nope--this is already based on light emanating from the object--so it's already reflected---this is light already coming towards the lens, so your 4.5 stops is not to be adjusted for since it's done already...the .2 or .25 has nothing to do with reflectivity since we start off assuming light already heading towards the lens from a source in front of the lens. in fact you can come up with different numbers besides .2 or .25 depending on how you assume the light strikes the lens coming in. the lens throughput is an exact energy transmission through the lens
the reason for the .2 or .25 or .3 or .7 (whatever it is) depends on the distance of the source and the collomation of the light coming from it. this assumes that the light has already been reflected or that the subject is a light source like a light bulb or something.
in other words the "perfect" lens would be f/0.5--this would give the same light meter reading outside the camera as inside the camera--so you meter FROM THE LENS pointed at the object with light coming towards the camera, take a reading--go inside the camera with an f/0.5 lens and meter and you get the same reading. This is assuming I think perfect collomation where the object is very huge to cover the entire view field...real objects are smaller than the view field and are closer than at infinity...so that's where the .25 starts getting towards .2 or something else.
see this here:
Last edited by johnielvis; 09-21-2012 at 04:43 AM. Click to view previous post history.
there is a quick and dirty method to come up with the 1/(4F^2) number but even that involves the concept of the radation "view factor" or "shape factor" used in radiation transfer theory.....the concept here is covered in the wikipedia article on etendue and there is a specific diagram showing how it works for refraction through a lens...see this...it's nasty hairy--if you did any radiation heat transfer problems, then this should look familiar
SO...the upshot is: the light inside is basically attenuated by this 1/(4F^2) factor...So--set the meter to f/1 to take care of the 1/F^2 and set the iso to 4 times more sensitive film and then the exposure time will read correctly inside the camera obscura.
oh...and USE the REFLECTED light meter...not the incident!!!!! perfect "spot meter" for zone system here---I believe sinar had such a back that had a probe that went in by the ground glass an did just this for super accurate spot metering.
Last edited by johnielvis; 09-21-2012 at 08:09 AM. Click to view previous post history.