Reference

Camera basics

There isn’t much you can do with a Camera object itself. Just as in real life, a camera is really just a light-tight box; it’s all the other components that make up its mechanisms that are interesting. When a Camera is instantiated, these other components are also instantiated, as attributes of it.

In addition, things like a roll of film and the physical environment in which the camera finds itself will also be instantiated, and available as attributes of the camera.

Let’s assume that we have created a camera with c = Camera(). In that case, your camera will include a number of components that allow you to perform various actions, or can be in various states, for example:

>>> from camera import Camera
>>> c = Camera()
>>> c.aperture = "A"

All the examples below are similarly attributes of a Camera instance.

Things you can set on a Camera instance

  • aperture - “A”, or a value between 1.7 and 16 (ƒ-number).

  • film_speed - 25-800 (ISO)

  • shutter_speed - 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500 (seconds; nominal values)

Things you do with a Camera instance

  • state(): get a report of the state of the camera and its sub-systems

  • film_advance_lever.advance()

  • shutter_button.press()

  • back.open() and back.close() - beware of opening the back in daylight with a half-exposed roll of film inside

Values you can read from a Camera instance

  • frame_counter: how many frames the camera indicates have been exposed

  • exposure_indicator()

Sub-systems

The sub-systems can be accessed directly. For example, you can do things like:

>>> from camera import Camera
>>> c = Camera()
>>> c.exposure_control_system.shutter.cock()
Cocking shutter
Applying aperture value ƒ/1.7 to iris
Cocked

which you wouldn’t or even couldn’t do, unless you had the camera partially disassembled in front of you. In normal use, such actions are only the effect of some other action (in this case, of advancing the film).

You can also make use of sub-systems completely independently of a camera object:

>>> from camera import Shutter
>>> s = Shutter()
>>> s.cock()
Cocking shutter
Cocked

As in a real camera, sub-systems are affected by each other, which is why in the first example above, cocking the shutter also applies the aperture value to the iris.

Some notable sub-sytem actions

Assuming c = Camera():

  • c.film_advance_mechanism.advance(): advance the film inside the camera and cock the shutter

  • c.film_rewind_mechanism.rewind(): wind the film back into the cartridge

  • c.exposure_control_system.shutter.trip(): release the shutter (i.e. actually take a photo)

  • c.exposure_control_system.meter(): depending on various things including its own mode, the ambient light, whether the lens cap is on, the film speed, the shutter speed, etc, will set the iris aperture

  • c.exposure_control_system.light_meter.reading(): takes a reading of the ambient light

Some states of the sub-system objects

Some of these states are ones that it makes sense for you to set (e.g. how bright it is or whether the lens cap is on) while others only make sense as reports from the system (such as whether the film has been ruined by something you did). Some are of both kinds.

  • c.film_advance_mechanism.advanced: True or False

  • c.exposure_control_system.shutter.timer: shutter speed

  • c.exposure_control_system.shutter.cocked: True or False

  • c.iris.aperture

  • c.lens_cap.on: True or False

  • c.film.frame: which frame we’re on

  • c.film.frames: how many frames in the roll

  • c.film.fully_rewound: True or False

  • c.film.ruined: True or False

  • c.environment.scene_luminosity: how bright it is

Exceptions

Exceptions occur when you try to do something that the camera refuses to do because it violates the logic of a mechanism of the camera. For example, if the shutter is already cocked, you can’t cock it again; it’s not logically possible to cock a shutter that’s already cocked. Some things are logically possible, but not physically possible, like selecting a shutter speed that this particular camera doesn’t have.

Exceptions should be fairly self-explanatory:

  • Camera

    • NonExistentShutterSpeed

    • ApertureOutOfRange

    • NonExistentFilmSpeed

  • FilmAdvanceMechanism.AlreadyAdvanced

  • Shutter.AlreadyCocked

  • Film.NoMoreFrames

  • ShutterButton.CannotBePressed

  • FilmAdvanceLever.CannotBeWound

On the other hand, attempting to call Shutter.trip() when the shutter is not cocked simply does nothing. It doesn’t violate any logic, it just doesn’t have any effect.