What Does Stacking Lenses Mean?
Lens to lens stacking, or stacking lenses, refers to using two fixed focal length lenses. The shorter lens is reversed so the lenses are face to face. They are connected via a male to male filter thread adapter that costs about $20 or less. Called lens to lens stacking rings, they are available in most filter sizes. Stacking short lenses on longer lenses can give 1X to 4X magnification.
Just use standard fixed focal length lenses. Don't use zoom to zoom or macro to macro lenses for stacking lenses since it will be difficult to focus, if it can be done at all.
With lens to lens stacking, the magnification equals the focal length of the long lens (next to the camera body) divided by the focal length of the shorter reversed lens. For example a 100mm lens with a reversed 50mm lens stacked on it gives 2X magnification. A 200mm lens with a reverse mounted 50mm lens will give 4X magnification. However, a 200mm lens with a 50mm lens hanging onto that is awkward to use and can vibrate too much even on a good tripod. If a 200mm lens is used, consider getting a tripod mount for your lens to balance the contraption better on your tripod. So to find out the amount of magnification, divide the focal length of longer lens attached to your camera to the shorter lens which is reverse mounted.
The shorter reversed lens should be set at its widest aperture. Some lenses don’t allow the aperture to be manually set when the lens is not mounted on the camera. Exposure is controlled by the shutter speed and the aperture of the normally mounted long lens. Some lenses will stop down when reverse mounted since the rear elements and connections are not connected to a camera. You can cut off the flat portion of a rear lens cap, making a kind of rear lens hood to try and keep the aperture open wide, and to protect the rear element of the lens which is now exposed with no protective filter on it. Scratching the rear element of a lens usually destroys it since rear element scratches are very noticeable.
Some combinations of lenses suffer from vignetting (black curves showing around the edges of the photograph). I use an 85mm Canon lens with a 58mm filter size that has a 50mm Nikon lens with a 52mm filter size stacked on and it has some vignetting I delete later in Photoshop®. I have gotten some very good detailed photographs of insects with this set up which is about 1.7X. (That was a mouthful). It doesn’t matter what brand is stacked on the front of your mounted lens since they are just connected by the filter threads.
A 200m lens with a 100m lens stacked on top of it gives 2x magnification and a good working distance. The disadvantage is this is really long and awkward to use.
A 100m with a 50mm stacked on it gives 2X magnification. A 200m lens with a 50mm lens gives 4X magnification.