pins connected to the arm of the escapement lock.

Before we go into the details check out the excellent animation of the Minox shutter blades by Tristan da Cunha:

Courtesy of Tristan da Cunha

Also note the two green pins on the right. They are then crucial for controlling the selected exposure times. The same in reality:

Minox A, front cover removed, shutter speed 1/2 s

The blades

The two blades lie one above the other in front of the lens. The opening blade (1) lies next to the lense, the closing blade lies above. They are made of 0.4 mm thick stainless steel and weigh just 0.08 g (own measurements).

Minox shutter blades: Opening blade (1), Closing blade (2)

The camera’s escape mechanism operates a lever, at the end of which there are two pins that lock the blades, see the two green pins on the first video. The next picture shows the lever with the two pins fixed on it. Note that the two pins of different lengths are attached to the same arm. So they can only be moved together. As we’ll see later, this is a brilliant idea.

Front view of the camera. Right: blade locking pins, viewfinder. Left: lens

By swinging the lever up or down, the two pins are moved up or down. In the upper position they hold the blades against the spring force, in the lower position they release them. The speed of the panning movement is determined by the escapement according to the selected shutter speed. Watch again the video above and have a look at the pins!

In the next image we see the same, but with shutter blades. You can clearly see how the pins hold the shutter blades in place. The top sheet of the plate guide has been removed so that you can see the blades better.

Front view of the cocked camera. Locking pins holding the blades.

The spring hooking arm connected to the tension springs is firmly hooked to the lower part of the housing, which releases the lens and viewfinder when the camera is pulled apart, thereby putting tension on the tension springs of the blades.

Spring hooking arm with opening blade attached
Attachment of the tension springs on spring hooking arm

The two tension springs each exert a spring force of approx. 1.6 N (own measurement) on the blades. This means that they are pulled into the end position when triggered.

The springs and blades run in a guide plate made of a brass alloy. The stainless steel – brass sliding pairing and the loose guide ensure very low friction without any lubricants. The guide consists of two parts that are connected with tabs. The channels for the tension springs are clearly visible on the left.

Blade guide, opened. Top: front side, bottom: lens side

The trigger cycle

The trigger cycle takes place in four steps, see photos below:

  1. The camera is pushed together. With that
    a) the blades are pushed all the way towards the viewfinder
    b) the shutter is cocked, i.e. the escapement is wound up and
    c) as soon as the eyelets of the blades have reached the locking pins, the pins are swiveled upwards.
    The window of the closing blade is now directly above the lens and that of the opening blade is to the right of the lens. Now the lens is covered only by the opening blade.
    Note that both locking pins are mounted on the same lever. The different heights of the pins are crucial. When triggered, the short pin is immediately pivoted downwards, releasing the opening blade 1. If the lever lowers further, the long pin releases the closing blade 2. The speed at which the lever lowers determines the shutter speed. This is controlled by the escapement mechanism.
  2. The camera is pulled apart. By this
    – the front of the camera is opened and
    – the tension springs of the blades are tensioned.

    The circle on the bottom blade tells the user that the camera is ready to shoot.
  3. The shutter button is pressed and
    – the lever begins to swing and lowers so far that the short pin releases the opening blade which runs to the left at maximum speed until it stops. The speed of the pins is controlled by the escapement.
    Now the windows of both blades are on top of each other in front of the lens and thus expose the lens. The exposure begins.
    – At this moment the escapement begins to run and brakes the movement of the lever according to the selected exposure time. This means that pin 2 initially remains in its eyelet and holds the closing blade in place.
  4. After the shutter time has elapsed, pin 2 has now swiveled down so far that it releases the closing blade, which now also runs into the left stop. Through this
    – the window of the closing blade slides from to the left, whereby
    the lens is closed, although the window of the opening blade remains in front of the lens.
Minox trigger cycle. Tension springs and upper guide plate removed.

Characteristics of the Minox shutter

In contrast to focal plane shutters the Minox the shutter window opens completely even at the shortest shutter speed of 1/1000 s. The kinematics of the shutter ensure that the closing blade only begins to close after the opening blade is in its end position.

The well-known image distortions (see Lartigue’s Le Grand Prix A.C.F.) caused by focal plane shutters at shortest shutter speeds are thus avoided. This is only possible because of the very small mass of the blades, which can therefore be accelerated extremely quickly. This is one reason why the Minox can cover the range from ½ to 1/1000 s with just one escapement.

The shutter speed in this design is therefore determined by the difference in the length of the two pins combined with the speed at which they move downward controlled by the escapement.

