Tamron SP 70-210mm F/3.5 Model 19AH: In my opinion, and for several reasons, this is the best 70-210 zoom lens ever designed and manufactured. Overall somewhat above average performance within its class, plus several notable design features. This later one-touch constant F3.5 aperture zoom features a unique "reverse" zooming cam in which the zoom collar moves forward to become a lens shade when zooming to the longer focal lengths. This, in conjunction with the bayonet lens hood, assures that the optics are always optimally shaded against stray light. The model 19AH has approximately 50% better overall resolution and contrast compared to the earlier two-touch model 52A. Macro optical performance is exceptional (for a zoom), reaching 1:2.66 life size at the 210mm position. Macro performance towards the corners of the field is superior to the earlier model. Achieving this high level of optical performance throughout the entire zooming and focusing ranges was no easy task.
Compare the optical design of this lens with the optical design for the earlier 70-210 model 52A. You will note that Tamron still uses a four element focus group to greatly control aberrations and to provide extremely good macro performance. The primary aberration which needs to be controlled within any continuous focusing telephoto macro zoom lens is spherical aberration. Several of Tamron's SP telephoto prime lenses use a special spherical aberration compensator group to virtually eliminate focus induced spherical aberration. Unfortunately, a spherical aberration compensator group is impossible to incorporate within a telephoto zoom lens since the zoom's variator and compensator groups occupy the space within the optical design where the spherical aberration compensator group would be placed. Tamron's optical engineers had to seek out other solutions. The only practical solution was to further refine the design of the four element focus group to not merely minimize focus induced spherical aberration, but to actually compensate for it to a fair degree. This is the purpose of the focus group's cemented doublet which looks like a meniscus lens. It allowed Tamron to optimize this lens for focus distances which are much closer to infinity rather than macro, yet also allows focus induced spherical aberration to be compensated for fairly well when close focusing. But there is a limit to this design approach, and this is why this lens is limited to a 1:2.66 macro magnification ratio compared to the earlier model's 1:2 macro magnification ratio. The result, however, is a 3:1 zoom lens which not only is sharp when photographing distant subjects but also is fairly sharp when used for macro photography.
In terms of overall optical performance, solid construction, ease of handling, zero zoom creep, and optimized variable shading when the lens is used with its lens hood, you can see why I think that this is the best 70-210 zoom lens ever produced by any manufacturer. In short, Tamron achieved the optimal balance of the overall optical and mechanical design features which Tamron wished to incorporate into this lens.
Text from http://www.adaptall-2.com/lenses/19AH.html