The longer the extension tube used, the higher the lens magnification becomes as you’re able to move the lens much closer to the subject and still achieve focus. What all this means is that you can turn a normal lens into something with much a much higher magnification factor to achieve macro-like images, without needing a dedicated macro lens.
A simple mathematical equation helps you determine what level of magnification you will achieve when using a specific size extension tube on a specific lens:
Additional magnification = Extension tube’s length in millimetres divided by the focal length of a lens
A 50mm extension tube combined with a 50mm lens (50 divided by 50) gives you a 1:1 ratio. Likewise, a 25mm extension tube combined with a 50mm lens (25 divided by 50) can offer a 1:2 (or half size) representation of your subject.
The important point is that “The diameter of an aperture is equal to the focal length, divided by the f-number”. This means that f-stop is directly related to the focal length and since focal length is related to various dimensions in the lens design, extension tubes also change the effective focal length of a lens. This in turn causes a change in effective f-stop
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