Here is a video tip from Jim Talkington wherein he explains how to achieve super macro shots with no special equipment. With this technique, the ratio of focal lengths of the main lens to the reverse lens is the magnification. Therefore with a 105mm lens and a 50mm reverse lens, the magnification is actually about 2:1. And since DSLRs have small sensors, it's actually more like 3:1, which should be obvious from the photo he provided. The screw looks huge!
1:1 is defined as such: imagine a film negative and you placed a screw on the negative and shined some light on it. The size of the silhouette of the screw on the negative will be it's actual size. This is called 1:1.
Generally, macro lenses are designed for full frame cameras. So they are 1:1 at the minimum focus distance for 35mm film. On a DSLR with a APS-C sized sensor a macro lens can produce an image that is 1.6:1, not 1:1.
If you use a longer lens as the main lens, you get more magnification. For instance a 300mm lens and a 50mm lens gives you 6:1! (Even more, really, because of the crop factor.)
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