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 (the three lenses to the right of the aperture tick marks in the optical above diagram) to introduce spherical aberration plus small amounts of off-axis coma and off-axis astigmatism. Some may be wondering, "How accurate is the zero click stop position of the SOFT ring in order to prevent extremely slight softness from being introduced even though the SOFT ring is set to the zero position?" The answer is that Tamron machined the cam for the the three rear floating elements to produce zero softness effect until you turn the SOFT ring to approximately 1/4 of the way between the 0 to 1 marks on the SOFT ring.

Our notes about using the SOFT ring (these notes were never described by Tamron in the lens's accompanying instruction manual):

You have two choices when using the SOFT ring...

• The first choice is NOT to adjust the focus after adjusting the SOFT ring. This results rapid softening toward the edges of the film plane. This is suitable in portraiture photography where the head and torso occupy approximately the central 1/3 of the image frame, yet you want the surrounding areas (towards the edges of the film plane) slightly to moderately blurred depending on the SOFT setting.
• The second choice is to adjust the focus after turning the SOFT ring. This results in a gentler and much more even softening across the entire film plane. The second choice results in mostly a pure spherical aberration softness effect being introduced across the entire film plane. The amount of softening across the entire film plane, of course, is dependent on the SOFT setting after you have refocused. This procedure is suitable for tightly cropped head and partial torso portrait photography where your subject occupies most of the film plane, and where you wish to soften the extremely fine details across the entire film plane.

Note that, when using the softness ring, you should remember to always manually stop down this lens and then look through your camera's viewfinder to verify the overall softness effect for the selected F/stop on the aperture ring. An even better method is to shoot several test shots of a subject at the SOFT 0, 1, 2 and 3 settings and at F/2.8, F/5.6 and F/8, and with no refocusing or after refocusing. Make careful notes, get the resulting prints (4"x6" is good enough) and label them. Break up the prints into two groups — one group without refocusing, and the other group after refocusing. Then you have a nice set of labeled "index card" prints which will show you what to expect. Once you have your "index card" prints and have learned what to expect from this lens, you will be amazed with your results.

Lens Specifications:

Lens Model	51A
Focal Length	70 — 150mm
Aperture Range	f/2.8 — 32, AE
Angle of View	34°~16°
Optical Construction (Groups / Elements)	10 / 14
Min. Focus from Film Plane	38.6" (0.98m)
Macro Mag. Ratio [w/2X Converter]	1:4.6 [1:2.3]
Filter Size	62mm
Diameter	2.7" (67.5mm)
Length at [w/Nikon mount]	5.8" (147mm) [6.0" (151.5mm)]
Weight	26.8 oz. (760g)
Lens Hood	Built-in type, retractable.
Accessory	Accepts SP 2X tele-converter #01F.

The SP 70-150 Dual SOFT Lens Which Never Went into Production

The pages below are from a Tamron news release distributed at the 1980 Photokina show. The lens, which never went into production and of unknown model number, was meant to replace the model 51A. We have no explanation for why this lens never went into production, but can only guess that the limited market niche combined with high manufacturing costs were significant factors in Tamron's decision to cancel production. It certainly would have been an extremely versatile lens.