The physical behavior of the Minox shutter

Movement of the locking pins

1 opening blade pin, 2: closing blade pin, 3 blade guide plate, 4 viewfinder, s = 1.7 mm


The length of the two locking pins differs by 1.7 mm. The pins must therefore cover this path within the selected shutter speed. The exact process and speeds are explained here.

At a selected shutter speed of 1/1000 s the pins run at 1.7 m/s ( ≈ 6 km/h = 3.7 mph), see here for the calculation. These are low speeds that can be easily controlled.

Movement of the blades, physical considerations

The speed of the blades themselves is also limited. The blade covers a total distance of 8mm on its path. A spring force of approx. 1.6 N acts on the blade’s mass of 0.008 g.

With an acceleration a, a body covers the distance s during t seconds
s = ½ * a * t2
With a spring force of 1.6 N and the blade’s mass of 0.008 g the accelaration becomes :
a = F / m = 1.6 N / 0.00008 kg = 20000 m/s2
From this and a distance s of 8 mm the time t is calculated to be
t = √(2 s / a) = √( 2 * 0.008 m / 200000 m/s2) = 0,0009 s ≈ 1/1000 s

This means that the blade needs approximately 1/1000 s for it’s path, which corresponds to an average speed of 8 m/s (≈ 30 km/h, calculated without friction).

This explains why even at the shutter speed of 1/1000 s the Minox shutter doesn’t behave like a focal plane shutter. As soon as the opening blade is released, the escapement starts. To ensure that exposure begins immediately, the opening blade has a wider window that extends to the edge of the lens:

1: Wider window of the opening blade.
2: Window of the closing blade

The lens is under the gray circle. You can see that the window (1) extends directly to the lens. This means that the exposure begins immediately after the opening blade starts to moveto the left.

After 1/1000 s, the closing blade is also released and also starts running. At exactly this moment, the opening blade has already reached the stop and the entire lens is open for this moment. After another 1/1000 s, the closing blade is at the stop and the lens is closed again.

3: blade stop bumper, 1: tension spring, 2: spring hooking arm, 4: blade, 5: eyelet

The blade hits the blade stop with a speed of 18 m/s ≈ 65 km/h. For comparison: A Formula 1 car needs a full 2 seconds from 0 to 65 km/h instead of 1/1000 s, see also Minox C advertisement July 1970: “Acceleration inside is over 3000 times faster than a starting racing car. In fact, the blades of the shutter system are so fast that you can accelerate from 0 to 50 km/h in 1/2000 of a second.)

In order to cushion the impact of the blade on the stop, a rubber buffer is installed between the spring hooking arm and the blade within the tension spring. The buffer has a cylindrical shape with 0.9 mm diameter and 2 mm length (own measurement).

Movement of the blades, experimental considerations

To show the actual conditions I carried out an optical measurement of the shutter opening (ShutterSpeed app / PhotoPlug using the reflex method).

Minox shutter speed test with ShutterSpeed Photoplug. White graph shows the amount of light coming through the lense. Inverse course of the graph: Falling course means increasing brightness.
Mint-colored area: shutter fully open, the width corresponds to approximately 1/1000 s.

In the experiment, the blades are illuminated from outside with a lamp. The reflected light is measured and recorded. Since the blades reflect strongly, high brightness values are measured when the lens is closed. When the lens is open, however, little light is reflected. The brightness progression is displayed graphically. The time during which the lens was fully open was measured in the graph (mint-colored area). The time scale can be derived from this.

The blades open and close within 1/2700 or 1/2800 s. In between there is 1/1000 s when the lens is fully opened. The acceleration phases of the two blades are clearly visible (blue arrows). The entire process takes 1/600 s. For the effective exposure time, the average of the opening and closing times must be added to the time in which the shutter is fully open. This results in an exposure time of approx. 1/(1/2750 +1/1000) = 1/730 s.

The deceleration of the closing blade can be seen in the purple arrow in the picture above. With the opening blade, the deceleration cannot be seen, as this happens when the shutter is fully opened. Therefore, the amount of light does not change when the opening blade is delayed.

Conclusion

In the Minox shutter four independent springs are used to
– drive the escapement,
– move the lever with the locking pins and
– drive the opening blade
– drive the closing blade.
This means that each spring can be optimized precisely for its intended purpose and unwanted mutual influence is ruled out. An ingenious construction!

After seeing how the shutter works and the shutter blades are controlled, we have to ask ourselves how the process can be delayed so that the lens can be kept open for longer than 1/1000 s. This is done by the escapement mechanism. You can read how it works in this article.

Back to Minox repair

References


Oleson, Rick: How it works – The Minox Shutter
The Camera Craftsman, Vol 23 No 6 1977: The Minox B

Leave a Reply

Your email address will not be published. Required fields are marked *