Harrzack Here's the formula for calculating magnification (or reproduction ratio if you prefer):
((Total Extension - Focal Length) / Focal Length) = Magnification
Where: "Total Extension" is the distance of the lens' principal plane from the film plane. By definition, a lens focused at infinity is extended by its focal length. Then as you rack the lens out to focus you add more extension, plus any tubes or macro bellows.
A 105mm lens that gives 1:1 has a total extension of 210mm:
((210 - 105) / 105) = 1
meaning the image on the film (forgive me for being so retro) will be the same size as the subject.
When you add another 68mm of extension via the Kenko tubes, you then have a total extension of 210mm plus the 68mm of the tubes, or 278mm. Inserting that into the equation:
(210 + 68 - 105) / 105 =~ 1.65x with the lens at max extension
(105 + 68 - 105) / 105 =~ 0.65x with the lens at infinity
As you can see from the math, the quoted 1:10 lower limit for your lens is not accurate, at least if the lens is capable of infinity focus, but is just a manufacturer's / marketer's shortcut to keep from confusing people with things like infinity and zeros.
Each time you extend the lens another multiple of its focal length, you add 1X to the magnification. A 105mm lens that is 210mm from the film plane gives 1X. If you extend the same lens to 315mm, you get 2X; 420mm gives 3X and so on. That's why you won't get a 10X range of reproduction ratios with the extended lens.
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