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Message: <blockquote data-quote="Revet" data-source="post: 387299" data-attributes="member: 17612">The mm I was depicting was not the f stop but the actual mm of the aperture opening.  Why even go there though since I believe this only would work if we had a single lens at a fixed focal length like we did in physics lab.  With a compound lens system, what you and Wayne say makes so much more sense.  f/1 = 100% light at the sensor, f/1.4 is half of that and so on.  Wayne, if I read you correctly, I am right in that at the 18 and 270 mm focal lengths, the actual aperture size in mm is pretty much the same (maximum opening capable by the lens blades controlling the aperture) but the amount of light reaching the sensor is less at the 270mm (f/6.3)</blockquote>