Understanding Camera

A camera is an instance of the Camera class. When a Camera is instantiated (c = Camera()), it’s initialised along with a number of objects.

For example, a camera has a shutter that is usually closed and blinks open to allow in light when you take a photograph; the shutter is c.shutter, and can be open or closed, cocked or uncocked, and has a timer (how long it remains open when it’s released).

Other examples are a film advance mechanism, and an exposure mechanism. Many of these subsystems interact with each other. For example in this camera, advancing the film also cocks the shutter.

See Get started for examples.

>>> c.film_advance_lever.wind()
On frame 0 (of 24)
Advancing film
On frame 1 (of 24)
Cocking shutter
Applying aperture value to iris
>>> c.shutter_button.press()
Shutter opening for 1/128 seconds
Shutter closes
Shutter uncocked

You can also interact with the mechanisms at a lower level:

>>> from camera import ExposureControlSystem
>>> ecs = ExposureControlSystem()
>>> ecs.shutter.cock()
Cocking shutter
>>> ecs.shutter.trip()
Shutter opening for 1/128 seconds
Shutter closes
Shutter uncocked

Modelled behaviour

  • Settings

    • You can only select valid film, shutter and aperture settings.

    • Although you select nominal shutter speeds, actual values apply.

  • Film advance mechanism

    • Winding the film advance lever advances the film and frame counter and cocks the shutter.

    • You can’t wind the lever multiple times without releasing the shutter.

    • You can’t wind the film past the last frame.

    • You can rewind the film.

  • Back

    • You can open and close the back of the camera.

    • If you open the back, the frame counter resets.

    • If you open the back in light when there is film in the camera, you will ruin the film, unless you have already rewound it.

  • Exposure and metering system

    • In A (auto-exposure shutter-priority mode) the exposure system responds accurately to ambient light.

    • In A mode, aperture is determined by the available light.

    • In A mode, you can’t take a photo unless the exposure is in range.

    • The exposure indicator shows the auto-exposure aperture.

    • The exposure indicator reports “Under” and “Over” appropriately.

    • The metering system only works when there is a battery installed.

    • You can’t get a meter reading with the lens cap on.

  • Iris control

    • The actual iris aperture responds to the aperture control in both directions when the shutter is cocked. When the shutter is uncocked, it can only decrease the aperture.

    • As soon as the shutter is cocked, the aperture setting is applied to the iris.

As well as external behaviour, internal mechanisms and objects that the user will never see are also modelled. In particular, the complex interactions within the exposure control system are modelled in some detail.

Behaviour still to be implemented (incomplete list)

  • The Canonet QL allows you to advance the film multiple times without releasing the shutter when first loading film.

  • A low battery should affect the light meter.

  • The button should admit of a half-press.

  • The exposure indicator should not show a value in manual mode.

  • The self-timer.

  • The self-test battery lamp.

  • Intermediate film speeds need to be selectable.

  • Only exposed frames should be spoiled by opening the back at the wrong time.

  • If you drop the camera, it might damage it.

Representing a physical device in software

Why does the Shutter default to a shutter speed of 1/128s, which is not a speed you’ll see indicated on any camera?

A film camera from the 1970s doesn’t have the same precision or accuracy as floating-point operations in software. In addition, cameras in any case use many nominal numbers in their controls, that only represent an approximation to some numerical ideal.

The traditional scale of shutter timings for example - 1s, 1/2s, 1/4s, 1/8s, 1/15s, 1/30s, 1/60s, 1/125s, 1/250s - proceeds roughly in sequential powers of 2, but breaks down twice in just a few steps, in order to provide easier numbers to work with.

If a shutter had a precise 1s speed and it followed the rule of 2 precisely, then it would have a 1/128s speed - which it does in software, even if no camera does in real life.

However, the real-life QL17 has shutter speed selector ring that that Camera also represents. When you apply a shutter speed:

c.shutter_speed = 1/125

two things happen. First, it checks whether the selected speed is one of those that the camera actually has, and raises a Camera.NonExistentShutterSpeed exception if not. If it’s a legitimate selection, it applies an actual shutter speed to the shutter (see the next section).

