The Minox exposure system

From the user’s perspective, exposure consists of three simple steps: set the exposure time, cock the shutter, and take the picture.

From a technical point of view, things look quite different. The shutter of the mechanical Minox (model A, B, BL, or AX ¹) is by far the most complex component in the camera. This is often the case with other camera types as well. However, it’s important to note that the entire Minox weighs at most half as much as the standard lens of a mechanical 35mm camera. All the Minox shutter components together weigh only about 5 g.

It is also noteworthy that the Minox exposure system described here is made almost entirely of metal. This means that wear and tear and aging of the materials are not an issue. Only three tiny parts are made of rubber and serve as buffer elements. Two of them slow down the blades in their end position, while the third serves as a stop for the escapement lever.

When you press the shutter release button on a Minox, a lot has to happen before the negative is exposed. A complicated, high-precision mechanism is set in motion so that the following can happen:

1. The shutter is opened so that light can fall on the film through the lens.
2. The set exposure time is measured, i.e., the camera waits until enough light has fallen on the film.
3. The shutter is closed again so that the exposed negative is protected from further light exposure.

To ensure that all of this can take place in a controlled manner, various mechanical assemblies are required inside the camera, which are interconnected and influence each other.

What happens before you press the shutter button?

Before releasing the shutter, the shutter must be cocked and the exposure time set.

Shutter speed and the exposure system

The exposure time is set by the user with the shutter speed dial. This may happen before or after the shutter is cocked. It depends on when the user decides on the shutter speed. There are three situations:

• The camera is closed and therefore the shutter is already cocked. This is the case when you pick up the camera to take a photo.
• The camera is open and the shutter is cocked. In this case, you have either just picked up the camera or you have just taken a picture, closed the camera and opened it again for the next picture.
• The camera is open after the last photo, so the shutter is not cocked. You want to take the next picture of a different subject that requires a different exposure time.

The position of the shutter speed dial determines the starting position of the escapement and thus the subsequent duration of the shutter opening.

Cocking the shutter and the exposure system

The Minox is cocked when the opened camera is pushed together completely and then opened again. This transfers the energy needed to move the shutter components to the camera. The energy generated by the user through the force and travel of pushing together and pulling open the camera together is stored in four springs inside the camera.

When cocking the shutter of the Minox, the following happens when pushing the camera together:

• The shutter release button is locked.
• The springs of drive lever and blade locking arm are cocked. These springs are torsion springs located in the respective pivot joints of the two levers. They are used to drive the rotary movement of the levers.
• Driven by the barrel spring in the escapement, the whole escapement and the sector gear rotate into the start position. The start position is determined by the preset position of the shutter speed dial. Afterwards he barrel spring is relaxed.
• The two shutter blades are pushed towards the viewfinder and thus brought into their starting position.
• The two pins at the end of the blade locking arm engage with the eyelets of the shutter blades and hold them in the start position.
• The film is advanced by one frame (approximately 13 mm). With the Minox A and B models, this happens each time the camera is pushed together; with the Minox BL, this only happens if the shutter release button has been pressed beforehand.
  

The following happens when pulling the camera open:

• The two tension springs that pull the blades toward the lens are tensioned. However, the blades remain in their starting position, held by the pins of the blade locking arm.
• The shutter button is unlocked.

The Minox is now ready to expose the film.

The subsystems of the Minox shutter

The Minox exposure system comprises five subsystems that come into play one after the other:

1. Speed dial preset
2. The trigger mechanism
3. The escapement mechanism
4. The blade control
5. The blade mechanism

We will now take a look at the interior of the Minox in the area relevant to us.

For your orientation: You are looking at the front panel with the shutter speed dial on the right-hand side. On the left-hand side, you can see the window with the exposure counter.

When you remove the front plate, you can see the five subsystems of the Minox exposure system in this area. Seeing this for the first time can be a bit overwhelming. So, let’s take a step-by-step look at each area and see how they work together.

We see the positions of the five main components and their relation to each other. Let’s take a closer look at the functions of the individual components:

1. Pin 1 is adjusted by the shutter speed dial according to the desired exposure time. This brings the escapement 3 into its starting position.
2. The green circle 2 describes the trigger mechanism. The shutter release button in the front plate presses the release lever at point 2a. The spring plate 2b pushes the release button back up after triggering.
3. The escapement is located in yellow area 3. The escapement lever is under spring tension and is held in place at position 3a by the release lever 2a. When triggered, 2a swings upward and releases the rotational movement of 3a. This rotation is slowed down by the escapement mechanism.
   Pin 3d is firmly connected to lever 3a and presses on the left edge of 3c, sliding along it. This pressure drives the escapement mechanism via the sector gear wheel 3c, which thus controls the rotational speed of the lever 3a/3d. The gears of the escapement rotate at the same constant speed for each shutter speed, which is determined by the interaction of anchor 3b with the escapement wheel. Here is a short video that illustrates this movement.
4. The shutter control functions are in the blue area 4. The locking blade arm 4a is driven by the escapement lever 3a/3d and thus also performs a pivoting movement. The two pins (not visible in the photo) at the end of the lever trigger the movement of the shutter blades one after the other. Here is a short video that illustrates this process.
   The two-part long exposure lever 4b implements the exposure times “B” and “T.” It blocks the swivel movement of blade locking arm 4a as soon as the first blade has been released. In this position, the lens is open. In position “B”, it immediately releases 4a when the shutter release button is released by the user. In position “T”, 4b engages when the user releases the shutter release button. Only pressing it again releases arm 4a. In both cases, the swivel movement of 3a/3d continues for 1/2 s before shutter blade 2 is released and the lens closes. This is because the starting position of the escapement for “B” and “T” is at 1/2 s.
5. The shutter blades 5 (not visible in the photo) open and close the lens, thereby controlling the amount of light that falls on the film. Here is a short video that illustrates this process.

This is just a brief overview focusing on how the five areas are connected and how the exposure system in the Minox works overall. The individual subsystems (with the exception of the shutter release button 2) are fascinating mechanical marvels. That’s why I’ve dedicated separate detailed articles to them.

For the user of the camera, it is simply a matter of turning the shutter speed dial to the desired time. But how is this setting transferred from the dial to the exposure system so that the shutter remains open for exactly the preset time? This is exactly what I explain in the article How the shutter speed dial works.

1. The Riga Minox is not included here, as it has a completely different shutter and winding mechanism. For details, see here. ↩︎