2026-08-04 300mm f/5.6 Bushnell Automatic Lens

There aren’t many lenses you come across sold under a distributor’s name that also clearly states who really made the lens. In this case this 300mm f/5.6 Bushnell Automatic has a serial number of "No.tam202707". Clearly the lens was made by Tamron.

A piece of Tamron’s earlier advertising said the then seven Adapt-a-Matic adapters would fit the seven lenses then available to 27 of the most popular cameras. As you will see the line reached nine adapters and nine lenses.

The 300mm f/5.6 was one of nine lenses made in the Tamron Adapt-A-Matic “LM” series for four years—

21mm f/4.5
28mm f/2.8
35mm f/2.8
135mm f/2.8
200mm f/3.5
300mm f/5.6
70-220mm f/4
80-250mm f/3.8
200-500mm f/6.9

These were the last lenses—

made without the aid of computer design
made without special glass, including low dispersion elements
all the screws were glued in place
they were so early they may even had hand milling/machining

The Adapt-A-Matic lens mounts were—

P-CS for Asahi PentaS2, SV and SP, Praktica, Edixa, Mamiya TL and Ricoh Singlex TL
N-IF for Nikon F, Fotomic, Nikkormat FS, FT and Nikkorex F
SR for Minolta SR1, SR2, and SR7
K-A for Konica Autoreflex
P-E for Petri FT, V6, 6F and V3
C-AF for Canon FP, FT, FX and Pellix
EX for Exakta cameras
M-A for Miranda F, G, Automex III and Sensorex
SR-T for Minolta SR T101
T-O for Topcon RE super, RE-2

Specifically the 300mm f/5.6 was designated as model 670Au introduced in 1972 and discontinued in 1973. Talk about simple designs, it has four elements in two groups (two in the front, two in the back). Magnesium fluoride coating was used internally and that is the only coating used.

Tamron 300mm f/5.6 (670Au) Specifications

Angle of view at infinity 8’ 12”
Minimum focus 2.5 m (8 feet)
Filter size        62mm, screw in
Magnification ratio 1:6
Maximum diameter 70mm
Length        200mm
Weight        795 g (1 lb. 12 oz)

Unlike later adapters, the Adapt-a-Matic adapters required a little in the way of instructions to install. There was just one coupling lever that sort of had to be maneuvered into place, but it wasn’t that hard to do.

The instructions are available on the internet.

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