> There is no "rule of thumb", just plain physics of light. > This case is in no way different than if you were using a glass lens.
I guess you could say the rule of thumb is to add 2 stops every time you double the focal lenngth. You can think of it this way... If you have a 50mm lens at f/16, divide 50 by 16 and the opening would is 3.125mm. If you had that same opening on a 100mm lens, take 100 divided by 3.125, and you get f/32. Two stops from f/16. John
