The Optomax 75-150mm f/3.9 was a compact 'Auto Zoom' telephoto, part of the wave of affordable Japanese-made zoom lenses reaching UK camera shops from the mid-1970s as one-lens alternatives to separate short and medium telephoto primes, similar in concept to the contemporary Hanimex and Ozeck zooms.
It carries automatic diaphragm coupling and was produced primarily in M42 screw mount, with Canon FD and Pentax K bayonet-fit versions also recorded on the secondhand market. Maximum aperture holds at f/3.9 through the range, and the barrel uses a one-touch push-pull zoom and focus collar typical of the class.
As with most consumer zooms of its generation, sharpness and contrast are modest next to fixed-focal-length lenses of the period, improving noticeably once stopped down a stop or two from maximum aperture. Its main appeal remains the flexibility of a single 2x zoom range in a small package.
Used examples are inexpensive, commonly under twenty pounds. Because these zooms are prone to sticky zoom/focus helicoids and hazy internal elements after decades of storage, check the zoom action for smoothness and the auto-diaphragm lever for a clean, full stop-down before buying.