Does anyone know what makes a lens able to handle a certain f stop? And how do you figure out what size opening is needed for an f-stop? Is there a formula for caluculating it? Edmonds optics(.com) sells good quality optics ($20 for a 250mm lens!) and leaf irises and I have a shutter. I would enjoy tinkering around with making my own lens but am a little short on the technical end. Can anyone fill in the blanks or suggest a book that might?