Equations Used in Pinhole Calculators

variables:

ph = pinhole size in mm

fl = focal length in mm

asa = ASA/ISO of film used

s16 = sunny 16: exposure in seconds for given f-stop and ASA film on a bright sunny day

f16 = exposure from a light meter for F16 at given ASA

fs = f-stop

m = magnification

sd = distance to subject

X = constant used for specific color

wl = wavelenth of the color in mm [example: green = 0.00055]

equations:

X = (wl/sqrt(wl))*1.562

ph = X * sqrt(fl/(m+1))

fl = ((ph/X)^2 ) * (m+1)

fs = fl/ph

s16 = ((fs*fs)/asa)*0.0039

sd = fl/m [note as m approaches zero, sd approaches infinity]

exposure for new fstop = (fs/16)^2 * f16

Reciprocity Calculations [only for exposure longer than 0.1 seconds]:

let ce = calculated exposure so far, then RE = corrected exposure

Tri-X and Plus-X 125

RE = 1.72*ce^1.2 + 0.55*ce^1.5 + 0.000218*ce^3 - 0.026

TMax 400

RE = 0.3*ce^0.8 + 0.9426*ce^1.15 - 0.014

TMax 100

RE = 0.137*ce^0.8 + 1.1*ce^1.15 + 0.0000751*ce^3

Ilford Films

RE = 1.5*ce^1.2 + 0.22*ce^1.5 + 0.0011*ce^3

Angle of view equations:

angle of view = ArcTan(0.5 * film width/focal length) * 360/Pi

film widths measured for different formats: [note individual cameras may vary]

Typical angles of view for 35mm camera lenses based on above: