How the Pentax MX Works

This section covers everything that happens from pushing the shutter button to the final 'thunk' of the mirror. At a high level, the process looks like this.

1. The shutter button releases the mirror, which flips up allowing the image to be projected onto the film plane.
2. The mirror, reaching the 'taking' position, releases the opening curtain latch.
3. The opening shutter curtain travels across the film gate beginning the exposure.
4. After an appropriate amount of time, determined by either the high speed mechanism or the slow speed governor, the closing shutter curtain begins its travel, ending the exposure.
5. The closing curtain reaches the end of its travel and releases the mirror return latch.
6. The mirror flips down into the 'viewing' position, allowing the user to see through the viewfinder again.

The camera uses this sequence (mirror releases shutter, then shutter releases mirror) so that the shutter can never open while the mirror is in the way. The following sections cover the process in greater detail, showing exactly which mechanisms are performing each action.

The shutter button houses a pin, which interacts with the shutter release plate inside the camera. The pin travels with the button during a normal press from the user. When a remote cable release is attached via the threaded connection, the pin travels independently from the button. The pin will also rotate with the shutter lock. When locked, the semi-circular portion of the pin moves above a rigid section of the camera frame, preventing the button from being pushed.

While the mechanism is lifting the mirror, it's also closing down the aperture in the lens to the set value. More specifically, it is releasing the aperture coupling lever, allowing the springs in the lens to close down the aperture.

When the mirror reaches the top of the mirror box, it closes the contact for the FP flash sync contact. See the section on Flash Sync for more details.

When the mirror completes its travel to the top of the mirror box a lever will strike the opening curtain latch, releasing the opening curtain and beginning the exposure.

It's important to note another release point in the shutter mechanism. The shutter release plate mentioned earlier also depresses the transport gear latch. When the latch is pushed down, it decouples the top gear from the bottom gear. This allows the top gear to spin freely as the opening curtain is traveling across the frame. They are then recoupled so when the camera is wound, the transport gears can bring the shutter curtains back.

The opening wind gear continues rotating until it hits the end stop. It's also caught by the curtain break, which prevents the curtain from bouncing off the end stop and back into the frame. If curtain bounce is detected, or if the wind gear is not reaching the end stop, the curtain break can be adjusted by turning the slotted screw, and changing the amount of engagement.

When the opening curtain reaches the end of its travel, a lever under the wind plate closes the X flash sync contact. See the section on Flash Sync for more details.

The delay of the closing curtain behind the opening curtain is determined by either the high speed mechanism or the slow speed governor, depending on what speed is selected. If the shutter speed is set to 60-1000, the high speed mechanism is used. If it is set to 1s-30, the slow speed governor is used.

As the opening curtain wind gear is spinning around, the high speed cam turns with it. After some amount of rotation, the radius of the cam becomes large enough that it pushes out the closing curtain latch.

Selecting different shutter speeds will put the high speed cam in different positions, resulting in different amounts of rotation before the closing curtain is released. Slower speeds require more rotation and faster speeds require lass. At 125, the opening curtain will travel almost the entire distance across the film gate before the closing curtain is released. At 1000, there are only a few millimeters between the edges of the two curtains.

For slow speeds, the closing curtain latch is disengaged immediately. But there is a second mechanism that delays the curtain. The slow speed governor is on the bottom of the camera and interacts with the closing curtain through a set of gears. The gear shown here is attached to the bottom of the closing curtain pinion shaft. The closing curtain starts its travel but runs into the governor plate before it can enter the film gate.

The slow speed governor houses a lengthy gear train that uses several methods to slow down the curtain. The first is simple inertia. Even though the gears are free to spin, it still takes time to get them moving, which is enough to create a delay of 1/30th to 1/15th of a second.

Next, if the governor needs even slower speeds, it engages an escapement. This creates even more resistance in the gear train. The escapement is used for speeds from 1s to 8.

Lastly, the governor can change the amount of overlap between the governor plates. Less overlap means the closing curtain doesn't have to push it as far and can get by more quickly. This is how the governor controls the smaller differences between shutter settings. It's also how the slow speed governor is tuned if the speeds are out of spec.

If a fast shutter speed is selected on the dial, the governor plate is moved out of the way and doesn't interact with the closing curtain.

Three things happen simultaneously when the user actuates the advance lever of the camera.

1. New, unexposed film is pulled out of the cannister and placed behind the shutter.
2. Springs in the mirror mechanism are charged, making them ready to rapidly flip the mirror up into the 'taking' position.
3. The shutter curtains are pulled back across the film plane to their starting positions.

The following sections describe in detail how each of the functions is achieved by the mechanisms in the camera.

The advance lever is directly connected to the frame counter assembly, which has a set of ratcheting pawls to turn the main wind gear. The wind gear has an additional ratcheting mechanism attached to the chassis, which prevents it from turning in reverse.

The main wind gear is connected to a shaft which runs to the bottom of the camera and drives the film transport gearing.

The bottom wind gear drives the cam gear, which in turn drives the sprocket gear. The sprocket provides the primary force for pulling new film.

The cam gear also drives the take-up spool through an idler gear. But the take-up spool is connected through a slipper clutch and cannot provide as much pulling force as the sprocket. As the take-up spool gathers more film, it doesn't need to rotate as much as the sprocket for each new frame. The slipper clutch helps to equalize the tension in the system and allows for different rates of rotation. The slipper clutch also allows the take-up spool to spin during film rewind, while the sprocket has to be fully disengaged (see the section covering film rewind for more details).

The mirror charging is also driven by mechanisms on the bottom of the camera. The cam gear mentioned previously also drives the long mirror charging lever.

The lever pushes a post on the bottom of the mirror box towards the front of the camera. The post is attached to a lever which charges two springs. One spring is responsible for flipping the mirror up into the taking position while the other returns it to the viewing position.

When both are fully charged, the levers will be latched and held in place.

Shutter charging is handled on the top of the camera, with a mechanism located underneath the shutter speed dial. The main wind gear drives the transport gears through an idler gear.

The transport gears drive the wind gears for the shutter curtains. There are two wind gears, one for the opening curtain (top) and one for the closing curtain (bottom). The transport gear only drives the top gear, which in turn drives the bottom gear with a protruding stud.

When the shutter curtains reach their starting position, they are held in place with latches. The opening curtain latch is located under the wind gear plate. It's not accessible without significant disassembly. The closing curtain latch is here. And finally, the transport gears are latched here.