josephs@ttidcb.UUCP (Bill Josephs) (01/29/85)
I've been meaning to post this for a while and keep
forgetting...for the mathematically inclined, the following formulae
can be used for calculating the ubiquitous difference in stops between
two f stops or two guide number (first formula) or between two shutter
speeds or two ASA/ISO ratings:
#stops = 2*(log(fg1/fg2))/log(2)
or:
#stops = log(sa1/sa2)/log(2)
where:
fg1 and fg2 are two f stops or two guide numbers,
and:
sa1 and sa2 are two shutter speeds or ASA/ISO ratings.
Bill Josephs
TTI
Santa Monica, Ca.ed@ISM780.UUCP (02/02/85)
> I've been meaning to post this for a while and keep >forgetting...for the mathematically inclined, the following formulae >can be used for calculating the ubiquitous difference in stops between >two f stops or two guide number (first formula) or between two shutter >speeds or two ASA/ISO ratings: > > #stops = 2*(log(fg1/fg2))/log(2) > > #stops = log(sa1/sa2)/log(2) >where: > fg1 and fg2 are two f stops or two guide numbers, >and sa1 and sa2 are two shutter speeds or ASA/ISO ratings. > > Bill Josephs > TTI > Santa Monica, Ca. >/* End of text from ISM780:net.rec.photo */ You didn't mention what log base to use, although upon closer inspection I see it doesn't matter. In that case, for most situations, base 2 will be easiest to calculate (sans calculator), so these simplify to: #stops = 2*log (fg1/fg2) 2 #stops = log (sa1/sa2) 2 Ed Lycklama decvax!cca!ima!ism780!ed
nishri@utcs.UUCP (Alex Nishri) (02/05/85)
Handy general equations to remember (known as APEX):
EV = SV + BV = AV + TV
Where:
EV = exposure value
SV = LOG2 kS (S arithmetic film speed; k constant)
BV = LOG2 B/6 (B in foot-candles)
AV = LOG2 f**2 (f-stop)
TV = LOG2 (1/T) (T exposure time in seconds)
The above equations are also sometimes presented as a simple SLAT table, where
SLAT stands for Speed + Light = Aperature + Time.
Alex Nishri
University of Toronto
UUCP: ... utcs!nishri
BITNET: alex at utoronto