I have always used the formula fl/f stop to determine the diameter of a lens ope ning. For example, a 55mm at 4.5 equals 12.2 mm diameter opening. A 75mm lens at f 6.15 also equals 12.2mm opeining. To anticipate the light on the gg for th e 55mm lens, I took my 75mm at f6.1 to see the amount of light a 55mm would let in at f4.5. I was told by a reliable source this was not an accurate way to mea sure such. I am confused why it would not be. The only difference as I see it, is the angles at which the light strikes the gg. They will be slightly greater with the 55mm, but the center should still duplicate the same amount of light, right? Any input would be appreciated... Thank you...
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