NAME

Photography::EV - Calculate exposure value (EV)

VERSION

version 0.07

SYNOPSIS

 use Photography::EV;
 my $ev = ev(5.6, 1/1000); # EV for f/5.6 at 1/1000s

DESCRIPTION

This module provides functions for calculating photographic exposure values. Some light meters can give readings in "Exposure Value" or EV. On some cameras the exposure can be locked into a specific Exposure Value (EV), such that changing the aperture or shutter speed will adjust the shutter speed or aperture to maintain the same exposure.

FUNCTIONS

ev

 my $ev = ev($aperture, $time);

Takes the aperture (f-stop) and shutter speed (in seconds). Returns the integer Exposure Value (EV).

aperture

 my $aperture = aperture($ev, $time);
 my $aperture = aperture($ev, $time, \@apertures);

Returns the correct aperture corresponding to the given EV and shutter speed (in seconds). By default returns the closest full stop aperture between 1 and 64.

If the optional third argument is given (a reference to a list of possible aperture values), then the returned aperture will be the closest possible from that list. This is helpful, for example, when you are using a lens that provides fractions of a stop. My Nikkor 50mm f/1.2 for example has stops at f/1.2, f/1.4, f/2, f/4 f/5.6, f/8, f/11 and f/16, so to get the correct aperture for 1/60 at EV 9 for that lens:

 my $aperture = aperture(9, 1/60, [1.2,1.4,2,4,5.6,8,11,16]);

shutter_speed

 my $time = shutter_speed($ev, $aperture);
 my $time = shutter_speed($ev, $aperture, \@times);

Returns the correct shutter speed (in seconds) corresponding to the given EV and aperture. By default returns the closest full stop between 1920s (32 minutes) and 1/8000s.

If the optional third argument is given (a reference to a list of possible shutter speeds), then the returned shutter speed will be the closest possible from that list. This is helpful for older cameras that have a different set of shutter speed stops, or newer cameras that use half stop shutter speeds. At least some Rolleiflex TLRs have shutter speeds of 1, 2, 5, 10, 25, 50, 100, 250, 500 instead of the modern values. To get the correct shutter speed for f/3.5 and EV 5:

 # map displayed shutter speed to 1/t to get time in seconds
 my $time = shutter_speed(6, 3.5, [map { 1/$_ } 1, 2, 5, 10, 25, 50, 100, 250, 500]);

CAVEATS

This module requires Perl 5.20 or better.

SEE ALSO

http://en.wikipedia.org/wiki/Exposure_value
Photography::DX

AUTHOR

Graham Ollis <plicease@cpan.org>

COPYRIGHT AND LICENSE

This software is copyright (c) 2015 by Graham Ollis.

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.