Ever since I started doing macro photography three years ago, I've wanted the 65mm 1-5x macro lens. Recently, though, I started thinking about diffraction, and concluded that using this lens only gives a slight advantage over using an ordinary macro lens, then cropping.


I'll explain what I mean (and if I'm wrong, I'd be happy if someone would set me straight). Right now I use a 100mm macro, and with extension tubes I can get 2x. If I want to take a picture at 4x, I could crop to half size, but then of course my linear resolution would be cut in half.


Compare that with getting 4x directly with the mp-e. Effective f number is f times (1 + magnification) (I think), so that means that at 2x, my diffraction circles are 3 times as big as they would be at infinity (and after I crop, they are 6 times as big). At 4x without cropping, they are 5 times as big.


In other words, using the lens at 4x only gives a 20% linear improvement over using the lens I have at 2x and cropping. I'm assuming, of course, that diffraction is the limiting factor, which I think is reasonable beyond 2x.


Is that right? Even if it is, it doesn't mean the lens isn't worthwhile (plenty of people pay bigger bucks for even more marginal improvements). It just means it won't help as much as I thought.