What magnification will I get with a particular combination of lens, extension tubes, and supplementary close-up lens?
Riccardo Polini’s web page called “Introduction to Macro Photography” has a wealth of information on these and related issues, and contains formulae that, while only strictly holding true for “simple” (thin, single element) lenses, are nonetheless useful in approximating the effect of adding tubes and diopter lenses to a lens. Bill Claff’s Simple Close-up Calculator gives a quick and easy way of calculating approximately what a particular combination will give you. There is also an Excel spreadsheet on Tony Jeffree’s website that can be used to calculate the effect of such combinations.
