I have a 50mm prime lens and a 18-55mm standard lens at my disposal. How do I measure the magnification of a reversed lens when used with a reverse mount. What are the factors that come into play to calculate the magnification?
Photograph a normal ruler, mm scale preferred. Then if your sensor is 24 mm wide (camera specs should say), and if the image shows say 20mm of that ruler, then the magnification is 24/20 or 1.2x (larger than 1:1 life size).
Normally lenses show smaller than life size, like showing 40 mm on a 24mm sensor would be 24/40 = 0.6x magnification.