P3D Re: Sunny 16 Rule (PHOTO-3D digest 3514)

[bobh@xxxxxxxxxxxxxxxxxx]

RE: Dr. T is right about the nearly constant illumination from the 
sun, but the formula  Exposure = f16 at 1/ASA , is NOT empirical. 
It is derived from the equation of illumination at a film plane, and 
when a brightness level is fixed all the other factors drop out. (This 
for the basic sun at back, 10am to 3pm, not yet considering offsets 
(= to Kodak Exposure guide.). 
As I recall the original equation for illumination at film plane is in 
Keith Henny's "Handbook of Photography", McGraw-Hill 1938. (He 
did Electrical Engineers' Handbook too). BUT by chance, a letter in 
the current Oct 1999 Popular Photography on page 69 under "Lots 
of LUX!" just happens in discussing camcorders, to give  a LUX 
version nearly like the Sunny 16 rule formulae. It is something to 
play with..substituting LUX for sunny day, a couple of examples are 
in the letter. Original inquiry was related to EV values, but this just 
says for a given light level and film speed, there are a number of 
f/stops and shutter speeds that are equivalent to produce the 
correct exposure.  The reason Sunny 16 works is that is has a 
scientfic basis. (The hidden factors in meters of field of view, color 
sensitivity including UV and IR, linearity of cells etc. almost make 
them the empirical method hi hi !) BobH 
Bob Howard (bobh@xxxxxxxxxxxxxxxxxx)
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