[bigkat007]

I love these but each time I come to use them I'm faced with a dilemma as to which one and which lens. Does anyone have a calculation method that can be adopted to work out focusing distances with different lenses - rather than slapping one on and trying it out each time!!?? Is this indeed possible? I don't have a head for sums I'm afraid - though I do try!!
Indeed, is there even an app for this???? (ever hopeful!!)