http://www.pentaxforums.com/reviews/tamron-90mm-macro-vs-pentax-100mm-wr/introduction.html
This reminds me of a forum thread that I came across some time ago in which someone made a quick calculation about the actual focal length of the Tamron 90. I've forgot and lost the link anyway. However, I still remember the rationale behind the calculations involved, which is really simple. As such, I don't mind to work it out again on my own. I think it would still be quite interesting to work it out and compare the two after all.
First of all, we need to learn the lens formula, i.e. the basic law for lenses and optics, which will be used:-
(For Convex Lenses)
Where 1/S1 + 1/S2 = 1/f and f is the unknown parameter that we are going to calculate, i.e., the true focal length.
Now, both macro lenses have the same maximum 1:1 macro capability as designed and specified. As such, we can simply make S1 = S2 = S under this case and the formula is further simplified as 2/S = 1/f, or simply: f = S/2.
And then, we need to look back at the specifications of the Tamron lens, here, and then that of the original Pentax lens, here, for what the PF reviewer has summarised.
Next, the object to film-plane distance, i.e., 2 x S, is to be found out. This total distance is the "minimum focus" + lens physical length when fully extended (i.e., at 1:1) + the K-mount flange to film distance (i.e., back focus register distance which is in 0.045m for K-mount - the 0.46mm tolerance is for the film thickness).
It should then be noted that whilst the closest focusing distances and the minimum physical lengths are all given in the two specs, the full extended physical lengths of the lenses are not given. But nevermind, PF has taken those comparison photos for the lenses when they are fully extended and retracted. So, I can simply derive the required extended lengths of the lenses by proportion, after some simple measurements on the figures.
Here we go:-
1. For the Tamron:-
2S = 0.29m + 0.097m x 76/53 + 0.045m => S = 0.2370m => f = 0.1185m, or 118.5mm.
2. For the Pentax:-
2S = 0.30m + 0.0805m x 80/50 + 0.045m => S = 0.2369 => f = 0.1185m, or 118.5mm.
Wow, what a coincidence! Both actual focals are actually more or less the same! :-o And, both lenses when focused at the 1:1 closest distance are actually having a longer true focal than the specified focal of 90mm and 100mm respectively, no matter what the lens is named. :-)
Related:-
Image Magnification Change Subject to Object Distance