sorry gnashings , only 1 reply was short and egotitical and having gotten so many of those from the other site I over reacted, yes, I have gotten some good answers on here, i got out of the pro bussiness because using formulas for figureing the size of the extension ring similar things took all the fun out of photoraphy for me, plus i spent all my time taking photos for other people of what they wanted, i have a friend that had rather do that type of stuff than actally take the photo, not me. my main reason for asking the question was i have 2 lenses, a 110 and 80, both 2.8, and it just seems odd that the mfr would make 2 lenses so close in focal length with the same max opening unless possibley the 80 let in a little more light. I don't need to learn a craft, i'm retired and do this for fun and relaxation, nothing more