What are some useful equations for Macro?
by Administrator
1. Effect on magnification
m’ = m + (L/f)
where m’ is the magnfication with the extension tube in place, m is the magnfication with no extension tube, L is the length of the extension tube in mm, and f is the focal length in mm.
Note that this will work for any (consistent) position of the focusing ring, but is usually of interest at the "closest focus" setting (which pertains to the greatest magnfications).
2. Effect on working distance
W’ = W - f((m’-m)/(m’m))
where W is the working distance with the extension tube in place, W is the working distance with no extension tube, and m’ and m’ are the magnifications with the extension tube in place and without.
As before, this will work for any (consistent) position of the focusing ring, but is usually of interest at the "closest focus" setting.
Do note that the "focus distance" often quoited for lenses is not the same as the working distance; the working distance is measured either from the front (’vertex") of the front element of the lens, or from the front of the entire lens. (The difference is pertinent only to lenses with an "inset" front element - "integral hood" lenses.) Thus one must be careful when dealing with the specifcations of "inset front element" lenses.
Equation (2) will work for either definition of working distance.
Doug
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