This is an amazing zoom lens from the 1970s intended as a portrait zoom, which has a variable soft focus feature. The lens is quite rare, and I was lucky to find one in excellent condition. The variable soft focus ring actually moves a group of elements in the lens to increase aberrations. This not only introduces a “glow” in the subject, but also strongly affects how the out-of-focus areas are rendered.
Here is how Tamron described it:
“Tamron SP 70-150mm F/2.8 Model 51A: Tamron’s fast 70-150 F/2.8 constant aperture zoom lens was specifically designed for portrait photography, and was the first compact telephoto zoom lens ever produced by any manufacturer which featured a built-in softness control. This lens is extremely sharp at all focal lengths when not using the softness control since a total of six lens elements are used in the variator and compensator groups to reduce zoom dependent aberrations to their absolute minimum. Although the optical performance is somewhat optimized for 105mm (the ideal portrait focal length), this lens’s optical performance nevertheless is very good throughout the entire zoom range.
How does the softness control work? Simply press the SOFT ring button and then dial in any amount of softness adjustment from zero to three. The SOFT ring works by shifting the rear-most elements of the master lens group to introduce spherical aberration plus small amounts of off-axis coma and off-axis astigmatism.”
So this is a very versatile lens, a fast zoom with variable “bokeh”. Here are some examples of what can be achieved.