Close-up lenses measure their power in diopters with the formula.
The technique of Stacking Lens typically involves using a telephoto lens on your camera as your normal lens, then taking a shorter focal length lens and using a filter to filter adapter, mount it to the telephoto in a reversed position. When you are doing this, you are essentially using the reversed lens as a supplementary close-up lens. So using the above formula reversing a 100mm lens would be a +10 Close-up; while a 50mm lens would be a +20 Close-up! You can easily see why 50mm and shorter lenses are a favorite for this technique.
To stack two lenses you need a filter to filter adapter. These are also sold as male to male adapters or as macro-coupling rings. If you can’t find an adapter with the filter size you need, you can mate it with a step-up or step-down ring, like the one pictured here. In this case it is a 58mm to 77mm adapter that is attached to a 77mm to 72mm step-down ring.
Magnification of Stacked Lenses
The above formula does not account for any extension that may already be built into the lens. You can also add your own extension to increase the magnification. You can also increase your magnification with a multiplier or tele-convertor. A multiplier will multiply your magnification.
A problem you can run into with stacked lenses is vignetting, or the darkening of the corners of the frame. This is why it is always a good idea to use a fast lens, like a 50mm f/1.8 as your reversed lens.
Continue with next section: Reversing Lenses