It was always my understanding that the stacked lens functioned as a multi-element very high quality close-up lens with a high diopter value. A 50mm lens has a diopter value of +20.
My experiments with stacked lenses have convinced me that the stacked lens needs to have a very wide aperture to minimize vignetting. I've used an A 50/1.4 stacked on an A 200/4 with some success. I still needed an extension tube behind the 200mm to avoid vignetting. The viewfinder gets awfully dark, depth of field is extremely shallow and exposures are long enough to make you wish you were never born if there is any breeze at all. Stacking lenses is an excellent way to drive yourself nuts. Tom Reese
