• 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:

www.ieee.li/pdf/viewgraphs/2d_barcodes.pdf