Minolta MC W.Rokkor-SI 28mm f/2.5

This is the radioactive one. The f/2.5 W.Rokkor uses thorium glass, a rare-earth element Minolta worked into the formula for its high refractive index, and decades later that decision is what gives the lens its signature. The thorium slowly yellows as it decays, so most surviving samples carry a warm, amber cast that you either correct for or learn to like. When it arrived in 1969 it was Minolta's fastest 28mm wide, sitting above the f/3.5 that had been the only option until then, with the f/2.8 and the faster f/2 not joining the lineup until later in the decade. A working photographer's wide, not exotic glass, but one that rewards a roll.

Wide open at f/2.5 it is soft in the corners and a touch low in contrast, the way most retrofocus wides from this period are. The center is already usable, though. Stop down to f/5.6 and the whole frame tightens up with the kind of crisp micro-contrast the Rokkors were known for. By f/8 it is flat-field sharp edge to edge and holds that through f/11. The color, on a yellowed sample, leans warm and pushes blues toward green rather than rendering them cleanly, so do not expect honest skies straight off a faded copy. A heavily yellowed lens can be partially bleached back by leaving it under UV or strong sun for a stretch, which is worth knowing before you write off the color.

The SI code follows Minolta's convention where the first letter is the group count and the second the element count: S (Sept, seven) groups and I (ninth letter, nine) elements, so nine elements in seven groups. It is a proper retrofocus design, built to clear the SR mount mirror box, and Minolta's all-metal construction from this period is dense and precise. Minolta marketed its own Achromatic multi-coating, introduced in 1958 and used through the 1970s, which is why these render with more saturation and less veiling flare than you would expect from glass this old. The focus throw is long and damped, geared for deliberate work rather than snap focusing. These survive because they were overbuilt.

Who shoots it: reportage, environmental portraits, street work on an SRT body. The 28mm length on full-frame is the documentary standard, wide enough to put context around a subject without the distortion drama of a 24mm. It flares predictably into a hard light source, a veil rather than colored ghosts, so keep a hood on it. The honest weakness is the wide-open corners combined with the falloff at f/2.5, on top of the color shift a yellowed copy brings. If you need sharp corners at maximum aperture, look elsewhere or stop down.

Used prices vary widely depending on how yellowed the glass is, and the radioactive reputation keeps it on collectors' radar. People cross-shop it against the Canon FD 28mm f/2.8 and the Nikkor 28mm f/3.5, and the Rokkor tends to win on character for the money once you have made peace with the warmth. Most live on mirrorless bodies now through a simple SR adapter.

One practical note. The fast f/2.5 maximum aperture is the reason to own this over the slower versions, so meter it wide open in dim interiors and let Zone Light Meter place your shadows before you decide where to stop down. The 55mm filter thread takes common screw-in glass, handy if you run a polarizer to cut glare or a grad for landscape frames.