Exposure

With Exposure you can compute the exposure for a photo camera based on settings of the camera and metrics of the environment.

Installation

pip install exposure

Usage

>>> from exposure import Av, Tv, Sv, Bv, Iv, Exposure
>>> from decimal import Decimal
>>> from fractions import Fraction

Aperture

>>> Av.from_stop(1)
Av f/1.4

>>> Av.from_focal_length_and_diameter(10, 5)
Av f/2.0

>>> Av.from_fstop(Decimal("1.4"))
Av f/1.4

>>> Av.from_fstop(Decimal("22")).stop
Fraction(9, 1)

>>> Av.from_fstop(Decimal("22")).fstop
Decimal('22')

Speed

>>> Sv.from_iso(100)
Sv 100 ISO

>>> Sv.from_stop(Fraction(1, 1))
Sv 200 ISO

>>> Sv.from_iso(100).stop
Fraction(0, 1)

>>> Sv.from_iso(100).iso
100

Shutter

>>> Tv.from_stop(1)
Tv 2 sec

>>> Tv.from_time(Fraction(10, 13))
Tv 10/13 sec

>>> Tv.from_time(Fraction(2, 1)).stop
Fraction(1, 1)

>>> Tv.from_time(Fraction(2, 1)).time
Fraction(2, 1)

Brightness

>>> Bv.from_foot_lamberts(1)
Bv 3.4 cd/m2

>>> Bv.from_candelas(109)
Bv 109.2 cd/m2

>>> Bv.from_stop(3)
Bv 27.4 cd/m2

>>> Bv.from_stop(3).stop
Fraction(3, 1)

>>> Bv.from_stop(3).candelas
27.4

>>> Bv.from_stop(3).foot_lamberts
8

Incident Light

>>> Iv.from_foot_candles(25)
Iv 269.2 lux

>>> Iv.from_lux(1076)
Iv 1076.0 lux

>>> Iv.from_stop(3)
Iv 537.6 lux

>>> Iv.from_lux(1076).stop
Fraction(4, 1)

>>> Iv.from_lux(1076).lux
1076.0

>>> Iv.from_lux(1076).foot_candles
99.9

Exposure

>>> tv = Tv.from_stop(1)
>>> av = Av.from_stop(2)
>>> sv = Sv.from_stop(2)
>>> bv = Bv.from_stop(1)
>>> iv = Iv.from_stop(1)

The exposure is in balance when

>>> tv + av == sv + bv
True

or

>>> tv + av == sv + iv
True

This can also be used to compute a needed setting, like

>>> Tv.from_exposures(sv, bv, av)
Tv 2 sec
>>> Tv.from_exposures(sv, bv, av) == tv
True

>>> Av.from_exposures(sv, bv, tv)
Av f/2.0

>>> Sv.from_exposures(tv, bv, av)
Sv 400 ISO

>>> Sv.from_exposures(tv, iv, av)
Sv 400 ISO

Future work:

• from_exif

Disclaimer

This package is based on http://dougkerr.net/Pumpkin/articles/APEX.pdf