How changing a camera setting changes other settings

As noted, when you apply value to c.shutter_speed, it also applies it to c.exposure_control_system.shutter.timer.

It does this with a shutter_speed() method of Camera, decorated to function as the setter for the attribute.

def shutter_speed(self, value):
    if not value in self.selectable_shutter_speeds:
        possible_settings = ", ".join([f"1/{int(1/s)}" for s in self.selectable_shutter_speeds.keys()])
        raise self.NonExistentShutterSpeed(f"Possible shutter speeds are {possible_settings}")

    self.exposure_control_system.shutter.timer = self.selectable_shutter_speeds[value]
    self._shutter_speed = value

Similarly, you can set c.aperture - but the setting will only be accepted if it’s one that’s actually available, and if not, you’ll get an ApertureOutOfRange exception.

Only valid values will then be applied to the subsystems.

Understanding the exposure control system

The most logically complex part of the camera is the exposure control system - ExposureControlSystem.

When the shutter release button is depressed, it moves the shutter release lever (part n. 19-0562, ShutterReleaseLever). As it travels down, it rotates the exposure level lever (ExposureLevelLever.activate()).

The exposure bounds lever (ExposureBoundsLever) is held against the exposure level lever by a spring. The movement of the exposure level lever allows (ExposureBoundsLever.activate()) the exposure bounds lever to follow it.

In turn, the shutter lock lever (ShutterLockLever, part n. 19-0566) is held against the exposure bounds lever by a spring, and the movement of the exposure bounds lever allows (ShutterLockLever.activate()) the shutter lock lever to follow it.

If the shutter lock lever is allowed to swing across far enough, it will prevent the shutter release lever from moving far enough to trigger the shutter release.

Two things can prevent the exposure bounds lever from moving far enough to allow the shutter lock lever to do this.

  • The camera is in manual mode: if a shutter speed is manually selected (i.e. the exposure system is in manual mode) then the EE lever (EELever, part n. Y13-5302) is locked into place. This prevents the exposure bounds lever and in turn the shutter lock lever from blocking the shutter release lever.

  • The camera is in aperture-priority mode and the exposure is within range: if aperture-priority mode is selected, the needle of the light meter can also prevent the exposure bounds lever from moving far enough. The needle’s opposite end is turned up through 90˚, and the exposure bounds lever is shaped so that if the needle is not in the range ƒ/1.7 to ƒ/16, the lever will move right the way across. However if the needle is within the range, the lever will be prevented from swinging across further (and also clamps the needle in place). This too prevents the shutter lock lever from blocking the shutter release lever.

If the camera is in aperture-priority mode and the exposure is within range, the exposure level lever moves across as the shutter release button is depressed, until it also hits the turned-up end of the light meter needle. Depending on the position of the needle, the the exposure level lever moves the EE lever to a certain position - and this determines the aperture of the iris.

Actuating the iris

The leaves of the iris only actually move under the following conditions:

  • manual mode, when:

    • shutter is cocked, any movement of the aperture ring adjusts iris

    • shutter is uncocked, iris can only be contracted by moving the aperture ring

    • cocking the shutter, the iris is immediately adjusted to the selected aperture

  • aperture-priority mode, when:

    • shutter is cocked, depressing the shutter release lever adjusts the iris to the auto-exposure aperture (if in range)

    • shutter is uncocked, depressing the shutter release lever only adjusts the iris to a smaller auto-exposure aperture (if in range)

    • cocking the shutter, the iris is immediately adjusted to the widest possible aperture

Why build a 40-year-old camera in Python?

I love film cameras and their mechanisms, and spend a lot of time repairing and servicing them. The mechanisms in a camera are full of functional logic, and thinking about how they change their own state and trigger changes in and depend on the mechanisms they are connected to is the same kind of thinking that goes on in object-oriented programming.

This project is an experiment in expressing the logic of a camera - in this case, a Canonet G-III QL17 rangefinder, one of my favourites - in code.