Does the Summarit have "optimal" f-stop for contrast and sharpness? I noticed that the f-stop scale is such that mid-point is between 2.8 and 4. And I thought maybe lens design would put the best performance in the center of the scale. Also I suppose a designer would naturally think someone buying this lens would intend on using it between 1.5 and 4 or 5.6? Anyone have experence with this lens that could give me a few ideas on how to get the best results out of it? Thankyou.